Bristle Software SQL Tips

Details of Tips:

MySQL Tips

SQL Tips

This section contains tips on SQL (Structured Query Language) statements in MySQL.
1. Top-N query
  Original Version: 2/20/2008
  Last Updated: 2/20/2008
  Applies to: MySQL 3+ (maybe even earlier)
  
  To limit the number of rows returned by a query, use the clause:
```
    LIMIT count
```
  For example:
```
    SELECT *
    FROM employee
    ORDER BY employee_id
    LIMIT 10;
```
  This can be much faster than querying all of the rows, not only because it sends less data back to the client (perhaps across a slow transmission line), but also because the query itself runs more quickly inside the database, using smaller internal data structures for sorting, etc.
  
  Unlike other databases (Oracle, Microsoft, etc.), in MySQL this "just works". There is no need for nested subqueries to ensure the correct order of operations, no need for "hints" to reduce the size of internal data structures, etc. Logically, it selects the specified rows, then sorts by the specified criteria, then finally limits the number of rows returned. Internally however, it makes some optimizations since it can discard rows that won't be returned. For example, while sorting, a LIMIT 10 clause allows it to avoid actually sorting all rows. Instead, it only keeps the 10 best at each step in the sort, and discards the rest unsorted.
  
  For more details of speed and efficiency, see:
  Efficiency of Top-N and pagination queries
  
  Thanks to Tom Stluka for contributing to this tip!
  
  --Fred
2. Pagination query
  Original Version: 2/20/2008
  Last Updated: 2/20/2008
  Applies to: MySQL 3+ (maybe even earlier)
  
  To page through a large data set, returning a specified number of rows at a time, use the clause:
```
    LIMIT offset, count
```
  For example:
```
    SELECT *
    FROM employee
    ORDER BY employee_id
    LIMIT 50, 10;
```
  This skips rows until it reaches the specified offset (50), then returns the specified number of rows (10). The offset is 0-based, so OFFSET 0 skips 0 rows and starts at the 1st row, while OFFSET 1 skips 1 row and starts at the 2nd row.
  
  As of MySQL 5 (or perhaps even MySQL 4 -- I'm not sure), an additional syntax was added as an option for better clarity. (Note that the order of count and offset are reversed from the previous syntax.)
```
    LIMIT count OFFSET offset
```
  For example:
```
    SELECT *
    FROM employee
    ORDER BY employee_id
    LIMIT 10 OFFSET 50;
```
  See the previous tip: Top-N query for additional details of how and why this can make your queries faster, and for assurances about not needing nested subqueries to force it to select, sort, and then limit the number of returned rows.
  
  For more details of speed and efficiency, see:
  Efficiency of Top-N and pagination queries
  
  For issues about paging accurately, see:
  Accuracy of pagination queries
  
  Thanks to Tom Stluka for contributing to this tip!
  
  --Fred
3. Choose a DB engine
  Original Version: 4/4/2015
  Last Updated: 4/4/2015
  Applies to: MySQL 5+
  
  MySQL supports various DB engines internally (InnoDB, MyISAM, etc.).
  
  The default engine was MyISAM until MySQL 5.5.5 when it changed to InnoDB.
  
  You can specify a different engine for each table. However, that's generally not a good idea because it interferes with foreign keys, leading to bogus ConstraintError and IntegrityError exceptions.
  
  One easy way to accidentally end up with a mix of engines is to dump a DB from one database server and load it into a second server. The dumped table definitions will specify the old engine, so those tables will use that engine, but newly created tables will use the default engine of the second server. It is especially easy when you move tables from a version of MySQL older than 5.5.5 to a newer version, since the default changes.
  
  Another easy way is to change the default engine after creating some tables, and not change the engine of those tables. Or upgrade an existing database server from a pre-5.5.5 version to a post-5.5.5 version.
  
  To see which engine is being used for a table:
```
    SHOW CREATE TABLE table1;
```
  To see a table of all tables and their engines:
```
    SHOW TABLE STATUS FROM database1;
```
  To change the engine of a table:
```
    ALTER TABLE table1 ENGINE=INNODB;
```
  To see which version of MySQL you are running:
```
    % mysql --version
```
  To change the default engine used for newly created tables that don't specify an engine, add this:
```
    [mysqld]
    default_storage_engine=MyISAM
```
  to the file:
```
    /etc/my.cnf
```
  Thanks to Hans Hermans for helping to diagnose the problem when it actually occurred to us on 10/9/2014!
  
  --Fred

See Also
Original Version: 2/20/2008
Last Updated: 2/20/2008
Applies to: MySQL 3+

The following are good sources of info about MySQL:
1. The MySQL Reference Manual, included in the free download of the software.
2. The MySQL links at my links page:
  http://bristle.com/~fred/#mysql_db
--Fred

Oracle Tips

SQL Tips

This section contains tips on standard SQL (Structured Query Language) statements in Oracle.
1. Top-N query
  Original Version: 1/24/2008
  Last Updated: 2/26/2008
  Applies to: Oracle 7+ (maybe even earlier)
  
  To limit the number of rows returned by a query, use the clause:
```
    WHERE ROWNUM <= count
```
  Warning: If you use an ORDER BY clause, as:
```
    SELECT *
    FROM employee
    WHERE ROWNUM <= 10
    ORDER BY employee_id;
```
  you may not get the rows you expected because, by default, it selects the first 10 rows in the database and sorts them, rather than sorting all rows and returning the first 10 of the sorted order. Instead, you must apply the ROWNUM test to an outer SELECT that occurs after the sorting, as:
```
    SELECT * 
    FROM (SELECT * 
          FROM employee
          ORDER BY employee_id)
    WHERE ROWNUM <= 10;
```
  This can be much faster than querying all of the rows, not only because it sends less data back to the client (perhaps across a slow transmission line), but also because the query itself can be made to run more quickly inside the database, using smaller internal data structures for sorting, discarding irrelevant rows earlier, etc. For details, see:
          General efficiency of Top-N and pagination queries
          Oracle efficiency of Top-N and pagination queries
  
  For complete gory details, see:
          http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html
  
  Thanks to Tom Luskin and Rob Baptista for contributing to this tip!
  
  --Fred
2. Pagination query
  Original Version: 1/24/2008
  Last Updated: 2/20/2008
  Applies to: Oracle 7+ (maybe even earlier)
  
  Based on the Top-N query tip above, you might assume that you can page through a large data set, returning a specified number of rows at a time, using multiple ROWNUM clauses as:
```
    SELECT *
    FROM (SELECT *
          FROM employee
          ORDER BY employee_id)
    WHERE ROWNUM >= 6
    AND ROWNUM <= 10;
```
  However, that doesn't work because of the details of how ROWNUM is computed. It is a counter that only gets incremented after each row is selected. It starts at 1, which is never >= 6, so no row ever gets selected, so it never gets incremented to 2. No rows are returned.
  
  Instead, you have to do the following to get ROWNUM calculated after the sort, for rows 1-10, and then filter out rows that are not >= 6:
```
    SELECT *
    FROM (SELECT e.*
                ,ROWNUM rnum
          FROM (SELECT *
                FROM employee
                ORDER BY employee_id) e
          WHERE ROWNUM <= 10)
    WHERE rnum >= 6;   -- Note: rnum, not ROWNUM
```
  For speed and efficiency concerns, see:
          General efficiency of Top-N and pagination queries
          Oracle efficiency of Top-N and pagination queries
  
  For complete gory details, see:
          http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html
  
  For issues about paging accurately, see:
          Accuracy of pagination queries
  
  Thanks to Tom Luskin and Rob Baptista for contributing to this tip!
  
  --Fred
3. Efficiency of Top-N and pagination queries
  
  Original Version: 1/24/2008
  Last Updated: 2/26/2008
  Applies to: Oracle 7+ (maybe even earlier)
  
  As described in General efficiency of Top-N and pagination queries, for very large tables (millions of rows or more), it can be pretty slow to page through the rows using a new query for each page of rows, or even to retrieve the first page of rows. You may need to tune your database to get acceptable performance, adding indexes, optimizer "hints", etc.
  
  For Oracle, the Top-N and pagination queries may run much faster if you use the FIRST_ROWS(n) hint, as:
```
    SELECT * 
    FROM (SELECT /*+ FIRST_ROWS(10) */
                 * 
          FROM employee
          ORDER BY employee_id)
    WHERE ROWNUM <= 10;
```
  and:
```
    SELECT *
    FROM (SELECT /*+ FIRST_ROWS(10) */
                 e.*
                ,ROWNUM rnum
          FROM (SELECT *
                FROM employee
                ORDER BY employee_id) e
          WHERE ROWNUM <= 10)
    WHERE rnum >= 6;   -- Note: rnum, not ROWNUM
```
  This hint allows the database to avoid fully sorting all rows. Instead, it only keeps the 10 best at each step in the sort, and discards the rest unsorted.
  
  Thanks to Tom Luskin and Rob Baptista for contributing to this tip!
  
  --Fred
4. SELECT * and more
  
  Last Updated: 6/6/1999
  Applies to: Oracle 7.3, 8 (and probably earlier versions)
  
  To select all columns of a table:
```
	select * from table
```
  However, to select all real columns, plus a pseudo-column like "user":
```
	select table.*, user from table
```
  The following does not work:
```
	select *, user from table
```
  --Fred
5. Materialized View
  
  Last Updated: 1/7/2002
  Applies to: Oracle 8+
  
  Oracle 8i introduced a new feature called a "materialized view". You define it just like any other view, except that you add the keyword MATERIALIZED:
```
	CREATE MATERIALIZED VIEW view_name
```
  A materialized view is like a combination of a table and a view. Like a view, it is defined as a logical view into the data of one or more tables. When you update the tables, subsequent queries of the view see the updated data. However, like a table, its data is stored in the database. Also, like a table, it is faster if you define indexes for it.
  
  A regular view is stored as a mapping of data from tables. When you modify the data in the tables, the view is completely ignored. When you access the view, it joins the data currently in the tables, and returns the data you requested. A materialized view is stored as such a mapping along with a copy of the actual data from the tables. When you modify the data in the tables, the view's copy of the data is also updated. When you access the view, the data is drawn directly from the copy.
  
  Thus a materialized view makes table updates a little slower, but makes view queries much faster. It also consumes additional space in the database.
  
  You could accomplish the same effect by defining an additional table instead of the view, and using triggers on the component tables to update it each time they are changed. However, using a materialized view is more convenient, more efficient, and clearer to the next person who has to maintain your database.
  
  Thanks to Andy Glick for sending me a sample of a materialized view from his application!
  
  --Fred
6. Search Source Code of Stored Procedures
  
  Original Version: 1/7/2006
  Last Updated: 1/20/2006
  Applies to: Oracle 7.3+
  
  Since the source code of all Oracle stored procedures, functions, and packages is stored as data in the database, you can do queries against it like any other data. For example, you can find all references to a given table in all stored procedures, functions, and packages with the following query:
```
	SELECT *
	FROM   all_source
	WHERE  LOWER(text) LIKE '%table_name%'
```
  If you don't have SELECT access to the all_source table, you can try user_source or dba_source instead.
  
  Thanks to Chris Boos, the most knowledgeable Oracle guru I know, for this and many other tips!
  
  Tip within a tip: To prevent the LIKE clause from treating underscore as a wildcard, use the ESCAPE clause, as:
```
	SELECT *
	FROM   all_source
	WHERE  LOWER(text) LIKE '%table\_name%' ESCAPE '\'
```
  --Fred

PL/SQL Tips

This section contains tips on PL/SQL statements -- the Oracle "procedural language" superset of SQL that you use to write stored procedures.

SQL Navigator Tips

This section contains tips on the SQL Navigator tool by Quest Systems. It is a graphical front end to the Oracle database, allowing you to create, delete, view, and modify all Oracle objects: tables, views, stored procedures, etc.

See Also
Original Version: 6/6/1999
Last Updated: 2/20/2008
Applies to: Oracle 7.3+

The following are good sources of info about Oracle:
1. Koch, George, and Kevin Loney. Oracle 8, The Complete Reference. Berkeley CA: For Oracle Press by Osborne McGraw-Hill, 1997. ISBN 0-07-882396-X.
  This book includes introductory database concepts as well as a complete reference to Oracle SQL and PL/SQL statements. The companion CD contains a complete copy of the book, so you can read it on-line, search it, etc.
2. Any of the O'Reilly books. I've been very impressed by all of the O'Reilly books since my early Unix and X-Windows days in the 80's, and they have a complete series on Oracle, covering PL/SQL, the standard packages, etc.
3. The Oracle links at my links page:
  http://bristle.com/~fred/#oracle_db
--Fred

MS SQL Server Tips

SQL Tips

This section contains tips on SQL (Structured Query Language) statements in MS SQL Server.
1. Top-N query
  Original Version: 1/24/2008
  Last Updated: 1/30/2011
  Applies to: MS SQL Server 2005+
  
  To limit the number of rows returned by a query, use the syntax:
```
    SELECT TOP n FROM
```
  Also, as of MS SQL Server 2005, there is a new Oracle-like ROW_NUMBER() function:
```
    ROW_NUMBER() OVER (ORDER BY ...)
```
  Since ROW_NUMBER() was added so many years after Oracle's ROWNUM clause, and even after MySQL's LIMIT clause, and since it requires you to specify the ORDER BY clause as part of its syntax, you might expect it to do a sub-select internally like MySQL does to get around the issues with Oracle's ROWNUM (see Oracle Top-N query). However, for some reason, it does not. The following:
```
    SELECT e.*
          ,ROW_NUMBER() OVER (ORDER BY employee_id) AS rnum
    FROM employee e
    WHERE rnum <= 10;
```
  selects the first 10 rows in the database and sorts them, rather than sorting all rows and returning the first 10 of the sorted order. To get the results you wanted, you must use the Oracle technique of applying the ROW_NUMBER() test to an outer SELECT that occurs after the sorting, as:
```
    SELECT *
    FROM (SELECT e.*
                ,ROW_NUMBER() OVER (ORDER BY employee_id) AS rnum
          FROM employee e)
    WHERE rnum <= 10;
```
  This can be much faster than querying all of the rows, because it sends less data back to the client (perhaps across a slow transmission line). Like Oracle and MySQL, it may also be possible to make it run more quickly inside the database, using smaller internal data structures for sorting, discarding irrelevant rows earlier, etc. Anyone know if it does so automatically, or how to cause it to?
  
  For speed and efficiency concerns, see:
          Efficiency of Top-N and pagination queries
  
  For complete gory details, see:
          http://www.asp.net/learn/data-access/tutorial-25-vb.aspx
          http://msdn2.microsoft.com/en-us/library/ms186734.aspx
  
  Thanks to Steve Weitzman, Rob Baptista, and Darin Strait for contributing to this tip!
  
  1/30/2011 Update: In MS SQL Server 2011, there's a new syntax. See:
          http://www.sqlservernation.com/home/paging-features-in-sql-server-and-postgresql.html
  
  Thanks to Steve Weitzman for the update!
  
  --Fred
2. Pagination query
  Original Version: 1/24/2008
  Last Updated: 2/20/2008
  Applies to: MS SQL Server 2005+
  
  As you may have guessed, once you've mastered the quirks of the ROW_NUMBER() clause (see MS SQL Server Top-N query), you can use the same syntax to page through a large data set, returning a specified number of rows at a time:
```
    SELECT *
    FROM (SELECT e.*
                ,ROW_NUMBER() OVER (ORDER BY employee_id) AS rnum
          FROM employee e)
    WHERE rnum <= 10
    AND   rnum >= 6;
```
  For speed and efficiency concerns, see:
          Efficiency of Top-N and pagination queries
  
  For complete gory details, see:
          http://www.asp.net/learn/data-access/tutorial-25-vb.aspx
          http://msdn2.microsoft.com/en-us/library/ms186734.aspx
  
  For issues about paging accurately, see:
          Accuracy of pagination queries
  
  Thanks to Steve Weitzman, Rob Baptista, and Darin Strait for contributing to this tip!
  
  --Fred
3. Dynamic SQL in a Stored Procedure
  Original Version: 2/7/1999
  Last Updated: 3/18/2006
  Applies to: MS SQL Server 6.5+
  
  A typical tradeoff for a database application is dynamic SQL (SQL commands embedded in the application -- for flexibility) vs. stored procedures (pre-compiled SQL procedures stored in the database and invoked by name from the application -- for speed and control over what SQL statements get executed). However, you can have the best of both worlds by using dynamic SQL inside your stored procedures. In a stored procedure, you can use the EXEC statement to execute a string of SQL statements that you built dynamically in the stored procedure or read from the database or any other data source.
  
  Warnings:
  1. Beware of security issues. One advantage of stored procedures is that they hide the underlying table structure, allowing you to grant access to the stored procedure and not to the tables. The stored procedure can access the tables using its own credentials. However, if the stored procedure uses dynamic SQL, the caller of the procedure will need permissions (select, update, delete, insert) on the tables.
  2. Beware of runtime errors, like exceeding the max string length when dynamically building a string of SQL to be executed.
  3. Beware of performance issues. For dynamic SQL, the query plan is only created at runtime, and may not even be cached if the dynamic query changes too much.
  4. Beware of debugging issues. Dynamic SQL is much harder to debug because you don't know for sure what string value is being passed to EXEC, unless you check it with the SQL Profiler.
  Thanks to:
  - Steve Rhoads for this tip.
  - Darin Strait for the warnings.
  --Fred
4. Search Source Code of Stored Procedures
  
  Last Updated: 1/10/2006
  Applies to: MS SQL Server 2000+
  
  Since the code of all MS SQL Server stored procedures is stored as data in the database, you can do queries against it like any other data. For example, you can find all references to a given table in all stored procedures in the current database with the following query:
```
	SELECT * 
	FROM   syscomments c, 
	       sysobjects o 
	WHERE  o.id = c.id
	AND    c.text LIKE '%table_name%'
```
  Thanks to Steve Weitzman for this tip!
  
  --Fred

This section contains tips on the SQL Enterprise Manager tool. It is a graphical front end to the database, allowing you to create, delete, view, and modify all MS SQL Server objects: tables, views, stored procedures, etc.

Keyboard Shortcuts

Original Version: 6/20/1999
Last Updated: 2/20/2006
Applies to: MS SQL Server 7.0+

Here is a list of some of the more useful shortcut keys in SQL Enterprise Manager.

Key	Function
F1	Help on SQL Enterprise Manager
Shift-F1	Help on syntax of current SQL statement
F4	Search for objects by name, type, etc.
Ctrl-E	Execute selected text in Query Analyzer
Ctrl-R	Hide/show results pane in Query Analyzer
Ctrl-Shift-C	Comment the currently selected text in Query Analyzer
Ctrl-Shift-R	Uncomment
Ctrl-Shift-L	Show the estimated query plan for the currently selected text.
Ctrl-K	Hide/show "Show execution plan, as executed". (The query has to be run for the query plan to be returned to Query Analyzer.)

Obviously, this list is far from complete. Please feel free to mail me your favorite shortcuts. I'll add to this list as time permits.

Contributors so far include: Darin Strait

For a comprehensive list of shortcuts, search for "shortcut keys" in the SQL Server Books Online.

General Tips

Efficiency of Top-N and pagination queries

Original Version: 2/20/2008
Last Updated: 2/26/2008

For very large tables (millions of rows or more), it can be pretty slow to retrieve the first few rows based on some sort criteria as described in:

        MySQL Top-N query
        Oracle Top-N query
        Microsoft Top-N query

because some databases may internally do a complete sort of all selected rows in order to determine which are the first few.

Also, it can be pretty slow to page through the rows using a new query for each page of rows as described in:

        MySQL pagination query
        Oracle pagination query
        Microsoft pagination query

Remember that the database is not holding open a single resultset created by a single query, and just returning the next page of rows. Instead, it is executing a new query for each page.

Presumably, you are doing this for the sake of an interactive user, to avoid overwhelming him, or his Web browser or other user interface, or his communication line, with too much data. Therefore, you only have to be fast enough to satisfy an interactive user. Less than a second or so is probably fast enough. If you were doing a massive batch operation on millions of rows, and needed much faster speeds, you would not have limited the query to one page at a time. You would have simply done one large query and iterated over the rows.

However, you must still be fast enough to satisfy the user, and must also avoid consuming massive resources in the database server. For these reasons, you may need to tune your database to get acceptable performance, adding indexes, optimizer "hints", etc.

Thanks to Art Graham and Rob Baptista for contributing to this tip!

--Fred

Accuracy of pagination queries
Original Version: 2/20/2008
Last Updated: 2/20/2008

If the contents of the DB are changing significantly during the pagination, a simple pagination query like:

        MySQL pagination query
        Oracle pagination query
        Microsoft pagination query

may not work reliably. Inserted rows can be skipped or can cause subsequent rows to be repeated. Deleted rows can cause subsequent rows to be skipped. Fortunately, for many applications, it doesn't matter much if you skip or repeat a row while paging through a long constantly changing list. However, when it does matter, you can avoid such problems by doing any of the following:
1. Lock the rows against update during the query. Yuck!
2. Copy the rows to a temporary table before paging. Yuck!
3. Select by a combination of row number and the sorted value that was displayed at the end of the previous page. Thus, you can resume at an appropriate place based on the changing table, but still get only the next n rows. If you do this, be careful about:
  1. Relying on a specific record continuing to exist.
    Don't rely on a specific record as your marker for where to resume. Be sure to test with "<", "<=", ">" or ">=", not "=" or "!=". You don't want to get tripped up by boundary conditions if someone deletes or updates that one record meanwhile.
  2. Sorting by highly duplicated values.
    If there are duplicate values in your sort column(s), you may need a tie breaker to distinguish among them and resume paging correctly after showing some of them, especially when there are more than a full page of duplicates. You could use row number as the tie breaker, but the order of the duplicate rows may change. It is better to specify more sort columns, to eliminate the duplicates.
Thanks to Art Graham for contributing to this tip!

--Fred

Portability Tips

Portable SQL syntax

Original Version: 12/19/2008
Last Updated: 12/19/2008
Applies to: All databases

Whenever possible, you should limit your SQL to standard portable syntax, as defined by the SQL-92 standard that is now implemented by most databases.

For links to a good SQL tutorial that focuses on portable syntax, and to the SQL-92 standard document itself, see:

http://bristle.com/~fred/#portable_sql

Thanks to Kevin Evans for telling me about the tutorial page!

--Fred

Top-N query
Original Version: 2/20/2008
Last Updated: 2/20/2008
Applies to: JDBC 1.0+

To limit the number of rows returned by a query, in a portable way, avoiding all of the different syntaxes and quirks described in:

        MySQL Top-N query
        Oracle Top-N query
        Microsoft Top-N query

use the JDBC/ODBC statement:
```
    stmtActiveStatement1.setMaxRows(n);
```
where 0 means no limit. Let the JDBC/ODBC drivers from the different database vendors deal with the differences among the databases.

However, if your resultset is large enough for efficiency to be a concern, beware of efficiency issues. Presumably, the database vendor uses the best possible algorithm for its own database in the driver it provides. However, that is not always the case. If the driver you are using does things inefficiently (like loading all rows of a huge resultset before discarding those beyond the first n, or sorting an entire huge resultset completely before discarding those beyond the first n), find a better driver or revert to the database-specific techniques.

Thanks to Art Graham for contributing to this tip!

--Fred

Differences Between Oracle and MS SQL Server

Concepts and Terminology

Original Version: 6/6/1999
Last Updated: 2/20/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows some differences in concepts and terminology between Oracle and MS SQL Server:

Concept/Term	Oracle	MS SQL Server 6.5	MS SQL Server 7+
Database engine (collection of databases)	Database	Database server	Instance
Database (collection of tables)	Schema	Database
Roles/Groups	Roles	Groups	Roles
Database administrator account	dba	sa	Member of appropriate server or database role Also: sa (deprecated)
Database owner	dba	dbo	Member of db_owner role
Data about the database	Data Dictionary - one per server	Database Catalog - one per database "Master" database - one per server	INFORMATION_SCHEMA views
Blocks and extents	Blocks and extents	Pages and extents
Tablespaces	Tablespaces	Not supported	File Groups
Network software	SQL*Net	Net-library
Data stream protocol	Transparent Network Substrate (TNS)	Tabular Data Stream (TDS)
Case sensitivity of names of tables, columns, etc.	Case-insensitive	Depends on character sort order, default is case-insensitive
Synonyms	Supported	Not supported
Readonly transaction	Supported	Not supported

Thanks to Darin Strait for the MS SQL Server 7+ info.

--Fred

Data Types

Last Updated: 6/6/1999
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows the corresponding data types in Oracle and MS SQL Server:

Data Type	Oracle	MS SQL Server
Fixed Length String	CHAR(n) - limit 2KB	CHAR(n), CHARACTER(n) - limit 255 (6.5) - limit 8KB (7.0)
Variable Length String	VARCHAR2(n), VARCHAR(n) - limit 4KB in a column - limit 32KB in a variable - VARCHAR is obsolete	VARCHAR(n), CHAR VARYING(n), CHARACTER VARYING(n) - limit 255 (6.5) - limit 8KB (7.0)
Integer	INTEGER, INTEGER(n), SMALLINT	INTEGER (4 bytes), INT (4 bytes), SMALLINT (2 bytes), TINYINT (1 byte), BIT (1 bit)
Fixed Point	NUMBER, NUMBER(n), NUMBER(n,d), FLOAT, FLOAT(n), FLOAT(n,d)	NUMERIC, NUMERIC(n), NUMERIC(n,d), DECIMAL, DECIMAL(n), DECIMAL(n,d), DEC, DEC(n), DEC(n,d), MONEY, SMALLMONEY
Floating Point	DECIMAL	FLOAT, FLOAT(n), DOUBLE PRECISION, REAL,
Date	DATE	DATETIME, SMALLDATETIME, TIMESTAMP - TIMESTAMP auto-updated
Binary	RAW(n) - limit 255 bytes	BINARY(n), VARBINARY(n), BINARY VARYING(n) - limit 255 (6.5) - limit 8KB (7.0)
Large String	LONG, LONG VARCHAR - limit 2GB - limit one per table row CLOB - limit 4GB	TEXT - limit 2GB
Large Binary	LONG RAW - limit 2GB - limit one per table row BLOB - limit 4GB	IMAGE - limit 2GB
Multi-byte chars	NCHAR(n) NVARCHAR(n) NCLOB - same limits as CHAR, VARCHAR, CLOB	NCHAR(n), NATIONAL CHAR(n), NATIONAL CHARACTER(n) NVARCHAR(n), NATIONAL CHAR VARYING(n), NATIONAL CHARACTER VARYING(n) NTEXT, NATIONAL TEXT - same limits as CHAR, VARCHAR, TEXT
OS File	BFILE	<not supported>
Row Identifier	implicit ROWID column	(use an IDENTITY column)
Secure OS Label	MLSLABEL, RAW MLSLABEL	<not supported>
128-bit Unique Number (UUID, GUID)	<not supported>	UNIQUEIDENTIFIER (version 7.0 only)

--Fred

Limits

Original Version: 6/7/1999
Last Updated: 2/20/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows differences in limits of Oracle and MS SQL Server:

Description	Oracle	MS SQL Server 6.5	MS SQL Server 7+
Columns per table	1000	250	1024
Row size	Unlimited	1962 bytes	8060 bytes - Includes pointers, but not data, for TEXT and IMAGE columns - With 2000, some text can be stored in-row
LONG and LONG RAW columns per row	1 (must be last column)	Unlimited (16-byte pointer per)
LOB, TEXT, and IMAGE columns per row	Unlimited (16-byte pointer per)
Clustered indexes per table	1
Non-clustered indexes per table	unlimited	249
Columns per index	16
Index row size	2K bytes	900 bytes
Identifier Length	30 chars		128 chars
Tables per SELECT	unlimited	16	256
Source code per stored procedure		64KB	250MB
Data type limits	(see Data Types)

For more info, search for "capacity" in the SQL Server Books Online.

Thanks to Darin Strait for updating the MS SQL Server 7+ info.

--Fred

Operators

Last Updated: 6/7/1999
Applies to: Oracle 7.3+, MS SQL Server 6.5+

Most operators are the same in Oracle and MS SQL Server. Here are some that differ:

Description	Oracle	MS SQL Server
String concatenation	string1 \|\| string2	string1 + string2

--Fred

Built-In Functions

Original Version: 6/7/1999
Fully Reviewed: 2/20/2006
Last Updated: 6/9/2009
Applies to: Oracle 7.3+, MS SQL Server 6.5+

Oracle and MS SQL Server offer many of the same built-in functions. For example, they both offer ABS, EXP, ROUND, UPPER, LOWER, AVG, COUNT, SUM, ASCII, etc. The following table shows some of the corresponding functions that don't have the same name. For a more complete list, see "Migrating Oracle Applications to SQL Server"

Description	Oracle	MS SQL Server
Smallest integer >= n	CEIL	CEILING
Modulus	MOD	%
Truncate number	TRUNC	<none>
Max or min number or string in list	GREATEST, LEAST	<none>
Translate NULL to n	NVL	ISNULL As of 7.0, also: COALESCE
Return NULL if two values are equal	DECODE As of Oracle 9i, also: NULLIF	NULLIF
String concatenation	CONCAT(str1,str2)	str1 + str2
Convert ASCII to char	CHR	CHAR
Capitalize first letters of words	INITCAP	<none>
Find string in string	INSTR	CHARINDEX
Find pattern in string	INSTR	PATINDEX
String length	LENGTH	DATALENGTH
Pad string with blanks	LPAD, RPAD	<none> Workaround: Use REPLICATE(' ', n) and string concatenation
Trim leading or trailing chars other than blanks	LTRIM(str,chars), RTRIM(str,chars)	<none>
Replace chars in string	REPLACE	STUFF, REPLACE
Convert number to string	TO_CHAR	STR, CAST
Convert string to number	TO_NUMBER	CAST
Get substring from string	SUBSTR	SUBSTRING
Char for char translation in string	TRANSLATE	<none>
Date addition	ADD_MONTH or +	DATEADD or +
Date subtraction	MONTHS_BETWEEN or -	DATEDIFF
Last day of month	LAST_DAY	<none>
Time zone conversion	NEW_TIME	<none>
Next specified weekday after date	NEXT_DAY	<none>
Convert date to string	TO_CHAR	DATENAME, CONVERT
Convert string to date	TO_DATE	CAST
Convert date to number	TO_NUMBER(TO_CHAR(d))	DATEPART
Parse date into parts (year, month, day, hour, min, sec, etc.)	TO_CHAR(date, format_string) Examples: TO_CHAR(SYSDATE, 'yyyy') TO_CHAR(SYSDATE, 'ss') etc. Also, as of Oracle 9.2: EXTRACT (date_part FROM date) Examples: EXTRACT(YEAR FROM SYSDATE) EXTRACT(SECOND FROM SYSDATE) etc.	DATEPART(date_part, date) Examples: DATEPART(YEAR, GETDATE()) DATEPART(SECOND, GETDATE()) etc. Also: YEAR(), DAY(), MONTH()
Date round	ROUND	CONVERT
Date truncate	TRUNC	CONVERT
Current date	SYSDATE	GETDATE or CURRENT_TIMESTAMP
Convert hex to binary	HEXTORAW	CAST
Convert binary to hex	RAWTOHEX	CONVERT
If statement in an expression	DECODE As of Oracle 9i, also: CASE ... WHEN or COALESCE	CASE ... WHEN or COALESCE
User's login id number or name	UID, USER	SUSER_ID (deprecated), SUSER_NAME (deprecated), SUSER_SNAME (preferred)
User's database id number or name	UID, USER	USER_ID, USER_NAME
Current user	USER	USER

Thanks to:

Jason Deacon for prompting me to update the Oracle column to reflect the new support for ANSI/ISO syntax.
Darin Strait for some MS SQL Server info.
Tor Helland for some date parsing functions.

--Fred

Differences in SQL Syntax

Original Version: 6/7/1999
Last Updated: 1/24/2008
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows the different syntax used in Oracle and MS SQL Server for the same SQL operations:

Description	Oracle	MS SQL Server
Left Outer Join	FROM table1 LEFT OUTER JOIN table2 ON (table1.column1 = table2.column2) Note: The following syntax is also supported, and was the only syntax prior to version 9i: WHERE column1 = column2(+) All rows from the right plus nulls to match additional rows from the left.	FROM table1 LEFT OUTER JOIN table2 ON table1.column1 = table2.column2 Note: The following syntax is also supported, but is no longer recommended: WHERE column1 *= column2
Right Outer Join	FROM table1 RIGHT OUTER JOIN table2 ON ( table1.column1 = table2.column2) Note: The following syntax is also supported, and was the only syntax prior to version 9i: WHERE column1(+) = column2 All rows from the left plus nulls to match additional rows from the right.	FROM table1 RIGHT OUTER JOIN table2 ON table1.column1 = table2.column2 Note: The following syntax is also supported, but is no longer recommended: WHERE column1 =* column2
Full Outer Join	As of Oracle 9i: FROM table1 FULL OUTER JOIN table2 ON (table1.column1 = table2.column2)	FROM table1 FULL OUTER JOIN table2 ON table1.column1 = table2.column2
SELECT without FROM	SELECT 'hello world' FROM DUAL	SELECT 'hello world'
SELECT data into a table	CREATE TABLE AS SELECT ...	SELECT ... INTO
Intersection of 2 SELECTS	SELECT ... INTERSECT SELECT ...	SELECT ... WHERE EXISTS (SELECT ...)
Subtraction of 2 SELECTS	SELECT ... MINUS SELECT ...	SELECT ... WHERE NOT EXISTS (SELECT ...)
INSERT into a JOIN	INSERT INTO SELECT ...	Create a VIEW and INSERT INTO it.
UPDATE data in a JOIN	UPDATE SELECT...	Create a VIEW and INSERT INTO it.
UPDATE one table based on criteria in another table	UPDATE table1 t1 SET field1 = (SELECT ... FROM table2 t2 WHERE <condition involving both t1 and t2>) WHERE EXISTS (SELECT ... FROM table2 WHERE <condition involving both t1 and t2>)	UPDATE table FROM ...
DELETE rows from one table based on criteria in another table	DELETE FROM table1 t1 WHERE EXISTS (SELECT ... FROM table2 WHERE <condition involving both t1 and t2>)	DELETE FROM table FROM ...
DROP a column from a table	As of Oracle 8i: ALTER TABLE table_name DROP COLUMN column_name	ALTER TABLE table_name DROP COLUMN column_name
Readonly VIEW	CREATE VIEW ... WITH READONLY	GRANT SELECT ...
Save point	SAVEPOINT	SAVE TRANSACTION
Table lock	LOCK TABLE...IN SHARE MODE	SELECT...table_name (TABLOCK)
Exclusive table lock	LOCK TABLE...IN EXCLUSIVE MODE	SELECT...table_name (TABLOCKX)
Reserving index space	PCTFREE=0	FILLFACTOR=100
Declaring a local variable	DECLARE varname type;	DECLARE @varname type
Initializing a local variable	DECLARE varname type := value;	<not supported>
Declaring a constant	DECLARE varname CONSTANT type := value;	<not supported>
Assigning to a variable	varname := value SELECT value INTO varname	SET @varname = value SELECT @varname = value
Assigning to a variable from a cursor	FETCH cursorname INTO varname	FETCH NEXT FROM cursorname INTO varname
Declaring a cursor	CURSOR curname (params) IS SELECT ...;	DECLARE curname CURSOR FOR SELECT ...
If statement	IF ... THEN ELSIF ... THEN ELSE ENDIF	IF ... BEGIN ... END ELSE BEGIN ... END
While loop	WHILE ... LOOP END LOOP	WHILE ... BEGIN ... END
Other loops	FOR ... END LOOP LOOP ... END LOOP	<not supported>
Loop exit	EXIT, EXIT WHEN	BREAK, CONTINUE
Print output	DBMS_OUTPUT.PUT_LINE	PRINT
Raise error	RAISE_APPLICATION_ERROR	RAISERROR
Statement terminator	Semi-colon (;)	None required, but semi-colons are tolerated as of 2000. GO is used to terminate a batch of statements.
INSERT with some columns getting their default values	As of Oracle 9i:	As of SQL Server 2000 (perhaps 7):
INSERT with some columns getting their default values	INSERT INTO table1 VALUES (123, DEFAULT, 'a string', NULL)
UPDATE resetting some columns back to their default values	As of Oracle 9i:	As of SQL Server 2000 (perhaps 7):
UPDATE resetting some columns back to their default values	UPDATE table1 SET col1 = 123, col2 = DEFAULT, col3 = 'a string', col4 = NULL
Limit number of rows returned	WHERE ROWNUM <= n Warning! See also: Oracle Top-N query Oracle Pagination query	SELECT TOP n FROM Warning! See also: Microsoft Top-N query Microsoft Pagination query

Thanks to:

Tom Johnston for catching a mistake in this tip. I had the FROM DUAL in the wrong column.
Jason Deacon for prompting me to update the Oracle column to reflect the new support for ANSI/ISO syntax.
Darin Strait for some MS SQL Server info.
Tom Luskin for the technique of updating one table from another in Oracle via 2 correlated subqueries.
Tom Luskin for the technique of limiting the number of rows returned.

--Fred

Differences in SQL Semantics

Original Version: 6/6/1999
Last Updated: 2/20/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows some semantic differences between Oracle and MS SQL Server:

Description	Oracle	MS SQL Server
Commit	Explicit COMMIT statement required	Automatic commit unless SET IMPLICIT_TRANSACTIONS ON
Reading uncommitted data	Database does temporary internal rollback to reconstruct most recently committed data for reader.	Depending on options, reader is allowed to read uncommitted data, or is forced to wait for writer to commit or rollback. As of 2005: New functionality designed to mimic Oracle's writers-don't-block-readers consistency model.
Releasing cursor data	CLOSE CURSOR releases all data. You can't re-open.	CLOSE CURSOR does not release data. You must explicitly call DEALLOCATE CURSOR. Until then, you can re-open the cursor.
Implicit data conversion in a statement like the following where vc is a column of type VARCHAR2: SELECT * FROM person WHERE vc =123	As each row is fetched from the table, an attempt is made to convert it to a number for the comparison with 123. If any row contains a value that cannot be converted to a number, a runtime error occurs.	The number 123 is converted to the string '123' once, and then the data is fetched from the table. If any row contains a value that cannot be converted to a number, it simply doesn't match '123' and is skipped without any error. Use the ISNUMERIC() function for more control, to search for expressions that do (or do not) evaluate to a valid numeric types.
Conversion to NULL	Setting a VARCHAR2 column to '' (the empty string) makes it NULL.	Setting a VARCHAR column to '' makes it the empty string (not NULL). Note: With text columns, this usually means that an 8KB page is allocated to store the empty string. Setting the column to NULL releases that storage.
Cursor performance		Typically, cursors are slower than joined select, insert, update or delete operations.

Thanks to Darin Strait for some MS SQL Server info.

--Fred

Differences in Managing Databases

Last Updated: 6/6/1999
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows some differences in how databases are managed in Oracle and MS SQL Server:

Description	Oracle	MS SQL Server
Model database	No model database	Newly created databases inherit characteristics (users, etc.) from the special database named "model".

--Fred

Differences in Managing Database Objects

Original Version: 6/6/1999
Last Updated: 3/18/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows some differences in how database objects (tables, views, stored procedures, etc.) are managed in Oracle and MS SQL Server:

Description	Oracle	MS SQL Server
Fully qualified name	[schema.]table [schema.]view	[[[server.][database].][owner].]table [[[server.][database].][owner].]view
Temp tables	Pre 8i: Temporary tables must be deleted explicitly 8i+: CREATE GLOBAL TEMPORARY TABLE	#table -- Any table named starting with a pound sign (#) is automatically dropped when the user logs off or the procedure ends. ##table -- Same as above, except that the table is accessible to other users and is dropped when the last user who referenced it logs off.
Re-creating an object that may or may not already exist	CREATE OR REPLACE ... (Oracle chooses correctly which action to take.)	You have to choose which action to take. If it doesn't already exist, use: CREATE ... If is does exist, use: ALTER ... or the sequence: DROP ... CREATE ...
Create view before dependent tables	CREATE FORCE VIEW	Not supported. Tables used by view must exist before view can be created.

Thanks to Darin Strait for some clarifications.

--Fred

Differences in Managing Users

Original Version: 6/6/1999
Last Updated: 2/20/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows some differences in how users are managed in Oracle and MS SQL Server:

Description	Oracle	MS SQL Server 6.5	MS SQL Server 7+
Membership in groups	Each user can be a member of any number of groups.	Each user can be a member of only one group other than "public".	Each user can be a member of any number of roles.

Thanks to Darin Strait for the MS SQL Server 7+ info.

--Fred

Differences in Integration with MS ADO, RDO, etc.

Original Version: 6/7/1999
Last Updated: 1/14/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows the different techniques used in Oracle and MS SQL Server to interact with MS ADO, RDO, etc.:

Description	Oracle	MS SQL Server
Return a recordset to the caller	Return a handle to a cursor. For more info: See MS KB article Q174679. Can return multiple recordsets as multiple output parameters.	SELECT with no INTO clause; Multiple such SELECTs return multiple recordsets

--Fred

Miscellaneous Differences

Original Version: 6/6/1999
Last Updated: 2/20/2006
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following table shows miscellaneous differences between Oracle and MS SQL Server:

Description	Oracle	MS SQL Server 6.5	MS SQL Server 7+
Generate unique numbers	CREATE SEQUENCE	IDENTITY column of a table
Cascaded DELETE	DELETE CASCADE ...	(use triggers)	DELETE CASCADE ...
Call a user-defined function from a SQL statement (as column of SELECT or expression in WHERE clause)	Supported	Not supported	Supported

Thanks to Darin Strait for the MS SQL Server 7+ info.

--Fred

See Also
Original Version: 6/7/1999
Last Updated: 3/3/2001
Applies to: Oracle 7.3+, MS SQL Server 6.5+

The following are good sources of info about differences between Oracle and MS SQL Server:
1. Bowman, Judith S., Sandra L. Emerson, and Marcy Darnovsky. The Practical SQL Handbook. Addison-Wesley Publishing Company, 1993. ISBN 0-201-62623-3.
  This book gives a good introduction to SQL, with a slight emphasis on Sybase, but with a useful summary in the back of the syntax for each of the SQL statements (SELECT, INSERT, UPDATE, DELETE, CREATE, DROP, GRANT, REVOKE, etc.) for each of the major databases (Oracle, Sybase, DB2, Informix, Ingres, etc.) The book pre-dates MS SQL Server, but the Sybase info is a good approximation since MS SQL Server is a derivative of Sybase.
2. "Migrating Oracle Applications to SQL Server" on MSDN CD, and at MS TechNet Web site:
  http://www.microsoft.com/resources/documentation/sql/2000/all/reskit/en-us/part2/c0761.mspx
  Microsoft clearly intended this to be used in one direction only, but I've used it quite successfully to translate my SQL Server knowledge to Oracle as well.
3. http://en.wikibooks.org/wiki/SQL_dialects_reference
--Fred

Bristle Software SQL Tips

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