Transact-SQL (T-SQL) is Microsoft's and Sybase's proprietary extension to the SQL (Structured Query Language) used to interact with relational databases. T-SQL expands on the SQL standard to include procedural programming, local variables, various support functions for string processing, date processing, mathematics, etc. and changes to the DELETE and UPDATE statements.
Transact-SQL is central to using Microsoft SQL Server. All applications that communicate with an instance of SQL Server do so by sending Transact-SQL statements to the server, regardless of the user interface of the application.
Stored procedures in SQL Server are executable server-side routines. The advantage of stored procedures is the ability to pass parameters.
Transact-SQL provides the following statements to declare and set local variables: <code>DECLARE</code>, <code>SET</code> and <code>SELECT</code>.
Keywords for flow control in Transact-SQL include <code>BEGIN</code> and <code>END</code>, <code>BREAK</code>, <code>CONTINUE</code>, <code>GOTO</code>, <code>IF</code> and <code>ELSE</code>, <code>RETURN</code>, <code>WAITFOR</code>, and <code>WHILE</code>.
<code>IF</code> and <code>ELSE</code> allow conditional execution. This batch statement will print "It is the weekend" if the current date is a weekend day, or "It is a weekday" if the current date is a weekday. (Note: This code assumes that Sunday is configured as the first day of the week in the <code>@@DATEFIRST</code> setting.)
<code>BEGIN</code> and <code>END</code> mark a block of statements. If more than one statement is to be controlled by the conditional in the example above, we can use <code>BEGIN</code> and <code>END</code> like this:
<code>WAITFOR</code> will wait for a given amount of time, or until a particular time of day. The statement can be used for delays or to block execution until the set time.
<code>RETURN</code> is used to immediately return from a stored procedure or function.
<code>BREAK</code> ends the enclosing <code>WHILE</code> loop, while <code>CONTINUE</code> causes the next iteration of the loop to execute. An example of a <code>WHILE</code> loop is given below.
In Transact-SQL, both the <code>DELETE</code> and <code>UPDATE</code> statements are enhanced to enable data from another table to be used in the operation, without needing a subquery:
This example deletes all who have been flagged in the table with the 'idle' flag.
<code>BULK</code> is a Transact-SQL statement that implements a bulk data-loading process, inserting multiple rows into a table, reading data from an external sequential file. Use of <code>BULK INSERT</code> results in better performance than processes that issue individual <code>INSERT</code> statements for each row to be added. Additional details are available in MSDN.
Beginning with SQL Server 2005, Microsoft introduced additional <code>TRY CATCH</code> logic to support exception type behaviour. This behaviour enables developers to simplify their code and leave out <code>@@ERROR</code> checking after each SQL execution statement.