Transact-SQL (T-SQL) is Microsoft's and Sybase's proprietary extension to the SQL language. Microsoft's implementation ships in the Microsoft SQL Server product. Sybase uses the language in its Adaptive Server Enterprise, the successor to Sybase SQL Server.
In order to make it more powerful, SQL has been enhanced with additional features such as:
Keywords for flow control in Transact-SQL include BEGIN and END, BREAK, CONTINUE, GOTO, IF and ELSE, RETURN, WAITFOR, and WHILE.
IF and ELSE allow conditional execution. This batch statement will print "weekend" if the current date is a weekend day, or "weekday" if the current date is a weekday.
IF DATEPART(dw, GETDATE()) = 7 OR DATEPART(dw, GETDATE()) = 1 PRINT 'It is the weekend.'
ELSE PRINT 'It is a weekday.'
BEGIN and END mark a block of statements. If more than one statement is to be controlled by the conditional in the example above, we can use BEGIN and END like this:
IF DATEPART(dw, GETDATE()) = 7 OR DATEPART(dw, GETDATE()) = 1BEGIN
PRINT 'It is the weekend.' PRINT 'Get some rest!'
ENDELSE
BEGIN PRINT 'It is a weekday.'
PRINT 'Get to work!'END
WAITFOR 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.
RETURN is used to immediately return from a stored procedure or function.
BREAK ends the enclosing WHILE loop, while CONTINUE causes the next iteration of the loop to execute. An example of a WHILE loop is given below.
Local variables are so named because they're local to the script executing them. Transact SQL doesn't support user-defined global variables.
DECLARE will declare a variable, giving it a name and a type. The SET statement can be used to provide a value, and the variable may be used in a statement by referencing its name.
This script declares a variable as an integer, initializes it, then uses WHILE to execute a loop.
DECLARE @Counter INTSET @Counter = 10WHILE @Counter > 0
BEGIN PRINT 'The count is ' + CONVERT(VARCHAR(10), @Counter)
SET @Counter = @Counter - 1END
The body of the loop will print a message including the value of the variable, and then decrement the counter.
A variable may be initialized as the result of a statement, like this:
DECLARE @ArticleCount INTSELECT @ArticleCount = COUNT(*) FROM Articles
INSERT INTO SizeLog (SampleTime, ArticleCount) VALUES (GETDATE(), @ArticleCount)
which will get the count of rows in the Articles table, then insert a row including that count and the current clock time into the SizeLog table.
In Transact-SQL, both the DELETE and UPDATE statements allow a FROM clause to be added which allows joins to be included.
This example delete all users who have been flagged with the 'Idle' flag.
DELETE users FROM users as u
JOIN user_flags as f ON u.id=f.id
WHERE f.name = 'Idle'
BULK INSERT is a Transact-SQL statement which implements a bulk data-loading process, inserting multiple rows into a table, reading data from an external sequential file. Use of BULK INSERT results in better performance than processes which issue individual INSERT statements for each row to be added. Additional details are available on Microsoft's MSDN page.
Critics of Transact-SQL say that not only do the additional features break compatibility with standard SQL, but they also break the discrete modular scope which SQL was intended to occupy as an exclusively declarative language (up until the 1999 version of the SQL standard). In other words, the additional functionality found in Transact-SQL would, according to the original concept of SQL, have been implemented with a separate imperative programming language and Embedded SQL, and it muddies the waters when flow control, for example, may be implemented either in a programming language layer or in SQL itself.