Transact-SQL

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.

Variables

Transact-SQL provides the following statements to declare and set local variables: DECLARE, SET and SELECT.

DECLARE @var1 NVARCHAR(30)    
SET @var1 = 'Some Name'       
SELECT @var1 = Name       
FROM Sales.Store       
WHERE CustomerID = 100

Flow control

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 "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 @@DATEFIRST setting.)

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()) = 1
BEGIN
   PRINT 'It is the weekend.'
   PRINT 'Get some rest on the weekend!'
END
ELSE
BEGIN
   PRINT 'It is a weekday.'
   PRINT 'Get to work on a weekday!'
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.

DECLARE @i INT
SET @i = 0

WHILE @i < 5
BEGIN
   PRINT 'Hello world.'
   SET @i = @i + 1
END

Changes to DELETE and UPDATE statements

In Transact-SQL, both the DELETE and UPDATE statements allow a FROM clause to be added, which allows joins to be included.

This example deletes all users who have been flagged with the 'Idle' flag.

DELETE u
  FROM users AS u
  INNER JOIN user_flags AS f
    ON u.id = f.id
    WHERE f.name = 'idle'

BULK INSERT

BULK 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 BULK INSERT results in better performance than processes that issue individual INSERT statements for each row to be added. Additional details are available in MSDN.

TRY CATCH

Beginning with SQL Server 2005,[1] Microsoft introduced additional TRY CATCH logic to support exception type behaviour. This behaviour enables developers to simplify their code and leave out @@ERROR checking after each SQL execution statement.

-- begin transaction
BEGIN TRAN

BEGIN TRY
   -- execute each statement
   INSERT INTO MYTABLE(NAME) VALUES ('ABC')
   INSERT INTO MYTABLE(NAME) VALUES ('123')

   -- commit the transaction
   COMMIT TRAN
END TRY
BEGIN CATCH
   -- roll back the transaction because of error
   ROLLBACK TRAN
END CATCH

See also

References

  1. ↑ "T-SQL Improvements in SQL Server 2012", Jonathan Allen on Mar 19, 2012, infoq.com
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