WAITFOR (Transact-SQL)

APPLIES TO: yesSQL Server yesAzure SQL Database noAzure SQL Data Warehouse noParallel Data Warehouse

Blocks the execution of a batch, stored procedure, or transaction until either a specified time or time interval elapses, or a specified statement modifies or returns at least one row.

Topic link icon Transact-SQL Syntax Conventions

Syntax

  
WAITFOR   
{  
    DELAY 'time_to_pass'   
  | TIME 'time_to_execute'   
  | [ ( receive_statement ) | ( get_conversation_group_statement ) ]   
    [ , TIMEOUT timeout ]  
}  

Arguments

DELAY
Is the specified period of time that must pass, up to a maximum of 24 hours, before execution of a batch, stored procedure, or transaction proceeds.

'time_to_pass'
Is the period of time to wait. time_to_pass can be specified either in a datetime data format, or as a local variable. Dates can't be specified, so the date part of the datetime value isn't allowed. time_to_pass is formatted as hh:mm[[:ss].mss].

TIME
Is the specified time when the batch, stored procedure, or transaction runs.

'time_to_execute'
Is the time at which the WAITFOR statement finishes. time_to_execute can be specified in a datetime data format, or it can be specified as a local variable. Dates can't be specified, so the date part of the datetime value isn't allowed. time_to_execute is formatted as hh:mm[[:ss].mss] and can optionally include the date of 1900-01-01.

receive_statement
Is a valid RECEIVE statement.

Important

WAITFOR with a receive_statement is applicable only to Service Broker messages. For more information, see RECEIVE (Transact-SQL).

get_conversation_group_statement
Is a valid GET CONVERSATION GROUP statement.

Important

WAITFOR with a get_conversation_group_statement is applicable only to Service Broker messages. For more information, see GET CONVERSATION GROUP (Transact-SQL).

TIMEOUT timeout
Specifies the period of time, in milliseconds, to wait for a message to arrive on the queue.

Important

Specifying WAITFOR with TIMEOUT is applicable only to Service Broker messages. For more information, see RECEIVE (Transact-SQL) and GET CONVERSATION GROUP (Transact-SQL).

Remarks

While executing the WAITFOR statement, the transaction is running and no other requests can run under the same transaction.

The actual time delay may vary from the time specified in time_to_pass, time_to_execute, or timeout, and depends on the activity level of the server. The time counter starts when the WAITFOR statement thread is scheduled. If the server is busy, the thread may not be immediately scheduled, so the time delay may be longer than the specified time.

WAITFOR doesn't change the semantics of a query. If a query can't return any rows, WAITFOR will wait forever or until TIMEOUT is reached, if specified.

Cursors can't be opened on WAITFOR statements.

Views can't be defined on WAITFOR statements.

When the query exceeds the query wait option, the WAITFOR statement argument can complete without running. For more information about the configuration option, see Configure the query wait Server Configuration Option. To see the active and waiting processes, use sp_who.

Each WAITFOR statement has a thread associated with it. If many WAITFOR statements are specified on the same server, many threads can be tied up waiting for these statements to run. SQL Server monitors the number of WAITFOR statement threads, and randomly selects some of these threads to exit if the server starts to experience thread starvation.

You can create a deadlock by running a query with WAITFOR within a transaction that also holds locks preventing changes to the rowset accessed by the WAITFOR statement. SQL Server identifies these scenarios and returns an empty result set if the chance of such a deadlock exists.

Caution

Including WAITFOR will slow the completion of the SQL Server process and can result in a timeout message in the application. If necessary, adjust the timeout setting for the connection at the application level.

Examples

A. Using WAITFOR TIME

The following example executes the stored procedure sp_update_job in the msdb database at 10:20 P.M. (22:20).

EXECUTE sp_add_job @job_name = 'TestJob';  
BEGIN  
    WAITFOR TIME '22:20';  
    EXECUTE sp_update_job @job_name = 'TestJob',  
        @new_name = 'UpdatedJob';  
END;  
GO  

B. Using WAITFOR DELAY

The following example executes the stored procedure after a two-hour delay.

BEGIN  
    WAITFOR DELAY '02:00';  
    EXECUTE sp_helpdb;  
END;  
GO  

C. Using WAITFOR DELAY with a local variable

The following example shows how a local variable can be used with the WAITFOR DELAY option. This stored procedure waits for a variable period of time and then returns information to the user as the elapsed numbers of hours, minutes, and seconds.

IF OBJECT_ID('dbo.TimeDelay_hh_mm_ss','P') IS NOT NULL  
    DROP PROCEDURE dbo.TimeDelay_hh_mm_ss;  
GO  
CREATE PROCEDURE dbo.TimeDelay_hh_mm_ss   
    (  
    @DelayLength char(8)= '00:00:00'  
    )  
AS  
DECLARE @ReturnInfo varchar(255)  
IF ISDATE('2000-01-01 ' + @DelayLength + '.000') = 0  
    BEGIN  
        SELECT @ReturnInfo = 'Invalid time ' + @DelayLength   
        + ',hh:mm:ss, submitted.';  
        -- This PRINT statement is for testing, not use in production.  
        PRINT @ReturnInfo   
        RETURN(1)  
    END  
BEGIN  
    WAITFOR DELAY @DelayLength  
    SELECT @ReturnInfo = 'A total time of ' + @DelayLength + ',   
        hh:mm:ss, has elapsed! Your time is up.'  
    -- This PRINT statement is for testing, not use in production.  
    PRINT @ReturnInfo;  
END;  
GO  
/* This statement executes the dbo.TimeDelay_hh_mm_ss procedure. */  
EXEC TimeDelay_hh_mm_ss '00:00:10';  
GO  

Here is the result set.

A total time of 00:00:10, in hh:mm:ss, has elapsed. Your time is up.

See Also

Control-of-Flow Language (Transact-SQL)
datetime (Transact-SQL)
sp_who (Transact-SQL)