How to wait for 2 seconds?

问题:

How does one cause a delay in execution for a specified number of seconds?

This doesn't do it:

WAITFOR DELAY '00:02';

What is the correct format?

回答1:

The documentation for WAITFOR() doesn't explicitly lay out the required string format.

This will wait for 2 seconds:

WAITFOR DELAY '00:00:02';

The format is hh:mi:ss.mmm.

回答2:

As mentioned in other answers, all of the following will work for the standard string-based syntax.

WAITFOR DELAY '02:00' --Two hours
WAITFOR DELAY '00:02' --Two minutes
WAITFOR DELAY '00:00:02' --Two seconds
WAITFOR DELAY '00:00:00.200' --Two tenths of a seconds

There is also an alternative method of passing it a DATETIME value. You might think I'm confusing this with WAITFOR TIME, but it also works for WAITFOR DELAY.

Considerations for passing DATETIME:

• It must be passed as a variable, so it isn't a nice one-liner anymore.
• The delay is measured as the time since the Epoch ('1900-01-01').
• For situations that require a variable amount of delay, it is much easier to manipulate a DATETIME than to properly format a VARCHAR.

How to wait for 2 seconds:

--Example 1
DECLARE @Delay1 DATETIME
SELECT @Delay1 = '1900-01-01 00:00:02.000'
WAITFOR DELAY @Delay1

--Example 2
DECLARE @Delay2 DATETIME
SELECT @Delay2 = dateadd(SECOND, 2, convert(DATETIME, 0))
WAITFOR DELAY @Delay2

A note on waiting for TIME vs DELAY:

Have you ever noticed that if you accidentally pass WAITFOR TIME a date that already passed, even by just a second, it will never return? Check it out:

--Example 3
DECLARE @Time1 DATETIME
SELECT @Time1 = getdate()
WAITFOR DELAY '00:00:01'
WAITFOR TIME @Time1 --WILL HANG FOREVER

Unfortunately, WAITFOR DELAY will do the same thing if you pass it a negative DATETIME value (yes, that's a thing).

--Example 4
DECLARE @Delay3 DATETIME
SELECT @Delay3 = dateadd(SECOND, -1, convert(DATETIME, 0))
WAITFOR DELAY @Delay3 --WILL HANG FOREVER

However, I would still recommend using WAITFOR DELAY over a static time because you can always confirm your delay is positive and it will stay that way for however long it takes your code to reach the WAITFOR statement.

回答3:

How about this?

WAITFOR DELAY '00:00:02';

If you have "00:02" it's interpreting that as Hours:Minutes.

回答4:

Try this example:

exec DBMS_LOCK.sleep(5);

This is the whole script:

SELECT TO_CHAR (SYSDATE, 'MM-DD-YYYY HH24:MI:SS') "Start Date / Time" FROM DUAL;

exec DBMS_LOCK.sleep(5);

SELECT TO_CHAR (SYSDATE, 'MM-DD-YYYY HH24:MI:SS') "End Date / Time" FROM DUAL;