SQL LIKE with no wildcards the same as '='

问题:

I know this is a pretty basic question, and I think I know the answer...but I'd like to confirm.

Are these queries truly equivalent?

SELECT * FROM FOO WHERE BAR LIKE 'X'
SELECT * FROM FOO WHERE BAR ='X'

Perhaps there is a performance overhead in using like with no wild cards?

I have an app that optionally uses LIKE & wild cards. The SP currently does the like and appends the wild cards -- I am thinking of just updating the query to use like but have the app append the wild cards as needed.

回答1:

As @ocdecio says, if the optimizer is smart enough there should be no difference, but if you want to make sure about what is happening behind the scenes you should compare the two query's execution plans.

回答2:

Original Answer by Matt Whitfield from here

There is a difference between = and LIKE. When you perform string comparisons by using LIKE, all characters in the pattern string are significant. This includes leading or trailing spaces.

So if you have a column that is char or nchar and not nvarchar or varchar, there will be different results due to trailing spaces.

Small example to reproduce this behaviour:

CREATE TABLE #temp (nam [varchar](MAX))
INSERT INTO [#temp] ([nam])
VALUES ('hello')
INSERT INTO [#temp] ([nam])
VALUES ('hello  ')

SELECT * FROM #temp WHERE [nam] = 'hello  '

SELECT * FROM #temp WHERE [nam] LIKE 'hello  '

回答3:

Any good optimizer would reduce the first expression to the second.