SQL默认的ELSE值

Question

in the following sql query i have a select case without else

 SET dateformat mdy 
    DECLARE @DATE1 DATETIME 
    SET @DATE1 = '12/31/2011' 
    DECLARE @DATE2 DATETIME 
    SET @DATE2 = '06/01/2012' 


    UPDATE tablx
    SET xdate = case WHEN xdate is null and odate BETWEEN @Date1 AND @Date2 then odate end

does the existance of else make difference, in the above case would the xdate value be filled with NULL if the condition is not met even if i didnt write ELSE NULL ?

Answer 1

It doesn't make any semantic difference ELSE NULL is the default if omitted.

Perhaps including the redundant ELSE explicitly might be clearer for potential readers unaware of this fact though.

Answer 2

From the MSDN documentation on CASE :

ELSE else_result_expression

Is the expression returned if no comparison operation evaluates to TRUE. If this argument is omitted and no comparison operation evaluates to TRUE, CASE returns NULL.

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On a side note, it's a better habit to get into to write your date literals as:

DECLARE @DATE1 DATETIME 
SET @DATE1 = '20111231'

Because such literals (YYYYMMDD, no separators) aren't ambiguous - you can then ignore the DATEFORMAT settings that are in effect.

Answer 3

Yes they would.

For example, if you have the values

create table tablx (xdate datetime, odate datetime)

insert into tablx values ('2012-01-01', '2012-01-02')
insert into tablx values (null,         '2012-01-02')
insert into tablx values (null,         '2013-11-12')
insert into tablx values ('2013-11-11', '2013-11-12')

The first, third, and fourth rows would be updated to null, and the second would be updated to '2012-01-02', so in effect

case WHEN <something> then odate end

is equivalent to

case WHEN <something> then odate else null end

But i would prefer the second version, as it is more explicit and obvious.