Return null for date_format when input is null in

问题:

I'm doing something like this:

SELECT date_format(mydate, '%d/%m/%Y') FROM xyz;

When mydate is NULL, date_format returns 00/00/0000. This is correct, but how can I make it so that it returns NULL when the input is NULL?

回答1:

SELECT IF(mydate,date_format(mydate, '%d/%m/%Y'),NULL) FROM xyz;

Source: http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html

回答2:

You can wrap this into a IF-Clause, like this:

SELECT IF(mydate,DATE_FORMAT(mydate, '%d/%m/%Y'),NULL) FROM xyz;

That said, if your variable mydate is not a date value, the query in your post should return (NULL) anyway.

回答3:

select case when isnull(mydate) then null else date_format(mydate, '%d/%m/%Y') end as mydate from xyz;

回答4:

case 
    when date_format(mydate, '%d/%m/%Y') = 00/00/0000 then null
    else ///
end as mydate,