问题
I have two fields that I'm comparing with MySQL's function COALESCE(). For example, COALESCE(Field1, Field2). The problem is, Field1 is sometimes blank but not null; since it's not null COALESCE() selects Field1, even though its blank. In that case, I need it to select Field2.
I know I can write a if-then-else (CASE) statement in the query to check for this, but is there a nice simple function like COALESCE() for blank-but-not-null fields?
回答1:
SELECT IFNULL(NULLIF(Field1,''),Field2)
NULLIF returns a NULL if Field1 is blank, while IFNULL returns Field1 if it's not blank or NULL and Field2 otherwise.
回答2:
You can use a CASE expression:
CASE WHEN Field1 <> '' THEN Field1 ELSE Field2 END
回答3:
I know I'm late to the party here, but there is a way to do this while still using COALESCE(). This would then work if your value was NULL or ''.
Select COALESCE(NULLIF(Field1,''), Field2)
来源：https://stackoverflow.com/questions/13235847/coalesce-for-blank-but-not-null-fields