MySQL-为什么在null和if为null的情况下结果不同？

Question

I have a query like:

SELECT IF(b.field IS NULL, c.field, b.field) as `field`
FROM aaaaa a
LEFT OUTER JOIN bbbbb b
    ON b.uid = a.b_id
LEFT OUTER JOIN ccccc c
    ON c.uid = a.c_id;

this query works. my question is why does the previous query work when the following, using a CASE , does not:

SELECT 
    CASE b.field 
        WHEN NULL THEN c.field
        ELSE b.field
    END as `field`
FROM aaaaa a
LEFT OUTER JOIN bbbbb b
    ON b.uid = a.b_id
LEFT OUTER JOIN ccccc c
    ON c.uid = a.c_id;

assume that either aaaaa.b_id or aaaaa.c_id will be null; never both and never neither. also assume foreign keys between aaaaa , bbbbb , and ccccc are all properly configured. mysql v5.1.60 on rehl x86_64

the first query properly returned results as expected, but when i use the CASE , i am only returned the values for when the ELSE is satisfied (no c.field s return, only b.field s). what gives?

Answer 1

That's because that form of case checks the condition b.field = null , and you can't compare null values that way.

Use this form of the case instead:

CASE
    WHEN b.field IS NULL THEN c.field
    ELSE b.field
END as `field`

You can also use coalesce for this:

COALESCE(b.field, c.field) as `field`

Or ifnull :

IFNULL(b.field, c.field) as `field`