MySQL SELECT CASE WHEN返回null

Question

Table who

wid--name-------father---mother  
1----Daisy------David----Liza  
2----Jenny------Joe------Judy  
3----Meggy------Mike-----Manuela  
4----Sarah------Joe------Judy  
5----Chelsea----Bill-----Hillary  
6----Cindy------David----Liza    
7----Kelly------Joe------Judy 

Table ages

aid---whoid---age  
1-----1--------0  
2-----2--------0  
3-----3-------14  
4-----4-------30  
5-----5-------22  
6-----6-------17  
7-----1-------18  

I want that list as a result:

id---name------age  
1----Meggy-----14  
2----Cindy-----17  
3----Daisy-----18 (Selected data that bigger than 0)  
4----Chelsea---22  
5----Sarah-----30  
6----Jenny-----30 (Her age is 0 on ages table and Sarah's age with same father and mother)  
7----Kelly-----30 (No data on ages table and Sarah's age with same father and mother)

I tried that query:

SELECT 
    *,  
    (CASE age  
        WHEN '0' THEN (
            SELECT age 
            FROM ages a 
            LEFT JOIN who w 
                ON w.wid = a.whoid 
            WHERE 
                w.father = father 
                AND 
                w.mother = mother 
            ORDER BY a.age DESC LIMIT 1
        )  
        ELSE age  
    END
    ) AS newage  
FROM who  
LEFT JOIN ages 
    ON wid = whoid  
ORDER BY  newage

What's wrong with that?

Answer 1

CASE … WHEN NULL will never match anything, and CASE NULL will always match the ELSE clause (which in your case returns age , ie NULL ).

Use this:

CASE COALESCE(age, 0) WHEN 0 THEN … ELSE age END

Update:

You also need to alias your tables and use the aliases in the field descriptions:

SELECT  *,
        CASE COALESCE(age, 0)
        WHEN '0' THEN
                (
                SELECT  MAX(age)
                FROM    who wi
                JOIN    ages ai
                ON      ai.whoid = wi.wid
                WHERE   wi.father = w.father
                        AND wi.mother = w.mother
                )
        ELSE
                age
        END AS newage
FROM    who w
LEFT JOIN
        ages a
ON      a.whoid = w.wid
ORDER BY
        newage