[英]MySQL SELECT CASE WHEN something THEN returning null
表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
表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
我希望該列表成為結果:
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)
我試過那個查詢:
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
這有什么問題?
CASE … WHEN NULL
永遠不會匹配任何內容, CASE NULL
將始終匹配ELSE
子句(在您的情況下返回age
,即NULL
)。
用這個:
CASE COALESCE(age, 0) WHEN 0 THEN … ELSE age END
更新:
您還需要對表進行別名並使用字段說明中的別名:
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
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