在 MYSQL 中执行 LEFT OUTER JOIN 时插入零而不是 NULL

Posted 2023-02-24

【中文标题】在 MYSQL 中执行 LEFT OUTER JOIN 时插入零而不是 NULL

【英文标题】：Inserting a Zero instead of NULL while performing LEFT OUTER JOIN in MYSQL

【发布时间】：2012-10-28 06:56:01

【问题描述】：

在对两个表执行 LEFT OUTER JOIN 时，有没有办法插入零而不是 NULL？

假设我有这样的查询：

SELECT * FROM
(SELECT uID from Class) T1
LEFT OUTER JOIN
(SELECT pID from University) T2
ON T1.uID = T2.pID
CASE WHEN T1.uID IS NULL
    THEN 0
ELSE T1.uID
END AS uID`

如果我错了，请纠正我

【参考方案1】：

使用ISNULL(field, 0)，如果字段为NULL，则插入零

【讨论】：

对于 mysql，您的意思是 ifnull() 而不是 isnull() =)

【参考方案2】：

类似：-

SELECT * FROM
(SELECT ifnull(uID,0) as uID from Class) T1
   LEFT OUTER JOIN
 (SELECT ifnull(pID,0) as pID from University) T2
   ON T1.uID = T2.pID

【参考方案3】：

CASE WHEN a.fieldname IS NULL 
       THEN 0
       ELSE a.fieldname
END AS fieldname

【讨论】：

SELECT * FROM (SELECT uID from Class) T1 LEFT OUTER JOIN (SELECT pID from University) T2 ON T1.uID = T2.pID CASE WHEN T1.uID IS NULL THEN 0 ELSE T1.uID END AS uID如果我错了请纠正我

如果你想在 T1.UID 为空时加入 0，请使用下面的代码：SELECT * FROM (SELECT CASE WHEN uID IS NULL THEN 0 ELSE uID END AS uID from Class) T1 LEFT OUTER JOIN (SELECT pID from University) T2 ON T1.uID = T2.pID