在MYSQL中执行LEFT OUTER JOIN时插入零而不是NULL

Question

在两个表上执行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

Answer 1

使用ISNULL（字段，0）如果字段为NULL，则将插入零

Answer 2

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

Answer 3

类似于： -

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