子查询 有两种 一种是在from中
在from中相当于省去一张字段表

SELECT ... FROM(subquery)name...

SELECT col 
FROM (
 SELECT a+b AS col
FROM t1)

SELECT t3.ol
ROM (

SELECT a+b AS col
FROM t1
UNION ALL
SELECT c+d AS cal
FROM t2

) t3

另外一种是在where中操作
在where中的子查询相当于条件单字段 所以做不了多字段表(纯属个人理解)

SELECT *
FROM A
WHERE A.a IN (SELECT foo FROM B);

其他支持的types 有 EXISTS and NOT EXISTS
SELECT A
FROM T1
WHERE EXISTS(SELECT B FROM )
.....