表a，id为1,2,3
表b，id为2,3,4
想找出不和b表id相等的数据。
（notin方法）
select * from a
where a.id not in (
select id from b
)
（leftjoin方法）
（1）
select * from a
left join b
on a.id=b.id
查出1,null ；2,2；3,3三条数据
上面sql加上where限制，where b.id=null, 就只剩下一条id=1的数据
UPDATE `取经四人组`
SET ending='齐天大圣' WHERE `取经四人组`.user_name in
(SELECT * FROM(SELECT `取经四人组`.user_name
FROM `取经四人组` INNER JOIN `孙悟空的朋友` ON `取经四人组`.user_name = `孙悟空的朋友`.user_name )as temp)
SELECT
t1.id,
t1.NAME,
t2.num,
count( 1 ) cnt
FROM
tbl_goods t1
LEFT JOIN tbl_sale t2 ON t1.id = t2.goods_id
LEFT JOIN tbl_sale t3 ON t2.goods_id = t3.goods_id
WHERE
t2.num <= t3.num
GROUP BY
1,
2,
3
HAVING
cnt <= 2
