表a，id為1,2,3
表b，id為2,3,4
想找出不和b表id相等的數據。
（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；3,3三條數據
上面sql加上where限制，where b.id=null, 就只剩下一條id=1的數據
UPDATE `取經四人組`
SET ending='齊天大圣' WHERE `取經四人組`.user_name in
(SELECT * FROM(SELECT `取經四人組`.user_name
FROM `取經四人組` INNER JOIN `孫悟空的朋友` ON `取經四人組`.user_name = `孫悟空的朋友`.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
