MySQL出现You can‘t specify target table for update in FROM clause错误
分析缘故原由在MySQL中，可能会遇到You can't specify target table '表名' for update in FROM clause这样的错误
它的意思是说，不能在同一语句中，先select出同一表中的某些值，再update这个表，即不能依据某字段值做判定再来更新某字段的值。
这个问题在MySQL官网中有提到解决方案：MySQL-UPDATE- 拉到文档下面
比方下面这张t_message表
+----+-----+-----------+---------------------+
| id | uid | content | addtime |
+----+-----+-----------+---------------------+
| 1 | 1 | content1 | 2022-04-26 00:00:01 |
| 2 | 2 | content2 | 2022-04-26 00:00:02 |
| 3 | 3 | content3 | 2022-04-26 00:00:03 |
| 4 | 1 | content4 | 2022-04-26 00:00:04 |
| 5 | 3 | content5 | 2022-04-26 00:00:05 |
| 6 | 2 | content6 | 2022-04-26 00:00:06 |
| 7 | 2 | content7 | 2022-04-26 00:00:07 |
| 8 | 4 | content8 | 2022-04-26 00:00:08 |
| 9 | 4 | content9 | 2022-04-26 00:00:09 |
| 10 | 1 | content10 | 2022-04-26 00:00:10 |
+----+-----+-----------+---------------------+
我想将每个用户第一条消息的内容更新为Hello World
UPDATE t_message
SET content = 'Hello World'
WHERE id IN (
SELECT min(id) FROM t_message GROUP BY uid
);
-- 会出现如下错误
-- ERROR 1093: You can't specify target table 't_message' for update in FROM clause
-- 解决方法：select 的结果再通过一个中间表 select 多一次，就可以避免这个错误
UPDATE message
SET content = 'Hello World'
WHERE id IN (
SELECT min_id FROM (
SELECT min(id) AS min_id FROM t_message GROUP BY uid
) AS a
);
-- 更新的其他方式
update fa_info fb, (
select
fb.member_id as oldMemberId,
mc.id as newMemberCardId
from
fa_info fb
inner join ott_member m on fb.member_id = m.id and m.is_deleted = 0
left join ott_card c on m.idcard_no = c.idcard_no and m.type = c.type and c.is_deleted = 0
inner join ott_member_card mc on mc.member_id = m.id and (mc.card_id = c.id or mc.card_id is null) and mc.is_deleted = 0
) mm
set fb.member_id = mm.newMemberCardId
where mm.oldMemberId = fb.member_id;
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