【mysql 】sql錯誤代碼 1093 You cannot specify target table xxxx for update in FROM clause

在mysql8.X中執行如下語句：

DELETE 
FROM
	`zjs-quality-control-qupai`.unsign_wocode 
WHERE
	id IN (


		SELECT
			a.id 
		FROM
			`zjs-quality-control-qupai`.unsign_wocode a,
			( SELECT wocode, max( in_time ) maxintime FROM `zjs-quality-control-qupai`.unsign_wocode GROUP BY wocode HAVING count(*) > 1 ) b 
		WHERE
			a.wocode = b.wocode 
			AND a.in_time < b.maxintime 

	);

報如下錯誤：

DELETE 
FROM
	`zjs-quality-control-qupai`.unsign_wocode 
WHERE
	id IN (


		SELECT
			a.id 
		FROM
			`zjs-quality-control-qupai`.unsign_wocode a,
			( SELECT wocode, max( in_time ) maxintime FROM `zjs-quality-control-qupai`.unsign_wocode GROUP BY wocode HAVING count(*) > 1 ) b 
		WHERE
			a.wocode = b.wocode 
			AND a.in_time < b.maxintime 

	)
> 1093 - You can't specify target table 'unsign_wocode' for update in FROM clause
> 時間: 0.002s

通過去官網mysql錯誤代碼查詢錯誤代碼1093得到信息如下：

Error number: 1093; Symbol: ER_UPDATE_TABLE_USED; SQLSTATE: HY000

Message: You can't specify target table '%s' for update in FROM clause

 

This error occurs for attempts to select from and modify the same table within a single statement. If the select attempt occurs within a derived table, you can avoid this error by setting the derived_merge flag of the optimizer_switch system variable to force the subquery to be materialized into a temporary table, which effectively causes it to be a different table from the one modified. See Section 8.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.

這個錯誤發生的原因是在單個sql語句中查詢並且修改了同一個表。如果嘗試從派生表中查詢，可以通過設置系統變量optimizer_switch的derived_merge屬性去強迫將子查詢的結果存儲到臨時表，從而避免這種錯誤發生。

 

下面是解決方案，不是通過上面的方式，而是網上的說法，在子查詢外再寫一層查詢

正確的源代碼如下：

 

DELETE 
FROM
	`zjs-quality-control-qupai`.unsign_wocode 
WHERE
	id IN (
	SELECT
		new.id 
	FROM
		(
		SELECT
			a.id 
		FROM
			`zjs-quality-control-qupai`.unsign_wocode a,
			( SELECT wocode, max( in_time ) maxintime FROM `zjs-quality-control-qupai`.unsign_wocode GROUP BY wocode HAVING count(*) > 1 ) b 
		WHERE
			a.wocode = b.wocode 
			AND a.in_time < b.maxintime 
		) new 
	);
	

 

最後，關於臨時表、派生表的解釋請參考如下地址：

https://blog.csdn.net/weixin_44863976/article/details/102789392