MySQL: [Err] 1093 - You can't specify target table 'bk' for update in FROM clause

错误的意思说，不能先select出同一表中的某些值，再update这个表(在同一语句中)。 例如下面这个sql：

delete from tbl where id in 
(
        select max(id) from tbl a where EXISTS
        (
            select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1
        )
        group by tac
)

改写成下面就行了：

delete from tbl where id in 
(
    select a.id from 
    (
        select max(id) id from tbl a where EXISTS
        (
            select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1
        )
        group by tac
    ) a
)

再如：UPDATE bk_ygsbk AS bk SET REPORT_VISIT = ( SELECT COUNT(*) FROM bk_ygsbk WHERE ZYID = '1' )  WHERE bk.ZYID = '1'

应改成：UPDATE bk_ygsbk AS bk SET REPORT_VISIT = (SELECT * FROM ( SELECT COUNT(*) FROM bk_ygsbk WHERE ZYID = '1' ) AS r ) WHERE bk.ZYID = '1'

也就是说将select出的结果再通过中间表select一遍，这样就规避了错误。注意，这个问题只出现于mysql，mssql和oracle不会出现此问题。