SQL报1093错误You can’t specify target table for update in FROM clause

update description set showcase_flag=1 where description.link in  (SELECT link FROM description where showcase_flag="是") ;

上述代码from后面跟的表description和update后面的表description是同一张表，这在sql语法里面是不允许的。所以会爆出#1093,You can't specify target table for update in FROM clause的错误，所以需要改为下面的代码：

第一种方法：涉及到as语法，用别名解决

实例1：
update description set showcase_flag=1 where description.link in (SELECT a.link FROM (SELECT link FROM description where showcase_flag="是")  as a);

实例2：
update wp_posts set post_content = REPLACE(post_content,"old","new")
where id in (select a.id from (select id from wp_posts,wp_term_relationships,wp_term_taxonomy 
where ID=object_id) as a)

主要是是后面的语句，where 字段1 in (select a.字段1 from (正常select语句) as a)，a是临时创建的一个表，不会创建在数据库中，as语句只能写在后面。

第二种方法：用case when 条件1 then 结果1 when 条件2 then 结果2.......else 结果x end

update description a set a.showcase_flag = case when a.showcase_flag = '是' then '1' when a.showcase_flag = '否' then '0' end;

以上两种代码都可以解决#1093,You can't specify target table for update in FROM clause的错误，大家可以灵活运用。