Question

查询是：

update products set sort_order = (SELECT max(sort_order) from products where branch_id = 1) + 1 where branch_id = 1

sort_order下的所有值当前= 0，我希望它们递增。显示的错误是：

You can't specify target table 'products' for update in FROM clause

表结构是

products:

product_id (int auto_increment),

product_category_id (int not unique), 

product_name (varchar), 

sort_order (int to specify which will be shown first sort_order ASC),

status (int 1 for active 0 for inactive), 

branch_id (int not unique)

那么子查询应该是什么？

Answer 1

<强>更新

啊，现在我得到了你想要的东西。试试这个：

set @row_counter := 0;
update products 
set sort_order = @row_counter := @row_counter + 1
where branch_id = 1
order by whatever_column_determines_the_sort_order;

详细了解用户定义的变量here。

原始回答：

操作顺序通常是

FROM clause
WHERE子句
GROUP BY子句
HAVING条款
SELECT条款
ORDER BY子句

用于select查询。 update也是如此。在update set等于select。这里成功的关键是在from子句中对要更新的表进行（子）查询。

update products 
cross join (select max(sort_order) + 1 mso from products where branch_id = 1) sq 
set sort_order = mso 
where branch_id = 1;

测试：

mysql>  create table products(branch_id int, sort_order int);
Query OK, 0 rows affected (0.03 sec)

mysql> insert into products values (1, 0), (2, 0);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> update products cross join (select max(sort_order) + 1 mso from products where branch_id = 1) sq set sort_order = mso where branch_id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from products;
+-----------+------------+
| branch_id | sort_order |
+-----------+------------+
|         1 |          1 |
|         2 |          0 |
+-----------+------------+
2 rows in set (0.00 sec)

子查询更新不起作用？

1 个答案: