使用 order by 和 limit 从多个表中删除 [英] Delete from multiple tables using order by and limit

问题描述

我正在尝试使用 ORDER BY DESC 和 LIMIT 从具有关系的两个表中删除.

I'm trying to delete from two tables which have a relationship using ORDER BY DESC and LIMIT.

DELETE FROM my_rel_table AS t1
LEFT JOIN my_photo_table AS t2 ON t2.typeid = t1.typeid
WHERE t1.relid = 1 
AND t1.type = 1
ORDER BY t1.id DESC
LIMIT 1

显然上述方法不起作用，因为 mysql 不接受使用内部联接的 order by 和限制.

Obviously the above does not work since mysql does not accept the order by and limit using an inner join.

表结构如下:

my_rel_table
id relid relno typeid type
int int int int tinyint

my_photo_table
typeid pos_x pos_y width height
int    int   int   int    int

推荐答案

对子选择使用 JOIN 来从 my_rel_table 中获取最高 id

Doing it using a JOIN against a subselect to get the highest id from the my_rel_table

DELETE my_rel_table, my_photo_table
FROM my_rel_table 
INNER JOIN 
(
    SELECT MAX(id) AS MaxId
    FROM my_rel_table 
    WHERE relid = 1 
    AND type = 1
) Sub1
ON my_rel_table.id = Sub1.MaxId
LEFT OUTER JOIN my_photo_table ON my_photo_table.typeid = my_rel_table.typeid
WHERE my_rel_table.relid = 1 
AND my_rel_table.type = 1

没有直接测试，因为我没有测试数据！

Not directly tested as I have no test data!

编辑 - 几次尝试前 5 名，但再次未测试

EDIT - Couple of attempts to do the top 5, but again not tested

DELETE my_rel_table, my_photo_table
FROM my_rel_table 
INNER JOIN 
(
    SELECT id
    FROM my_rel_table 
    WHERE relid = 1 
    AND type = 1
    ORDER BY id DESC
    LIMIT 5
) Sub1
ON my_rel_table.id = Sub1.id
LEFT OUTER JOIN my_photo_table ON my_photo_table.typeid = my_rel_table.typeid
WHERE my_rel_table.relid = 1 
AND my_rel_table.type = 1

或者换一种方式.

DELETE my_rel_table, my_photo_table
FROM my_rel_table 
INNER JOIN 
(
    SELECT id, @Counter:=@Counter+1 AS ItemCounter
    FROM my_rel_table 
    CROSS JOIN (SELECT @Counter:=0) Sub1
    WHERE relid = 1 
    AND type = 1
    ORDER BY id DESC
) Sub1
ON my_rel_table.id = Sub1.id
AND Sub1.ItemCounter <= 5
LEFT OUTER JOIN my_photo_table ON my_photo_table.typeid = my_rel_table.typeid
WHERE my_rel_table.relid = 1 
AND my_rel_table.type = 1

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