0

我有一个对 tuple 具有唯一约束的映射表(c_id, t_id)。

以下是一些示例数据来说明情况： 

id  c_id    t_id  
----------------
1   10      2
2   10      3
3   10      7
4   12      2
5   13      3

t_ids我为(x,y -> z OR x,y -> x)编写了一个合并函数。如果我的内容 ( c_id) 两者都有t_ids，那么我当然违反了使用以下语句的约束： 

UPDATE mapping_table
SET t_id = '$target_tid'
WHERE t_id = '$t1_id' OR t_id = '$t2_id';

结果将是： 

id  c_id    t_id
----------------
1   10      4
2   10      4       /* violates unique constraint */
3   10      7

现在我想出了这个： 

/* delete one of the duplicate entries */
DELETE FROM mapping_table
WHERE   ( SELECT count(c_id)
          FROM mapping_table
          WHERE t_id = '$t1_id' OR t_id = '$t2_id'
        ) > 1;

/* update the remaining row */
UPDATE mapping_table
SET t_id = '$target_tid'
WHERE t_id = '$t1_id' OR t_id = '$t2_id';

现在我收到以下错误：
You can't specify target table 'mapping_table' for update in FROM clause

我的问题是：

  1. 这里到底出了什么问题？该DELETE语句是否被视为更新并且不能在WHERE子句中使用？
  2. 这有没有更有效的方法来做到这一点？
4

3 回答 3

1

您遇到的错误是 MySQL 的一个特性。您可以使用一组双重子查询来解决此问题：

DELETE FROM mapping_table
WHERE  (select *
        from ( SELECT count(c_id)
               FROM mapping_table
               WHERE t_id = '$t1_id' OR t_id = '$t2_id'
             ) > 1
        ) t

但是，要解决您的问题，只需删除除最小值之外的所有 id。我认为这也可能有效：

delete from mapping_table
where id > (select minid from (select min(id) from mapping_table mt2
                               where mt2.c_id = mapping_table.c_id and
                                     mt2.t_id = mapping_table.t_id
                              )
           )

您还可以将 id 列表存储在临时表中，并在查询中使用它：

create temporary table minids as
     select c_id, t_id, min(id) as minid
     from mapping_table
     group by c_id, t_id;

delete from mapping_table
where exists (select 1 from minids
              where mt2.c_id = mapping_table.c_id and
                    mt2.t_id = mapping_table.t_id and
                    mt2.minid > mapping_table.id
             )
于 2013-02-23T17:37:19.473 回答
0

试试这个

DELETE FROM mapping_table
    WHERE   ( SELECT count(c_id)
      FROM mapping_table
      WHERE t_id = '$t1_id' OR t_id = '$t2_id'
      Having count(c_id) > 1
    );

编辑：

在你的更新声明中试试这个

 UPDATE mapping_table
 SET t_id = '$target_tid'
WHERE t_id in (select t_id from mapping_table where t_id= '$t1_id' OR t_id = '$t2_id')
于 2013-02-23T17:26:29.113 回答
0

我一直在寻找这个解决方案。性能可能低得惊人，但至少我找到了一个可行的解决方案（并学到了一些东西）。

/* actually delete rows that will become duplicate after the update */
DELETE FROM mt1 USING mapping_table AS mt1 WHERE id IN (

    /* sub-query to allow `mapping_table` in the DELETE statement */
    SELECT * FROM (

        /* select ids/rows with one t_id available */
        SELECT id
        FROM mapping_table AS mt2
        WHERE mt2.tag_id = $t1_id AND c_id IN (

            /* select ids/rows with both t_id available */
            SELECT c_id
            FROM mapping_table AS mt3
            WHERE mt3.c_id = mt2.c_id AND mt3.tag_id = $t2_id)

    /* alias needed for every derived table */
    ) as mres
)

/* Update to merge t_ids */
UPDATE mapping_table
SET t_id = '$target_id'
WHERE t_id = '$t1_id' OR t_id = '$t2_id';
于 2013-02-26T21:40:26.070 回答