CASE 切换内部联接 table mysql

Question

所以我有一个 table，其中包含对其他三个 table 的编辑，我需要根据值 ce.entity_id 切换内部联接。是的，我本可以规范化此编辑 content_edits table，但这似乎容易得多。

        SELECT ce.id, ce.file, ce.date_init, ce.content_type, ce.created_by_id, ce.status, ce.date_decide, u.username
                         FROM content_edits AS ce
                         INNER JOIN users AS u
                         ON ce.created_by_id=u.id
                             CASE ce.entity_type
                                WHEN ce.entity_type=0 THEN
                                  INNER JOIN m_articles AS m
                                  ON ce.entity_id=markers.id
                                WHEN 1 THEN
                                  INNER JOIN groups AS g
                                  ON ce.entity_id=groups.id
                                WHEN 3 THEN
                                  INNER JOIN e_news AS e
                                  ON ce.entity_id=events.id
                         WHERE ce.status=1
                         LIMIT 10

衷心感谢您的帮助。非常感谢。

Answer 1

CASE WHEN 函数可用于有条件地过滤记录但不能构造记录集。具体来说，这意味着它可以用在 SELECT、WHERE 甚至 JOIN ON 子句中。但是，它不能根据动态标准选择要连接的表。

查询优化器的第一个任务是读取 FROM 和 JOIN 行，然后过滤、聚合、select、and/or 排序记录.

或者，您可以使用 UNION 查询。它可能是重复的，但它更易读，并且可以理解表的结构。并且根据ce_entity_id，只有一个联合表会输出。

 SELECT ce.id, ce.file, ce.date_init, ce.content_type, 
        ce.created_by_id, ce.status, ce.date_decide, u.username
 FROM content_edits AS ce
 INNER JOIN users AS u ON ce.created_by_id=u.id
 INNER JOIN m_articles AS m ON ce.entity_id=markers.id
 WHERE ce.entity_type = 0 AND ce.status=1
 LIMIT 10;

 UNION 

 SELECT ce.id, ce.file, ce.date_init, ce.content_type, 
        ce.created_by_id, ce.status, ce.date_decide, u.username
 FROM content_edits AS ce
 INNER JOIN users AS u ON ce.created_by_id=u.id
 INNER JOIN groups AS g ON ce.entity_id=groups.id
 WHERE ce.entity_type = 1 AND ce.status=1
 LIMIT 10;

 UNION 

 SELECT ce.id, ce.file, ce.date_init, ce.content_type, 
        ce.created_by_id, ce.status, ce.date_decide, u.username
 FROM content_edits AS ce
 INNER JOIN users AS u ON ce.created_by_id=u.id
 INNER JOIN e_news AS e ON ce.entity_id=events.id
 WHERE ce.entity_type = 3 AND ce.status=1
 LIMIT 10;

Answer 2

连接不能依赖于 CASE。您可以在内部查询中将所有三个表联合在一起，然后像下面的查询一样一起使用它们

SELECT ce.id, ce.file, ce.date_init, ce.content_type, ce.created_by_id, ce.status, ce.date_decide, u.username,S.details
                         FROM content_edits AS ce
                         INNER JOIN users AS u
                         ON ce.created_by_id=u.id
                         INNER JOIN (
                           /*add details required columns*/
                           select 0 as entity_type,a.id as entity_id,a.art_details as details
                            from m_articles AS a
                            union all 
                           select 1 as entity_type,g.id as entity_id,g.grp_details as details
                            from groups AS g
                           union all 
                           select 3 as entity_type,n.id as entity_id,n.n_details as details
                            from e_news AS n
                           )S
                                  ON ce.entity_id=S.entity_id
                                  AND ce.entity_type = S.entity_type
                         LIMIT 10;

http://sqlfiddle.com/#!9/2b984/1

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