Mysql 联接将是快速的，并且始终使用索引： (select * from Children1 c join Parent p1 ON p1.id = c.parentId) union (select * from Children1 c join Parent2

联接将是快速的，并且始终使用索引：

(select * from Children1 c join Parent p1 ON p1.id = c.parentId)
union
(select * from Children1 c join Parent2 p2 ON p2.id = c.parentId)

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你可能是对的。为什么我要问这个问题：我有一个Hibernate模型，它可以生成这些查询，但比这里复杂得多。我考虑了如何根据MySQL的功能改进Hibernate对union子类继承映射的查询生成。但是由于Hibernate和MySQL在这个方向上有它们的缺点，我从那时起就放弃了努力，只是重新安排了数据库结构。谢谢你花时间。你可能是对的。为什么我要问这个问题：我有一个Hibernate模型，它可以生成这些查询，但比这里复杂得多。我考虑了如何根据MySQL的功能改进Hibernate对union子类继承映射的查询生成。但是由于Hibernate和MySQL在这个方向上有它们的缺点，我从那时起就放弃了努力，只是重新安排了数据库结构。谢谢你抽出时间。

select * from Children1 c 
inner join (
select id as parentId from Parent
union 
select id as parentId from Parent2
) p on p.parentId = c.parentId

+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------+
| id | select_type  | table      | type  | possible_keys | key     | key_len | ref  | rows | Extra                                               |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY      | NULL       | NULL  | NULL          | NULL    | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables | 
|  2 | DERIVED      | Parent     | index | NULL          | PRIMARY | 4       | NULL |    1 | Using index                                         | 
|  3 | UNION        | Parent2    | index | NULL          | PRIMARY | 4       | NULL |    1 | Using index                                         | 
| NULL | UNION RESULT | <union2,3> | ALL   | NULL          | NULL    | NULL    | NULL | NULL |                                                     | 
+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------+
4 rows in set (0.00 sec)

mysql> explain select * from Children1 c  inner join ( select id as parentId,name from Parent union  select id as parentId,name from Parent2 ) p on p.parentId = c.parentId;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables | 
|  2 | DERIVED      | Parent     | ALL  | NULL          | NULL | NULL    | NULL |    1 |                                                     | 
|  3 | UNION        | Parent2    | ALL  | NULL          | NULL | NULL    | NULL |    1 |                                                     | 
| NULL | UNION RESULT | <union2,3> | ALL  | NULL          | NULL | NULL    | NULL | NULL |                                                     | 
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
4 rows in set (0.00 sec)

select * from Children1 c 
inner join (
select id as parentId from Parent where Parent.id = c.parentId
union 
select id as parentId from Parent2 where Parent.id = c.parentId
) p 

Result: "Unknown column 'c.parentId' in 'where clause'.

select *, IFNULL(p1.name, p2.name) AS name from Children1 c
left join Parent p1 ON p1.id = c.parentId
left join Parent2 p2 ON p2.id = c.parentId

(select * from Children1 c join Parent p1 ON p1.id = c.parentId)
union
(select * from Children1 c join Parent2 p2 ON p2.id = c.parentId)