Question

说我有下表：

=================================================
| color_id | parent_id | language_id | name     |
=================================================
| 1        | 50        | 1           | Black    |
-------------------------------------------------

然后说我需要行WHERE parent_id = 50 AND language_id = 2。显然，根据我的示例表，我什么也得不到。但是，我仍然需要一个结果 - 可能是这样的：

=================================================
| color_id | parent_id | language_id | name     |
=================================================
| NULL     | 50        | 2           | NULL     |
-------------------------------------------------

有没有办法在SQL中执行此操作？

Answer 1

您可以对潜在有效记录和默认值进行联合查询，然后选择第一个：

SELECT * FROM
(SELECT color_id, parent_id, language_id, name, 1 as order_rank
 FROM some_table
 WHERE parent_id = %parent_id% AND language_id = %language_id%
 UNION
 SELECT NULL, %parent_id%, %language_id%, NULL, 2 as order_rank
)
ORDER BY order_rank
LIMIT 1

（根据OMG小马的建议编辑静态值）

Answer 2

尝试使用LEFT JOIN语句。我可能不是100％这样做，但你的一些试验和错误应该使这项工作。

SELECT table1.field1, table1.field2, table2.field3, table2.field4
FROM my_table table1 
LEFT JOIN my_table table2 ON table1.field1=table2.field1 OR table1.field2=table2.field2

Answer 3

强制固定值第一个表上的左连接应该可以工作。

select 
      YourTable.color_id,
      ForcedSQL1Record.parent_id,
      ForcedSQL1Record.language_id,
      YourTable.name
   from 
      ( select 50 as Parent_ID,
                2 as Language_ID
            from YourTable
            limit 1 ) ForcedSQL1Record
      left join
         YourTable
            on ForcedSQL1Record.Parent_ID = YourTable.Parent_ID
            AND ForcedSQL1Record Language_ID = YourTable.Language_ID

SQL：如何返回不存在的行？

3 个答案: