[英]How can I join together two queries on the same table?
我有一个MySQL表,名为related_post ,看起来像这样:
+----+--------+--------+
| id | post_1 | post_2 |
+----+--------+--------+
| 1 | 25 | 26 |
| 2 | 25 | 27 |
| 3 | 25 | 28 |
| 4 | 26 | 27 |
| 5 | 26 | 28 |
| 6 | 27 | 28 |
| … | … | … |
+----+--------+--------+
我需要检索给定帖子ID的所有相关帖子。
如果提供“ 27”,则希望它返回一个以“ 25”,“ 26”和“ 28”作为值的列。
现在,我需要将其分为2个查询:
SELECT post_1 FROM related_post where post_2 = 27;
SELECT post_2 FROM related_post where post_1 = 27;
我想知道是否可以仅执行一个查询来完成相同的结果。
尝试UNION
:
SELECT post_1 AS post FROM related_post where post_2 = 27
UNION
SELECT post_2 FROM related_post where post_1 = 27
UNION
还将删除重复项; 如果您不希望这样做,请使用UNION ALL
您可以使用CASE语句在一个查询中执行此操作
SELECT CASE WHEN post_2 = 27 THEN post_1 ELSE post_2 END as post
FROM related_post
WHERE post_2 = 27 OR post_1 = 27;
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