[英]How can I join together two queries on the same table?
我有一個MySQL表,名為related_post ,看起來像這樣:
+----+--------+--------+
| id | post_1 | post_2 |
+----+--------+--------+
| 1 | 25 | 26 |
| 2 | 25 | 27 |
| 3 | 25 | 28 |
| 4 | 26 | 27 |
| 5 | 26 | 28 |
| 6 | 27 | 28 |
| … | … | … |
+----+--------+--------+
我需要檢索給定帖子ID的所有相關帖子。
如果提供“ 27”,則希望它返回一個以“ 25”,“ 26”和“ 28”作為值的列。
現在,我需要將其分為2個查詢:
SELECT post_1 FROM related_post where post_2 = 27;
SELECT post_2 FROM related_post where post_1 = 27;
我想知道是否可以僅執行一個查詢來完成相同的結果。
嘗試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
還將刪除重復項; 如果您不希望這樣做,請使用UNION ALL
您可以使用CASE語句在一個查詢中執行此操作
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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