Question

我有三个表，即test1，test2，test3

test1 data
===========
id test_id q_id 
1    25      2
2    27      2

test2 data
===========
id test_id q_id 
1    25      2
2    34      2

test3 data
===========
id test_id q_id 
1    34      2

如何在不重复数据的情况下从这三个表中获取test_id q_id = 2值？

即25,27,34

Answer 1

如果你真的无法摆脱三个结构相同的表中的两个，请看一下UNION运算符。默认行为是UNION DISTINCT ，它会从结果中删除重复项。

  SELECT test_id FROM test1 WHERE q_id=2
UNION DISTINCT
  SELECT test_id FROM test2 WHERE q_id=2
UNION DISTINCT
  SELECT test_id FROM test3 WHERE q_id=2

Answer 2

@just somebody - 你的查询正在做3选择*可能是密集的，最好限制所有三个以避免不必要的行数：

    SELECT test_id, 'test1' AS tableName FROM test1 WHERE q_id = 2
    UNION
    SELECT test_id, 'test2' AS tableName FROM test2 WHERE q_id = 2
    UNION
    SELECT test_id, 'test3' AS tableName FROM test3 WHERE q_id = 2

修改了上述查询以反映每个q_id来自哪个表。

Answer 3

SELECT test_id
FROM (
    SELECT * FROM test1 UNION
    SELECT * FROM test2 UNION
    SELECT * FROM test3
) tests
WHERE q_id = 2

Mysql查询疑问

3 个答案: