Question

所以我有一个简单的查询如下：

Select DISTINCT A.col1 as col1 
    FROM Table A join Table B on(A.col2 = B.col2)
    JOIN table C on(C.col1 = A.col3)
UNION
SELECT DISTINCT A.col1 as col1 
    FROM Table A 
    JOIN Table B on (A.col2 = B. col2) where A.col4 != 'somevalue';

因为它是可见的每次都在同一列上执行表A和表B连接。我无法找到一种可以利用这些细节并使查询更快的方法

Answer 1

尝试将连接转换为EXISTS和子查询逻辑，如下所示：

SELECT DISTINCT A.col1 AS col1
FROM TABLE A
WHERE EXISTS
  (SELECT 1 FROM TABLE B WHERE A.col2 = B.col2
  )
AND EXISTS
  (SELECT 1 FROM TABLE C WHERE A.col1 = C.col3
  )
UNION
SELECT DISTINCT A.col1 AS col1
FROM TABLE A
WHERE EXISTS
  (SELECT 1 FROM TABLE B WHERE A.col2 = B.col2
  )
AND A.col4 != 'somevalue');

Answer 2

这应该是等价的，但它没有多大意义：

Select DISTINCT A.col1 as col1 
FROM Table A
JOIN Table B  on (A.col2 = B.col2)
WHERE EXISTS ( SELECT * FROM C WHERE C.col1 = A.col3 )
   OR A.col4 != 'somevalue';

mysql union，独特的查询优化

2 个答案: