Question

如果下面的第一个查询返回1377行，第二个查询返回765行，为什么第三个查询返回1806行而不是预期的1377 + 765 = 2142行？

SELECT DISTINCT item_id FROM MyTable 
WHERE(COMMENTS LIKE '%Created by Upload #90323%')
ORDER BY item_id

SELECT DISTINCT item_id FROM MyTable
WHERE (COMMENTS LIKE '%Created by Upload #90324%')
ORDER BY item_id

SELECT DISTINCT item_id FROM MyTable
where (COMMENTS LIKE '%Created by Upload #90323%') 
   or (COMMENTS LIKE '%Created by Upload #90324%')
ORDER BY item_id

Answer 1

由于DISTINCT，两个item_id值将具有相同的COMMENT值。为了证明这一点，请从所有查询中删除DISTINCT，然后将看到行数匹配。

为什么该第三查询不返回第一查询和第二查询的总行数？

1 个答案: