Question

我需要比较2个表来查找重复项，但这些字段允许重复。

For Example: 
Tbl1:
455
7991
7991

Tbl2: 
455
7991
7991
7991

我需要编写一个查询，显示表2中有一个额外的7991.我尝试了一个正确的连接，但它仍然找到匹配，因此它无法正常工作。如果有人能指出我正确的方向，我会非常感激。谢谢！

Answer 1

SELECT  t1.colname
FROM    ( SELECT    colname
                  , COUNT(colname) AS colCount
          FROM      tbl1
          GROUP BY  colname
        ) t1
        INNER JOIN ( SELECT   colname
                      , COUNT(colname) AS colCount
               FROM     tbl2
               GROUP BY colname
             ) t2 ON t1.colname = t2.colname
                     AND t1.colCount <> t2.colCount

Answer 2

如果您的数据库支持FULL OUTER JOIN（不是MySQL），这将为您提供两个表之间的所有差异：

SELECT
    T1.x,
    COALESCE(T1.cnt, 0) AS T1_count,
    COALESCE(T2.cnt, 0) AS T2_count
FROM
(
    SELECT x, COUNT(*) AS cnt
    FROM Tbl1
    GROUP BY x
) T1
FULL OUTER JOIN 
(
    SELECT x, COUNT(*) AS cnt
    FROM Tbl2
    GROUP BY x
) T2
ON T1.x = T2.x
WHERE T1.cnt <> T2.cnt

SQL：将2个表与重复项进行比较

2 个答案: