SQL：比較兩個表與重複

Question

我需要比較2個表來找到重複，但這些字段允許重複。

For Example: 
Tbl1: 
455 
7991 
7991 

Tbl2: 
455 
7991 
7991 
7991 

我需要編寫一個查詢，顯示有一個額外的7991在表2中我嘗試了右連接，但它仍然找到了比賽，因此不能正常工作。如果有人能指出我正確的方向，我將不勝感激。謝謝！！

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

如果你的數據庫支持FULL OUTER JOIN（不是MySQL的）這會給你所有的兩個表之間的差異：

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