sql-server - 一个简单的查询正在复制记录

Question

我有两（2）张桌子。第一个命名TABLE有 N 行，唯一的 _A (KEY)，第二个命名TABLE_AUX有 0 行，然后我只是运行下一个查询：

INSERT INTO TABLE_AUX(_A,_B,_C) 
SELECT A,B,C FROM TABLE;

评估查询后，我有：

a) N = SELECT COUNT(1) FROM TABLE = SELECT COUNT(1) FROM TABLE_AUX

b) K = SELECT SUM(Q.CNT-1) 
       FROM ( SELECT COUNT(1) as CNT, _A 
              FROM TABLE_AUX GROUP BY _A HAVING COUNT(1)>1
       ) Q;

c) M = SELECT COUNT(Q._A) 
       FROM (SELECT DISTINCT _A 
             FROM TABLE_AUX) Q

, WHERE N = M + K, then M < N 

d) Zero = 0 = SELECT COUNT(1) 
              FROM TABLE T 
              WHERE NOT EXISTS ( SELECT * 
                                 FROM TABLE_AUX TA 
                                 WHERE TA._A = T._A );

e) P = SELECT COUNT(1) FROM TABLE T, TABLE_AUX TA WHERE T._A = TA._A;

, WHERE P = N + K, then N < P

很奇怪，在某些情况下会出现重复的结果。但并非在所有情况下。有人知道会发生什么吗？

Answer 1

在 Select 查询中使用 Distinct 关键字。

INSERT INTO TABLE_AUX(_A,_B,_C) 
SELECT DISTINCT A,B,C FROM TABLE;