混合RAND（）和UNION給出隨機rowcount？

Question

我測試了一個查詢在MySQL中得到一個隨機整數，我注意到了這個問題：

mysql> SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)); 
+-------------------------+ 
| FLOOR(0 + (RAND() * 5)) | 
+-------------------------+ 
|      1 | 
|      2 | 
|      4 | 
+-------------------------+ 
3 rows in set (0.00 sec) 

mysql> SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)); 
+-------------------------+ 
| FLOOR(0 + (RAND() * 5)) | 
+-------------------------+ 
|      4 | 
+-------------------------+ 
1 row in set (0.00 sec) 

mysql> SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)) UNION SELECT FLOOR(0 + (RAND() * 5)); 
+-------------------------+ 
| FLOOR(0 + (RAND() * 5)) | 
+-------------------------+ 
|      1 | 
|      4 | 
|      0 | 
+-------------------------+ 
3 rows in set (0.00 sec) 

我似乎得到行的結果集中的隨機數！那裏有3 UNION ed SELECT s。到底是怎麼回事？我在想，從蘭特語句的結果也許有沒有因爲RAND()的元組，但沒有任何意義 - 應該有一些結果，一個NULL至少！

mysql> select version(); 
+-------------------+ 
| version()   | 
+-------------------+ 
| 5.0.51a-24+lenny4 | 
+-------------------+ 
1 row in set (0.00 sec) 

Answer 1

UNION消除了重複結果，因爲它是一個集合運算符。如果您想保留重複項，請使用UNION ALL。也許我錯誤地理解了你的問題，但是這看起來正是我期望的查詢結果的類型。