MySQL聚合函數如何與MySQL IF()函式結合？

Combining MySQL aggregate functions with MySQL IF() function can be very helpful to get the specific output we want. Consider the following queries which combine SUM() and COUNT() aggregate functions with IF() function.

Example

mysql> Select SUM(IF(Language = &#39;English&#39;, 1, 0)) As English, SUM(IF(Language <> &#39;English&#39;,1,0)) AS "Non-English" from Students;
+---------+-------------+
| English | Non-English |
+---------+-------------+
| 5       | 4           |
+---------+-------------+
1 row in set (0.00 sec)

上述查詢將SUM()聚合函數與IF()函數結合使用，從「學生」表中取得英文母語學生與非英語母語學生的輸出。

mysql> Select COUNT(IF(country = 'USA', 1, NULL))AS USA,
    -> COUNT(IF(country = 'UK', 1, NULL))AS UK,
    -> COUNT(IF(country = 'France', 1, NULL))AS France,
    -> COUNT(IF(country = 'Russia', 1, NULL))AS Russia,
    -> COUNT(IF(country = 'Australia', 1, NULL))AS Australia,
    -> COUNT(IF(country = 'INDIA', 1, NULL))AS INDIA,
    -> COUNT(IF(country = 'NZ', 1, NULL))AS NZ FROM Students;
+-----+----+--------+--------+-----------+-------+----+
| USA | UK | France | Russia | Australia | INDIA | NZ |
+-----+----+--------+--------+-----------+-------+----+
| 2   | 1  | 1      | 1      | 1         | 2     | 1  |
+-----+----+--------+--------+-----------+-------+----+
1 row in set (0.07 sec)

上面的查詢將COUNT()聚合函數與IF()函數結合起來，以從「Students」表中取得國家數量的輸出。

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