単一のMySQLクエリで複数のCOUNT（）を実装する

これには、CASEステートメントを使用します。まずテーブルを作成しましょう-

mysql> create table DemoTable
   -> (
   -> LastName varchar(100)
   -> );
Query OK, 0 rows affected (0.60 sec)

挿入コマンド-

mysql> insert into DemoTable values('Smith');
Query OK, 1 row affected (0.17 sec)

mysql> insert into DemoTable values('Brown');
Query OK, 1 row affected (0.17 sec)

mysql> insert into DemoTable values('Brown');
Query OK, 1 row affected (0.13 sec)

mysql> insert into DemoTable values('Smith');
Query OK, 1 row affected (0.15 sec)

mysql> insert into DemoTable values('Taylor');
Query OK, 1 row affected (0.15 sec)

mysql> insert into DemoTable values('Taylor');
Query OK, 1 row affected (0.11 sec)

mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.11 sec)

mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.12 sec)

mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.12 sec)

selectステートメントを使用してテーブルのすべてのレコードを表示する-

mysql> select *from DemoTable;

出力

これにより、次の出力が生成されます-

+----------+
| LastName |
+----------+
| Smith    |
| Brown    |
| Brown    |
| Smith    |
| Taylor   |
| Taylor   |
| Miller   |
| Miller   |
| Miller   |
+----------+
9 rows in set (0.00 sec)

以下は、1つのクエリに複数のCOUNT（）メソッドを実装するためのクエリです-

mysql> select COUNT(CASE WHEN LastName='Smith' THEN 1 END) AS TotalFrequency,
-> COUNT(CASE WHEN LastName='Brown' THEN 1 END) AS TotalFrequency,
-> COUNT(CASE WHEN LastName='Taylor' THEN 1 END) AS TotalFrequency,
-> COUNT(CASE WHEN LastName='Miller' THEN 1 END) AS TotalFrequency
-> from DemoTable;

出力

これにより、次の出力が生成されます-

+----------------+----------------+----------------+----------------+
| TotalFrequency | TotalFrequency | TotalFrequency | TotalFrequency |
+----------------+----------------+----------------+----------------+
| 2              | 2              | 2              | 3              |
+----------------+----------------+----------------+----------------+
1 row in set (0.00 sec)