在 MySQL 中使用 GROUP BY 限制計數

我們首先建立一個表 -

mysql> create table DemoTable
(
   UserId int,
   UserMessage varchar(100)
);
Query OK, 0 rows affected (0.51 sec)

使用 insert 命令在表中插入一些記錄 -

mysql> insert into DemoTable values(1,'Hi');
Query OK, 1 row affected (0.09 sec)
mysql> insert into DemoTable values(2,'Hello');
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable values(2,'Good');
Query OK, 1 row affected (0.20 sec)
mysql> insert into DemoTable values(1,'Nice');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable values(1,'Awesome');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values(1,'Amazing');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values(1,'Good Morning');
Query OK, 1 row affected (0.12 sec)

使用 select 語句從表中顯示所有記錄 -

mysql> select *from DemoTable;

這將產生以下輸出 -

+--------+--------------+
| UserId | UserMessage  |
+--------+--------------+
|      1 | Hi           |
|      2 | Hello        |
|      2 | Good         |
|      1 | Nice         |
|      1 | Awesome      |
|      1 | Amazing      |
|      1 | Good Morning |
+--------+--------------+
7 rows in set (0.00 sec)

以下是使用 GROUP BY 限制計數的查詢 -

mysql> select UserId, case when count(*) < 4 then count(*) else 'Greater Than 4' end as 'NumberOfMessage' from DemoTable group by UserId;

這將產生以下輸出 -

+--------+-----------------+
| UserId | NumberOfMessage |
+--------+-----------------+
|      1 | Greater Than 4 |
|      2 | 2              |
+--------+-----------------+
2 rows in set (0.06 sec)

更新於: 26-Sep-2019

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