MySQL 如何在MySQL数据库中计算来自一列的唯一记录？

为此，使用聚合函数count(*)计算分组的GROUP BY。首先创建一个表−

mysql> create table DemoTable
(
   UserName varchar(100),
   UserPostMessage text
);
Query OK, 0 rows affected (0.54 sec)

使用插入命令向表中插入一些记录−

mysql> insert into DemoTable values('Chris','Hi');
Query OK, 1 row affected (0.09 sec)
mysql> insert into DemoTable values('David','Hello');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Chris','Awesome');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values('Chris','Amazing');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('David','Nice Place');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Chris','Amazing');
Query OK, 1 row affected (0.12 sec)

使用select语句显示表中的所有记录−

mysql> select *from DemoTable;

这将产生以下输出−

+----------+-----------------+
| UserName | UserPostMessage |
+----------+-----------------+
| Chris    | Hi              |
| David    | Hello           |
| Chris    | Awesome         |
| Chris    | Amazing         |
| David    | Nice Place      |
| Chris    | Amazing         |
+----------+-----------------+
6 rows in set (0.00 sec)

下面是在MySQL数据库中计算来自一列唯一记录的查询−

mysql> select UserName,count(DISTINCT UserPostMessage) AS NumberOfMessagesPostByUser from DemoTable group by UserName;

这将产生以下输出−

+----------+----------------------------+
| UserName | NumberOfMessagesPostByUser |
+----------+----------------------------+
| Chris    |                          3 |
| David    |                          2 |
+----------+----------------------------+
2 rows in set (0.00 sec)

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