如何计算 MySQL 数据库中某一列的唯一记录?
mysqlmysqli database更新于 2025/12/24 3:24:17
为此，使用聚合函数 count(*) 进行计数，以 GROUP BY 进行分组。让我们首先创建一个表 −
mysql> create table DemoTable ( UserName varchar(100), UserPostMessage text ); Query OK, 0 rows affected (0.54 sec)
使用 insert 命令在表中插入一些记录 −
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)
以下查询用于计算 MySQL 数据库中某一列的唯一记录数 −
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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