获取MySQL列中唯一行的计数？
在MySQL中，COUNT()将显示行数。DISTINCT用于忽略重复的行并仅获取唯一行的计数。
让我们首先创建一个表：
mysql> create table DemoTable ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, FirstName varchar(10) );
以下是使用insert命令在表中插入一些记录的查询：
mysql> insert into DemoTable(FirstName) values('Larry');
mysql> insert into DemoTable(FirstName) values('John');
mysql> insert into DemoTable(FirstName) values('Sam');
mysql> insert into DemoTable(FirstName) values('John');
mysql> insert into DemoTable(FirstName) values('John');
mysql> insert into DemoTable(FirstName) values('Larry');
mysql> insert into DemoTable(FirstName) values('Mike');
mysql> insert into DemoTable(FirstName) values('Robert');
mysql> insert into DemoTable(FirstName) values('Carol');mysql> insert into DemoTable(FirstName) values('Mike');以下是使用select命令显示表中记录的查询：
mysql> select *from DemoTable;
这将产生以下输出：
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | Larry | | 2 | John | | 3 | Sam | | 4 | John | | 5 | John | | 6 | Larry | | 7 | Mike | | 8 | Robert | | 9 | Carol | | 10 | Mike | +----+-----------+ 10 rows in set (0.00 sec)
以下是忽略COUNT个重复行并仅获取唯一行数的查询：
mysql> select count(distinct FirstName) from DemoTable;
这将产生以下输出：
+---------------------------+ | count(distinct FirstName) | +---------------------------+ | 6 | +---------------------------+ 1 row in set (0.03 sec)