我可以同时使用 MySQL COUNT() 和 DISTINCT 吗?
mysqlmysqli database
是的，您可以同时使用 COUNT() 和 DISTINCT 来显示仅不同行的计数。
语法如下 −
SELECT COUNT(DISTINCT yourColumnName) AS anyVariableName FROM yourTableName;
为了理解上述语法，让我们创建一个表。
创建表的查询如下 −
mysql> create table CountDistinctDemo -> ( -> Id int NOT NULL AUTO_INCREMENT, -> Name varchar(20), -> PRIMARY KEY(Id) -> ); Query OK, 0 rows affected (3.11 sec)
使用 insert 命令在表中插入一些记录。 查询语句如下 −
mysql> insert into CountDistinctDemo(Name) values('Carol');
Query OK, 1 row affected (0.48 sec)
mysql> insert into CountDistinctDemo(Name) values('Bob');
Query OK, 1 row affected (0.43 sec)
mysql> insert into CountDistinctDemo(Name) values('Carol');
Query OK, 1 row affected (0.26 sec)
mysql> insert into CountDistinctDemo(Name) values('John');
Query OK, 1 row affected (0.27 sec)
mysql> insert into CountDistinctDemo(Name) values('Bob');
Query OK, 1 row affected (0.35 sec)
mysql> insert into CountDistinctDemo(Name) values('Carol');
Query OK, 1 row affected (0.98 sec)
mysql> insert into CountDistinctDemo(Name) values('John');
Query OK, 1 row affected (0.26 sec)
mysql> insert into CountDistinctDemo(Name) values('Sam');
Query OK, 1 row affected (0.14 sec)
mysql> insert into CountDistinctDemo(Name) values('Mike');
Query OK, 1 row affected (0.53 sec)
mysql> insert into CountDistinctDemo(Name) values('Carol');
Query OK, 1 row affected (0.31 sec)
mysql> insert into CountDistinctDemo(Name) values('David');
Query OK, 1 row affected (0.40 sec)
使用 select 语句显示表中的所有记录。
查询如下。
mysql> select *from CountDistinctDemo;
以下是输出。
+----+-------+ | Id | Name | +----+-------+ | 1 | Carol | | 2 | Bob | | 3 | Carol | | 4 | John | | 5 | Bob | | 6 | Carol | | 7 | John | | 8 | Sam | | 9 | Mike | | 10 | Carol | | 11 | David | +----+-------+ 11 rows in set (0.07 sec)
如果您不使用 DISTINCT，则 COUNT() 函数会计算所有行的数量。
查询如下 −
mysql> select count(Name) as TotalName from CountDistinctDemo;
以下是输出 −
+-----------+ | TotalName | +-----------+ | 11 | +-----------+ 1 row in set (0.04 sec)
这是同时使用 COUNT() 和 DISTINCT 的查询 −
mysql> SELECT COUNT(DISTINCT Name) as UniqueName FROM CountDistinctDemo;
下面是输出结果
+------------+ | UniqueName | +------------+ | 6 | +------------+ 1 row in set (0.00 sec)
