我可以同时使用MySQL COUNT()和DISTINCT吗？
是的，您可以COUNT()一起使用DISTINCT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) -> );
使用insert命令在表中插入一些记录。查询如下-
mysql> insert into CountDistinctDemo(Name) values('Carol');
mysql> insert into CountDistinctDemo(Name) values('Bob');
mysql> insert into CountDistinctDemo(Name) values('Carol');
mysql> insert into CountDistinctDemo(Name) values('John');
mysql> insert into CountDistinctDemo(Name) values('Bob');
mysql> insert into CountDistinctDemo(Name) values('Carol');
mysql> insert into CountDistinctDemo(Name) values('John');
mysql> insert into CountDistinctDemo(Name) values('Sam');
mysql> insert into CountDistinctDemo(Name) values('Mike');
mysql> insert into CountDistinctDemo(Name) values('Carol');
mysql> insert into CountDistinctDemo(Name) values('David');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()函数将给出所有行的计数。
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)
这是要COUNT()一起使用的查询和DISTINCT-
mysql> SELECT COUNT(DISTINCT Name) as UniqueName FROM CountDistinctDemo;
以下是输出
+------------+ | UniqueName | +------------+ | 6 | +------------+ 1 row in set (0.00 sec)