我们可以在COUNT中忽略重复行吗？

是的，我们可以使用DISTINCT在COUNT中忽略重复行。以下是语法：

select count(distinct yourColumnName) from yourTableName;

在MySQL中，COUNT()将显示行数。DISTINCT用于忽略重复行，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)
);
Query OK, 0 rows affected (0.47 sec)

以下是使用insert命令将一些记录插入表中的查询：

mysql> insert into DemoTable(FirstName) values('Larry');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable(FirstName) values('Sam');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable(FirstName) values('Larry');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable(FirstName) values('Mike');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable(FirstName) values('Robert');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(FirstName) values('Carol');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(FirstName) values('Mike');
Query OK, 1 row affected (0.12 sec)

以下是使用select命令从表中显示记录的查询：

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中重复行并仅获取唯一行计数的查询：

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)

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