获取 MySQL 列中仅出现一次的值的数量?
mysqlmysqli database
要获取列中仅出现一次的值的数量，请使用 GROUP BY HAVING。让我们首先创建一个表:
mysql> create table DemoTable ( Name varchar(20) ); Query OK, 0 rows affected (0.55 sec)
下面是使用 insert 命令在表中插入一些记录的查询:
mysql> insert into DemoTable values('Larry');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable values('Larry');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Sam');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('Chris');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('Sam');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('Mike');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable values('Sam');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Larry');
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable values('Mike');
Query OK, 1 row affected (0.32 sec)
mysql> insert into DemoTable values('Mike');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values('Carol');
Query OK, 1 row affected (0.27 sec)
mysql> insert into DemoTable values('Robert');
Query OK, 1 row affected (0.27 sec)
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 −
+--------+ | Name | +--------+ | Larry | | Larry | | Sam | | Chris | | Sam | | Mike | | Sam | | Larry | | Mike | | Mike | | Carol | | Robert | +--------+ 12 rows in set (0.00 sec)
以下查询用于计算列中仅出现一次的值，即“Name”列中仅出现一次的名称的数量:
mysql> SELECT COUNT(Name) FROM ( SELECT Name FROM DemoTable GROUP BY Name HAVING COUNT(*) = 1 ) AS APPEAR_FIRST_TIME;
这将产生以下输出 −
+-------------+ | COUNT(Name) | +-------------+ | 3 | +-------------+ 1 row in set (0.00 sec)
