在 MySQL 中使用条件计数?
mysqlmysqli database更新于 2024/2/9 14:30:00
要计数，请使用聚合函数 SUM()；要使用条件计数，则需要使用 WHERE 设置条件。让我们首先创建一个表 −
mysql> create table DemoTable1515 -> ( -> ClientId varchar(10), -> ClientName varchar(20) -> ); Query OK, 0 rows affected (0.53 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable1515 values('CLI-101','Chris');
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1515 values('CLI-110','David');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable1515 values('CLI-101','Bob');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable1515 values('CLI-101','Sam');
Query OK, 1 row affected (0.19 sec)
mysql> insert into DemoTable1515 values('CLI-101','Sam');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable1515 values('CLI-101','Mike');
Query OK, 1 row affected (0.19 sec)
mysql> insert into DemoTable1515 values('CLI-130','Mike');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable1515 values('CLI-101','Chris');
Query OK, 1 row affected (0.35 sec)
mysql> insert into DemoTable1515 values('CLI-101','Sam');
Query OK, 1 row affected (0.18 sec)
使用 select 语句显示表中的所有记录 −
mysql> select * from DemoTable1515;
这将产生以下输出 −
+----------+------------+ | ClientId | ClientName | +----------+------------+ | CLI-101 | Chris | | CLI-110 | David | | CLI-101 | Bob | | CLI-101 | Sam | | CLI-101 | Sam | | CLI-101 | Mike | | CLI-130 | Mike | | CLI-101 | Chris | | CLI-101 | Sam | +----------+------------+ 9 rows in set (0.00 sec)
这是带条件的计数查询 −
mysql> select ClientId,sum(ClientName='Sam') as NameOn101Id,count(*) as TotalNameOn101 from DemoTable1515 -> where ClientId='CLI-101';
这将产生以下输出 −
+----------+-------------+----------------+ | ClientId | NameOn101Id | TotalNameOn101 | +----------+-------------+----------------+ | CLI-101 | 3 | 7 | +----------+-------------+----------------+ 1 row in set (0.00 sec)
