如何在 WHERE 子句中使用实际行数 (COUNT(*))，而无需在 MySql 中编写与子查询相同的查询?
mysqlmysqli database更新于 2023/12/8 9:53:00
借助 where 子句实现此目的。
语法如下
SELECT yourColumnName1,yourColumnName2,...N FROM yourTableName WHERE ( SELECT COUNT(*) FROM yourTableName )=2;
为了理解这个概念，让我们创建一个表。创建表的查询如下
mysql> create table CountWithSubqueryDemo - > ( - > Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, - > EmployeeName varchar(20) - > ); Query OK, 0 rows affected (2.09 sec)
使用 insert 命令在表中插入一些记录。
查询如下
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('John');
Query OK, 1 row affected (0.54 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values(NULL);
Query OK, 1 row affected (0.36 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('Carol');
Query OK, 1 row affected (0.23 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('');
Query OK, 1 row affected (0.31 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('Bob');
Query OK, 1 row affected (1.24 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values(NULL);
Query OK, 1 row affected (0.30 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('Sam');
Query OK, 1 row affected (0.18 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('Mike');
Query OK, 1 row affected (0.18 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('James');
Query OK, 1 row affected (0.26 sec)
使用 select 语句显示表中的所有记录。
查询如下
mysql> select *from CountWithSubqueryDemo;
以下是输出 −
+----+--------------+ | Id | EmployeeName | +----+--------------+ | 1 | John | | 2 | NULL | | 3 | Carol | | 4 | | | 5 | Bob | | 6 | NULL | | 7 | Sam | | 8 | Mike | | 9 | James | +----+--------------+ 9 rows in set (0.19 sec)
The following is how you can use actual row count in where clause
mysql> select Id,EmployeeName from CountWithSubqueryDemo - > where - > ( - > select count(*) from CountWithSubqueryDemo - > )=2; Empty set (0.03 sec)
在上述查询中，我们得到空集，因为如果表只有两条记录，它就会返回行。因此，让我们从表中删除所有记录并在表中插入 2 条记录。
查询如下
mysql> truncate table CountWithSubqueryDemo; Query OK, 0 rows affected (1.95 sec)
使用 insert 命令在表中插入一些记录。
查询如下
mysql> insert into CountWithSubqueryDemo(EmployeeName) values('James');
Query OK, 1 row affected (0.15 sec)
mysql> insert into CountWithSubqueryDemo(EmployeeName) values(NULL);
Query OK, 1 row affected (0.46 sec)
使用 select 语句显示表中的所有记录。
查询如下
mysql> select *from CountWithSubqueryDemo;
以下是输出 −
+----+--------------+ | Id | EmployeeName | +----+--------------+ | 1 | James | | 2 | NULL | +----+--------------+ 2 rows in set (0.00 sec)
现在以下是在 where 子句中对行数的查询
mysql> select Id,EmployeeName from CountWithSubqueryDemo - > where - > ( - > select count(*) from CountWithSubqueryDemo - > )=2;
以下是输出 −
+----+--------------+ | Id | EmployeeName | +----+--------------+ | 1 | James | | 2 | NULL | +----+--------------+ 2 rows in set (0.00 sec)
