子查询嵌套中的问题

子查询中where查询条件要用IN（**在里面）关键字 或者=ANY（任意**）关键字，来表示值在这个子查询中。
注意：使用ANY/ALL谓词时必须同时使用比较运算符

例子：

SELECT * FROM employees
WHERE DepartmentID = (SELECT DepartmentID FROM departments WHERE DepartmentName='财务部')

当子查询中的结果只有一条时，where的查询条件可以用=等号，否则会报错。需要用IN关键字

SELECT * FROM employees
WHERE employees.EmployeeID = (SELECT EmployeeID FROM salary WHERE InCome < 2500)

1242 - Subquery returns more than 1 row
子查询多于一行
要改为

SELECT * FROM employees
WHERE employees.EmployeeID IN (SELECT EmployeeID FROM salary WHERE InCome < 2500)

或者在子查询前加any关键字

SELECT * FROM employees
WHERE employees.EmployeeID =ANY(SELECT EmployeeID FROM salary WHERE InCome < 2500)