There are cases we want to execute a query includes a WHERE clause which must look up in a subquery and join tables is not prefferable . Find this example below.
We have a table includes the payroll of employees consist of 2 fields:
Table: EmployeesPayroll
Field 1:EmployeeId
Field 2: Amount
The query must create a list includes all employees live in London.
This is the statement
Select EmployeeId, Amount from EmployeesPayroll
where EmployeeId in (SELECT Employeeid FROM Employees Where City=’London’)