Subqueries can appear in different clauses of an outer query, or in the set operation.
They must be enclosed in parentheses
If the result of the subquery is compared to something else, the number of columns must match.
Table aliases are required for subqueries in the FROM clause to name the temporary table.
Use a subquery to filter the result set. For example this will return all employees with a salary equal to the highest paid employee.
SELECT * FROM Employees WHERE Salary = (SELECT MAX(Salary) FROM Employees)
A subquery in a
FROM clause acts similarly to a temporary table that is generated during the execution of a query and lost afterwards.
SELECT Managers.Id, Employees.Salary FROM ( SELECT Id FROM Employees WHERE ManagerId IS NULL ) AS Managers JOIN Employees ON Managers.Id = Employees.Id
SELECT Id, FName, LName, (SELECT COUNT(*) FROM Cars WHERE Cars.CustomerId = Customers.Id) AS NumberOfCars FROM Customers
You can use subqueries to define a temporary table and use it in the FROM clause of an "outer" query.
SELECT * FROM (SELECT city, temp_hi - temp_lo AS temp_var FROM weather) AS w WHERE temp_var > 20;
The above finds cities from the weather table whose daily temperature variation is greater than 20. The result is:
The following example finds cities (from the cities example) whose population is below the average temperature (obtained via a sub-qquery):
SELECT name, pop2000 FROM cities WHERE pop2000 < (SELECT avg(pop2000) FROM cities);
Here: the subquery (SELECT avg(pop2000) FROM cities) is used to specify conditions in the WHERE clause. The result is:
Subqueries can also be used in the
SELECT part of the outer query. The following query
shows all weather table columns with the corresponding states from the cities table.
SELECT w.*, (SELECT c.state FROM cities AS c WHERE c.name = w.city ) AS state FROM weather AS w;
This query selects all employees not on the Supervisors table.
SELECT * FROM Employees WHERE EmployeeID not in (SELECT EmployeeID FROM Supervisors)
The same results can be achieved using a LEFT JOIN.
SELECT * FROM Employees AS e LEFT JOIN Supervisors AS s ON s.EmployeeID=e.EmployeeID WHERE s.EmployeeID is NULL
Correlated (also known as Synchronized or Coordinated) Subqueries are nested queries that make references to the current row of their outer query:
SELECT EmployeeId FROM Employee AS eOuter WHERE Salary > ( SELECT AVG(Salary) FROM Employee eInner WHERE eInner.DepartmentId = eOuter.DepartmentId )
SELECT AVG(Salary) ... is correlated because it refers to
eOuter from its outer query.