# Subqueries

# Subquery in FROM clause

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

# Subquery in WHERE clause

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)

# Subquery in SELECT clause

SELECT
  Id, 
  FName, 
  LName,
  (SELECT COUNT(*) FROM Cars WHERE Cars.CustomerId = Customers.Id) AS NumberOfCars
FROM Customers

# Correlated Subqueries

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
    )

Subquery SELECT AVG(Salary) ... is correlated because it refers to Employee row eOuter from its outer query.

# Subqueries in FROM clause

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:

city temp_var
ST LOUIS 21
LOS ANGELES 31
LOS ANGELES 23
LOS ANGELES 31
LOS ANGELES 27
LOS ANGELES 28
LOS ANGELES 28
LOS ANGELES 32

.

# Subqueries in WHERE clause

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:

name pop2000
San Francisco 776733
ST LOUIS 348189
Kansas City 146866

# Filter query results using query on different table

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

# Subqueries in SELECT clause

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;

# Remarks

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.