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
...
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 i...
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 s...
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.E...
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.Dep...
