The easiest and simplest way to explain what a subquery is to say that it's a query within a query. For example if you want to get the employee that belongs to specific territory in the Northwind database without a join, you would have to use a subquery. Like the following subquery.

Things You Should Know About Subqueries:

SELECT EmployeeID, (FirstName + ' ' + LastName) AS Name FROM Employees WHERE EmployeeID IN (SELECT EmployeeID FROM EmployeeTerritories WHERE TerritoryID=01581)

Things You Should Know About Subqueries:
- They are not the most efficient performance wise
- You can only retrieve a single column in the subquery, retrieving multiple columns will throw an error
SELECT CategoryName, (SELECT AVG(UnitPrice) FROM Products WHERE CategoryID = 1) AS AvgPrice FROM Categories WHERE CategoryID = 1

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