Hello and welcome to the fourth part of our SQL Practice Set in MS SQL Server! Today we'll work with subqueries! We'll start with simple, uncorrelated subqueries. (We'll revisit correlated subqueries later in this part.) Here's a brief reminder:

A subquery is a query within another query.

We can use subqueries in the WHERE clause to compare a given column with the result of a whole query. When comparing with the result of the subquery, you can use comparison operators by themselves:

SELECT CatId
FROM Cats
WHERE Age > (SELECT Age FROM Cats WHERE CatName = 'Kitty')

or comparison operators with the ANY or ALL keywords, if your subquery can return multiple rows:

SELECT CatId
FROM Cats
WHERE Age > ANY (SELECT Age FROM Cats WHERE CatName = 'Kitty')

or the operator IN, if the value of the column compared with the subquery has to be in the result of particular subquery, e.g.

SELECT CatId
FROM Cats
WHERE Age IN (SELECT Age FROM Cats WHERE CatName LIKE 'K%')

We can also use the subqueries in the FROM clause, and filter our rows in this way. The subquery in the FROM clause has to have an alias.

SELECT MAX(NumberOfCats)
FROM 
  (SELECT Breed, COUNT(*) AS NumberOfCats
  FROM Cat
  GROUP BY Breed) BreedCount