What is the difference between an ''or'' query and an ''and'' query?

Answer 1

It is the same difference between an OR Boolean and AND Boolean. In an OR query, you are looking for something that matches one of several possible values, while in an AND query, you are looking for something that matches all of several possible values.

-- Find the first name of anyone in the USA, or have a last name starting with A.

SELECT FirstName FROM Contacts WHERE Country = 'USA' OR LastName LIKE 'A%'

-- Find the first name of anyone in the USA that also has a last name starting with A

SELECT FirstName FROM Contacts WHERE Country = 'USA' AND LastName LIKE 'A%'

The difference between these two is that the first one will return people outside the USA if their last name starts with an A, while in the second query, only people inside the USA with the last name starting with A will be returned.

AND will always return the same or fewer values than OR.