SQL - AND & OR

The AND clause is used when you want to specify more than one condition in your WHERE clause, and at the same time you want all conditions to be true.
For example if you want to select all customers with FirstName "John" and LastName "Smith", you will use the following SQL expression:

SELECT * FROM Customers WHERE FirstName = 'John' AND LastName = 'Smith'

The result of the SQL query above is:

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222

The following row in our Customer table, satisfies the second of the conditions (LastName = 'Smith'), but not the first one (FirstName = 'John'), and that's why it's not returned by our SQL query:

FirstName	LastName	Email	DOB	Phone
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

The SQL OR statement is used in similar fashion and the major difference compared to the AND is that OR clause will return all rows satisfying any of the conditions listed in the WHERE clause.

If we want to select all customers having FirstName 'James' or FirstName 'Paula' we need to use the following SQL statement:

SELECT * FROM Customers WHERE FirstName = 'James' OR FirstName = 'Paula'

The result of this query will be the following:

FirstName	LastName	Email	DOB	Phone
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

You can combine AND and OR clauses anyway you want and you can use parentheses to define your logical expressions.
Here is an example of such a SQL query, selecting all customers with LastName 'Brown' and FirstName either 'James' or 'Paula':

SELECT * FROM Customers WHERE (FirstName = 'James' OR FirstName = 'Paula') AND LastName = 'Brown'

The result of the SQL expression above will be:

FirstName	LastName	Email	DOB	Phone
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232