Question

I came across the following SQL statement and I was wondering if it was valid:

SELECT COUNT(*) 
    FROM 
        registration_waitinglist, 
        registration_registrationprofile 
    WHERE 
        registration_registrationprofile.activation_key = "ALREADY_ACTIVATED"

What does the two tables separated by a comma mean?

Answer 1

When you SELECT data from multiple tables you obtain the Cartesian Product of all the tuples from these tables. It can be illustrated in the following way:

This means you get each row from the first table paired with all the rows from the second table. Most of the time, it is not what you want. If you really want it, then it’s clearer to use the CROSS JOIN notation:

SELECT * FROM A CROSS JOIN B;