Learn

So far, we’ve focused on matching rows that have some information in common.

Sometimes, we just want to combine all rows of one table with all rows of another table.

For instance, if we had a table of shirts and a table of pants, we might want to know all the possible combinations to create different outfits.

Our code might look like this:

SELECT shirts.shirt_color,
   pants.pants_color
FROM shirts
CROSS JOIN pants;

The first two lines select the columns shirt_color and pants_color.
The third line pulls data from the table shirts.
The fourth line performs a CROSS JOIN with pants.

Notice that cross joins don’t require an ON statement. You’re not really joining on any columns!

If we have 3 different shirts (white, grey, and olive) and 2 different pants (light denim and black), the results might look like this:

shirt_color	pants_color
white	light denim
white	black
grey	light denim
grey	black
olive	light denim
olive	black

3 shirts × 2 pants = 6 combinations!

This clothing example is fun, but it’s not very practically useful.

A more common usage of CROSS JOIN is when we need to compare each row of a table to a list of values.

Let’s return to our newspaper subscriptions. This table contains two columns that we haven’t discussed yet:

start_month: the first month where the customer subscribed to the print newspaper (i.e., 2 for February)
end_month: the final month where the customer subscribed to the print newspaper

Suppose we wanted to know how many users were subscribed during each month of the year. For each month (1, 2, 3) we would need to know if a user was subscribed. Follow the steps below to see how we can use a CROSS JOIN to solve this problem.

Instructions

Eventually, we’ll use a cross join to help us, but first, let’s try a simpler problem.

Let’s start by counting the number of customers who were subscribed to the newspaper during March.

Use COUNT(*) to count the number of rows and a WHERE clause to restrict to two conditions:

start_month <= 3
end_month >= 3

Don’t remove the previous query.

The previous query lets us investigate one month at a time. In order to check across all months, we’re going to need to use a cross join.

Our database contains another table called months which contains the numbers between 1 and 12.

Select all columns from the cross join of newspaper and months.

Don’t remove your previous queries.

Create a third query where you add a WHERE statement to your cross join to restrict to two conditions:

start_month <= month
end_month >= month

This will select all months where a user was subscribed.

Don’t remove your previous queries.

Create a final query where you aggregate over each month to count the number of subscribers.

Fill in the blanks in the following query:

SELECT month,
  COUNT(*)
FROM ________
CROSS JOIN ________
WHERE ________ AND ________
GROUP BY ________;

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