CROSS JOIN and UNION ALL

Let’s say you have the following table:

user_id	start_date	end_date
1	2023-01-01	2023-01-10
2	2023-01-05	2023-01-15
3	2023-02-01	2023-02-10

Why CROSS JOIN?

1. Purpose of the CROSS JOIN:
   • The CROSS JOIN generates all possible pairs of rows from the Subscription table.
   • By pairing each row u1 with every other row u2, it enables comparing subscription ranges between different users (u1.user_id != u2.user_id).
2. Condition Application:
   • After generating all combinations, the WHERE clause filters the results to include only those where:
     • The users are different (u1.user_id != u2.user_id).
     • The subscription ranges overlap:sqlCopy codeu2.start_date BETWEEN u1.start_date AND u1.end_date OR u2.end_date BETWEEN u1.start_date AND u1.end_date
3. Use Case for CROSS JOIN:
   • It’s ideal for scenarios where you need pairwise comparisons between rows in the same table.
   • Example: Checking for overlapping date ranges, finding duplicate rows, or performing similarity comparisons.

Why Not UNION ALL?

1. Purpose of UNION ALL:
   • Combines the results of multiple SELECT queries into a single result set.
   • Does not compare rows against each other—it simply stacks results from different queries.
2. Why UNION ALL Doesn’t Work Here:
   • To detect overlaps, you need to compare subscriptions against each other (row by row). A UNION ALL cannot perform such comparisons because it doesn’t create pairs of rows.
   • Example

SELECT user_id, start_date, end_date FROM Subscription
UNION ALL
SELECT user_id, start_date, end_date FROM Subscription;

• This just duplicates the table without any comparisons.

Key Limitation:

• UNION ALL is not designed for pairwise comparisons or filtering based on relationships between rows.

Alternative to CROSS JOIN (More Efficient Approach):

If you want to avoid the computational cost of a CROSS JOIN (it generates a large number of row pairs), you can use a self-join:

SELECT DISTINCT u1.user_id, 1 AS is_overlap
FROM Subscription u1
JOIN Subscription u2
  ON u1.user_id != u2.user_id
 AND (u2.start_date BETWEEN u1.start_date AND u1.end_date
      OR u2.end_date BETWEEN u1.start_date AND u1.end_date);

This performs the same logic as the CROSS JOIN but limits the number of row combinations by applying the join condition during the pairing step.

What CROSS JOIN Does:

A CROSS JOIN generates all possible pairs of rows for comparison:

SELECT u1.user_id, u2.user_id
FROM Subscription u1
CROSS JOIN Subscription u2;

Resulting pairs:

u1.user_id	u2.user_id
1	1
1	2
1	3
2	1
2	2
2	3
3	1
3	2
3	3

You can then apply conditions to filter meaningful comparisons:

WHERE u1.user_id != u2.user_id
AND (u2.start_date BETWEEN u1.start_date AND u1.end_date OR
     u2.end_date BETWEEN u1.start_date AND u1.end_date)

UNION ALL:

SELECT user_id, start_date, end_date FROM Subscription
UNION ALL
SELECT user_id, start_date, end_date FROM Subscription;

The result simply duplicates the rows (or combines rows from multiple queries). Adding a WHERE clause like:

WHERE start_date BETWEEN ... OR end_date BETWEEN ...

would filter rows independently, but it wouldn’t allow you to compare one row against another because UNION doesn’t pair rows.

Example to Clarify:

Task: Find Overlapping Subscriptions Between Users

user_id	start_date	end_date
1	2023-01-01	2023-01-10
2	2023-01-05	2023-01-15
3	2023-02-01	2023-02-10

Using CROSS JOIN:

SELECT u1.user_id AS user1, u2.user_id AS user2
FROM Subscription u1
CROSS JOIN Subscription u2
WHERE u1.user_id != u2.user_id
AND (u2.start_date BETWEEN u1.start_date AND u1.end_date
     OR u2.end_date BETWEEN u1.start_date AND u1.end_date);

Result:

user1	user2
1	2
2	1

Using UNION:

If you try to use UNION ALL:

SELECT user_id, start_date, end_date
FROM Subscription
UNION ALL
SELECT user_id, start_date, end_date
WHERE start_date BETWEEN ... OR end_date BETWEEN ...;

It simply duplicates rows. There’s no pairing or comparison between rows because UNION ALL just stacks rows from the two queries.

1. What is UNION ALL?

• Purpose: Combines the result sets of two or more queries into a single result.
• Key Feature: Keeps all rows from all queries, including duplicates (unlike UNION, which removes duplicates).

Example:

Table A:

id	name
1	Alice
2	Bob

Table B:

id	name
2	Bob
3	Charlie

Query:

SELECT * FROM A
UNION ALL
SELECT * FROM B;

Result:

id	name
1	Alice
2	Bob
2	Bob
3	Charlie

• Combines the rows from both tables without comparing or filtering relationships.