Understanding the SQL CROSS JOIN Clause

The SQL CROSS JOIN clause is used to combine rows from two or more tables, such that each row from the first table is combined with each row from the second table. This type of join doesn't need a join condition, unlike the other types of JOINs in SQL. Instead, it performs what is known as a Cartesian product: a mathematical operation that returns all possible combinations of all rows in all tables, irrespective of any matching criteria.

The CROSS JOIN is particularly useful in situations where you want to create a combination of every row from two tables. Imagine you have a table of products and a table of locales. If you want to see all possible combinations of products and locales - perhaps for a global pricing matrix or for testing - CROSS JOIN is a practical way to achieve that.

However, the CROSS JOIN operation may result in a vast number of result rows and it can be very expensive in terms of computing resources. For example, if table A has 1000 rows and table B has 1000 rows, a CROSS JOIN will result in 1,000,000 rows! Therefore, it should be utilized with caution.

Here is an example of how the SQL CROSS JOIN clause works:

SELECT A.column_name, B.column_name...
FROM table_A
CROSS JOIN table_B;

In this query, table_A and table_B are the names of the tables that you want to join. The query will return a table consisting of every combination of rows from table_A and table_B.

As a best practice, always make sure you know exactly how many rows each table in your CROSS JOIN contains. If one or both of the tables have a large number of rows, the resulting table could be unwieldy and potentially slow down your database. If you only need to combine certain rows, consider using a different type of join that allows you to specify a condition to limit the results.