What SQL clause is primarily used to combine results from two or more SELECT queries?

The clause that is primarily used to combine results from two or more SELECT queries is the JOIN clause. This is essential when you need to retrieve data that is spread across multiple tables within a database.

Using JOIN allows you to specify how the tables are related, enabling you to create a single result set that includes columns from both tables based on a logical relationship. There are various types of JOINs, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN, each serving different purposes depending on how you want to combine the data.

For example, if you have a table of customers and a separate table of orders, you can use a JOIN to combine these tables and produce a result that shows which customers made which orders. This is invaluable for queries that require a comprehensive view of related data.

The other clauses serve different purposes: WHERE is used to filter records based on specific conditions, GROUP BY organizes rows that have the same values into summary rows, and ORDER BY is utilized to sort the result set based on one or more columns. Each of these plays a crucial role in SQL queries, but they do not focus on combining results from multiple SELECT operations as the JOIN clause does.