They're fairly rare, but they do come up. It allows you to very quickly create data, and since it's multiplication it can very quickly create a lot of data. It'

They're fairly rare, but they do come up. It allows you to very quickly create data, and since it's multiplication it can very quickly create a lot of data. It's mostly a way of thinking about things rather than something you actually do. Usually they can be written as an INNER JOIN because what you really want is a non-standard join or a partial CROSS JOIN. You want a Cartesian product. For example, maybe you want to join an employee table to a calendar table to produce a list of dates for each employee. Usually you want a subset of the calendar though, so you end up with something like: SELECT * FROM Employee e INNER JOIN Calendar c ON c.Cal_Date BETWEEN '2021-01-01' AND '2021-12-31' They also come up when you define an inline table with a VALUES statement, which is like creating a virtual table in a query. So if you want one row for M-F, you could do something like this: SELECT * FROM Employee e CROSS JOIN (VALUES ('M'),('T'),('W'),('R'),('F')) d (Day_of_week) Note that we have to give the system an alias for the "table" we're making and a name for the column we created, too. Another common use is with a tally table, which is a utility table that stereotypically has a single column with integers from 1 to some arbitrary value (10,000 is common). It's got many uses. Say you want to look for non-English letters in names. You have a UNICODE() function that returns the character code of a single character, and a SUBSTRING() function that returns a substring of a character field. SELECT * FROM Employee e INNER JOIN Tally t ON t.N BETWEEN 1 AND LEN(e.Name) WHERE UNICODE(SUBSTRING(e.Name,t.N,1)) NOT BETWEEN 32 AND 126 Note again that it's not actually written as a CROSS JOIN here, but it's the same line of logic that leads to CROSS JOINs. We're creating a Cartesian product. You could write it as a CROSS JOIN, but this method makes it a bit more readable to understand the logic. (The above is written with SQL Server or T-SQL. Other RDBMSs have different syntax, but can generally do the same things.)