How to Use FROM Clauses in SQL
The FROM clause in SQL is fairly easy to understand if you need to specify only one table. To observe this simple statement in action, check out the following example:
SELECT * FROM SALES ;
This statement returns all the data in all the rows of every column in the SALES table. You can, however, specify more than one table in a FROM clause. Consider the following example:
SELECT * FROM CUSTOMER, SALES ;
This statement forms a virtual table that combines the data from the CUSTOMER table with the data from the SALES table. Each row in the CUSTOMER table combines with every row in the SALES table to form the new table.
The new virtual table that this combination forms contains the number of rows in the CUSTOMER table multiplied by the number of rows in the SALES table. If the CUSTOMER table has 10 rows and the SALES table has 100, then the new virtual table has 1,000 rows.
This operation is called the Cartesian product of the two source tables. The Cartesian product is a type of JOIN.
In most applications, when you take the Cartesian product of two tables, most of the rows that are formed in the new virtual table are meaningless. That’s also true of the virtual table that forms from the CUSTOMER and SALES tables; only the rows where the CustomerID from the CUSTOMER table matches the CustomerID from the SALES table are of interest.
You can filter out the rest of the rows by using a WHERE clause.