Read this SQL tutorial to learn when to use SELECT, JOIN, subselects and UNION to access multiple tables with a single statement.

A join combines two or more tables side by side. If you do not specify how to join the tables, you get a Cartesian product. This means that SQL combines each row from the first table with every row ...

SQL is a convenient way to manage and query your data, but badly written queries can tie up your database. Here are seven common SQL traps and how to avoid them.