Learn more about temporary, or derived tables.
Subqueries are one of the most advanced concepts to learn in SQL query writing. Seeing lots of examples and getting lots of practice will help solidify the concept.
As you begin using subqueries, be advised that a poorly structured subquery written against a large table or tables can impact overall query performance. Be careful if you are writing subqueries against data sets that consume tables with row counts in the million-plus row range.
This course does not cover the concept called Correlated Subqueries, which are slightly different in how they tie in with the outer query. Most of the time a regular subquery will do what you need, but correlated subqueries can be handy, too. These are especially heavy weight on the database engine, as they run the subquery many times; once per row in the outer query.
See all of the SQL used in Querying Relational Databases in the Querying Relational Databases Cheat Sheet.
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