Learning MySQL, 2nd Edition

Chapter 5. Advanced Querying

Over the previous two chapters, you’ve completed an introduction to the basic features of querying and modifying databases with SQL. You should now be able to create, modify, and remove database structures, as well as work with data as you read, insert, delete, and update entries. Over this and the next two chapters, we’ll look at more advanced concepts and then will proceed to more administrative and operations-oriented content. You can skim these chapters and return to read them thoroughly when you’re comfortable using MySQL.

This chapter teaches you more about querying, giving you skills to answer complex information needs. You’ll learn how to do the following:

• Use nicknames, or aliases, in queries to save typing and allow a table to be used more than once in a query.

• Aggregate data into groups so you can discover sums, averages, and counts.

• Join tables in different ways.

• Use nested queries.

• Save query results in variables so they can be reused in other queries.

Aliases

Aliases are nicknames. They give you a shorthand way of expressing a column, table, or function name, allowing you to:

• Write shorter queries.

• Express your queries more clearly.

• Use one table in two or more ways in a single query.

• Access data more easily from programs.

• Use special types of nested queries, discussed in “Nested Queries”.

Column Aliases

Column aliases are useful for improving the expression of your queries, reducing the number of characters ...