An expression is a combination of values that are interpreted by MySQL to produce another value. Expressions can be used in SELECT statement clauses, the WHERE clause, the ORDER BY clause, the HAVING clause (covered in Chapter 10, Grouping and Summarizing Data), or in a SET statement (covered in Chapter 12, Programmable Objects).
Expressions include column values, operators, literal values, built-in functions, NULL values, user-defined functions, and stored procedures. User-defined functions and stored procedures are covered in Chapter 12, Programmable Objects.
You can combine literals, operators, and built-in functions in countless ways to produce expressions. Your imagination may be the only limit on the ways you can combine these into expressions.
