Expression Evaluation and Type Conversion
MySQL allows you to write expressions that include constants, function calls, and references to table columns. These values may be combined using different kinds of operators, such as arithmetic or comparison operators, and terms of an expression may be grouped with parentheses.
Expressions occur most commonly in the column selection list and WHERE clause of SELECT statements:
SELECT CONCAT(last_name, ", ", first_name), (TO_DAYS(death) - TO_DAYS(birth) / 365) FROM president WHERE birth > "1900-1-1" AND DEATH IS NOT NULL
Each column selected represents an expression, as does the content of the WHERE clause. Expressions also occur in the WHERE clause of DELETE and UPDATE statements, the VALUES() clause ...