Expressions are composed of various operators and operands that evaluate to a single value or a set of values. Some expressions do not use operators; for example, a column name is an expression. Expressions are used in many contexts, such as specifying values to be retrieved (in a SELECT clause) or compared (in a WHERE clause).

In the following example, empname and empage are expressions representing the column values to be retrieved, salary is an expression representing a column value to be compared, and 75000 is an integer literal expression.

Expressions that contain aggregate functions can appear only in SELECT and HAVING clauses. Aggregate functions cannot be nested.

An expression can be enclosed in parentheses without affecting its value.