1.  You can combine literals, operators, and built-in functions in countless ways to produce expressions.
2. A constant value such as a string, a number, or a NULL value.
3. Perform math in your query.
4. An important concept in mathematics is operator precedence, which means the order of operations. Precedence means that higher-level mathematical operations will be performed first so that multiplication and division are done before addition and subtraction. You can add parentheses around calculations to impact the precedence.

5. String, numeric, and date.
6. Converts text to lowercase.
7. Converts a value into the specified data type.
8. The query may no longer use the indexes on the table.
9. STDDEV(column)
10. Virtual and stored.