Sams Teach Yourself SQL in 21 Days, Fourth Edition

Working with Query Expressions

The definition of an expression is simple: An expression returns a value. Expression types are very broad, covering different data types such as String, Numeric, and Boolean. In fact, pretty much anything following a clause (SELECT or FROM, for example) is an expression. In the following example, AMOUNT is an expression that returns the value contained in the AMOUNT column:

SELECT amount FROM checks;

In the following statement NAME, ADDRESS, PHONE, and ADDRESSBOOK are expressions:

SELECT NAME, ADDRESS, PHONE
FROM ADDRESSBOOK;

Now, examine the following WHERE clause:

WHERE NAME = 'BROWN'

It contains a condition, NAME = 'BROWN', which is an example of a Boolean expression. NAME = 'BROWN' will be either TRUE ...