AND OR NOT

One predicate is rarely enough. AND, OR and NOT combine predicates into richer search conditions - the everyday logic of real queries.

AND, OR and NOT in one line

• AND - both predicates must be true. Narrows the result.
• OR - at least one predicate must be true. Widens the result.
• NOT - reverses a predicate: NOT (SALARY > 40000) means salary is 40000 or less.
• You can chain as many as you need: A AND B OR C AND NOT D.

Evaluation order and parentheses

• DB2 evaluates NOT first, then AND, then OR.
• So A OR B AND C means A OR (B AND C) - not (A OR B) AND C.
• Parentheses override the default order. Use them whenever OR and AND mix.
• Reading the condition aloud helps: say 'and'/'or' exactly where they appear.

NOT and NULL need care

• NOT (SALARY > 40000) is true for salaries of 40000 or below - but not for NULL salaries.
• NOT flips true to false and false to true; unknown stays unknown.
• If NULLs matter, add an explicit IS NULL or IS NOT NULL test.
• Prefer positive conditions where possible - they are easier to read and to index.

Example