Overview of logical operators and three-valued logic - PolarDB

PolarDB for PostgreSQL (Compatible with Oracle) supports three logical operators: AND, OR, and NOT. SQL uses a three-valued logic system where values can be true, false, or null (unknown). Understanding how these operators behave with null inputs helps you write correct filter conditions.

Operators

Syntax	Returns	Behavior with null
`Boolean AND Boolean → Boolean`	`true` only when both operands are `true`	Returns `false` if either operand is `false`; otherwise returns `null`
`Boolean OR Boolean → Boolean`	`true` when either operand is `true`	Returns `true` if either operand is `true`; otherwise returns `null`
`NOT Boolean → Boolean`	Negates the operand	Returns `null` if the operand is `null`

Truth tables

AND and OR

a	b	a AND b	a OR b
true	true	true	true
true	false	false	true
true	null	null	true
false	false	false	false
false	null	false	null
null	null	null	null

NOT

a	NOT a
true	false
false	true
null	null

Examples

-- AND: both conditions must be true
SELECT true AND true;   -- true
SELECT true AND false;  -- false
SELECT true AND null;   -- null
SELECT false AND null;  -- false

-- OR: at least one condition must be true
SELECT true OR false;   -- true
SELECT false OR null;   -- null
SELECT true OR null;    -- true

-- NOT: negates the value
SELECT NOT true;   -- false
SELECT NOT false;  -- true
SELECT NOT null;   -- null

Usage notes

AND and OR are commutative: switching the left and right operands does not change the result. However, the database does not guarantee that the left operand is evaluated before the right operand. If your query depends on evaluation order — for example, to guard against a division-by-zero error — use a CASE expression instead.