NULL values are special values that are used to represent missing data in SQL. Columns of any type can contain NULL values. Logically, a NULL value can be seen as “the value of this field is unknown”.

-- insert a null value into a table
CREATE TABLE integers(i INTEGER);
INSERT INTO integers VALUES (NULL);

NULL values have special semantics in many parts of the query as well as in many functions:

Any comparison with a NULL value returns NULL, including NULL=NULL.

You can use IS NOT DISTINCT FROM to perform an equality comparison where NULL values compare equal to each other. Use IS (NOT) NULL to check if a value is NULL.

SELECT NULL=NULL;
-- returns NULL
SELECT NULL IS NOT DISTINCT FROM NULL;
-- returns true
SELECT NULL IS NULL;
-- returns true

NULL and Functions

A function that has input argument as NULL usually returns NULL.

SELECT COS(NULL);
-- NULL

COALESCE is an exception to this. COALESCE takes any number of arguments, and returns for each row the first argument that is not NULL. If all arguments are NULL, COALESCE also returns NULL.

SELECT COALESCE(NULL, NULL, 1);
-- 1
SELECT COALESCE(10, 20);
-- 10
SELECT COALESCE(NULL, NULL);
-- NULL

NULL and Conjunctions

NULL values have special semantics in AND/OR conjunctions. For the ternary logic truth tables, see the Boolean Type documentation.

NULL and Aggregate Functions

NULL values are ignored in most aggregate functions.

Aggregate functions that do not ignore NULL values include: FIRST, LAST, LIST, and ARRAY_AGG. To exclude NULL values from those aggregate functions, the FILTER clause can be used.

CREATE TABLE integers(i INTEGER);
INSERT INTO integers VALUES (1), (10), (NULL);

SELECT MIN(i) FROM integers;
-- 1

SELECT MAX(i) FROM integers;
-- 10