-- WRONG: WHERE column = NULL (returns 0 rows)

-- CORRECT: WHERE column IS NULL

-- NULL-safe calculation:

SELECT salary + COALESCE(bonus, 0) FROM employees;

-- Aggregates:

COUNT(*) includes NULLs, COUNT(column) excludes NULLs

NULL represents unknown, not empty or zero

Use IS NULL / IS NOT NULL for checks

NULL = NULL returns UNKNOWN, not TRUE

WHERE excludes UNKNOWN results

NULL propagates in calculations

Strict NULL handling, NULLS FIRST/LAST in ORDER BY

NULL sorts first in ASC, last in DESC

NULL sorts first by default

Core references in this topic include WHERE, =, != / <>. Learn what each one does, when to use it, and the execution or engine rules that matter.

WHERE

Filters rows before projection and sorting. It decides which rows continue through the query pipeline.

SELECT ... FROM table WHERE condition;

Most performance issues start with a weak WHERE clause or a missing supporting index.

=

Returns rows where the left and right values are exactly equal.

column = value

Use with exact matches. Do not use = NULL.

!= / <>

Returns rows where the compared values are not equal.

column <> value

SQL supports both <> and != in many engines, but <> is the portable form.

<, >, <=, >=

Range comparison operators for less-than, greater-than, and inclusive boundary checks.

salary >= 80000

OR

Matches rows when at least one condition is TRUE.

condition_a OR condition_b

Use parentheses when mixing OR with AND.

NOT

Negates a boolean condition and returns the opposite truth value.

NOT condition

IS NULL / IS NOT NULL

Tests whether a value is missing. SQL NULL semantics require dedicated NULL predicates.

manager_id IS NULL