-- WRONG: Returns nothing

SELECT * FROM employees WHERE manager_id = NULL;

-- CORRECT: Returns rows with NULL manager_id

SELECT * FROM employees WHERE manager_id IS NULL;

-- Find employees WITH managers

SELECT * FROM employees WHERE manager_id IS NOT NULL;

SELECT

salary,

bonus,

salary + COALESCE(bonus, 0) AS total_comp

FROM employees;

SELECT

COUNT(*) AS total_employees,

COUNT(bonus) AS employees_with_bonus,

AVG(bonus) AS avg_bonus

FROM employees;

-- Convert empty string to NULL

SELECT NULLIF(phone, '') AS phone_clean

FROM customers;

NULL is not a value - it means "unknown"

NULL = NULL returns FALSE (not TRUE)

Always use IS NULL or IS NOT NULL

NULL in calculations returns NULL

COALESCE returns first non-NULL value

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

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.

OR

Matches rows when at least one condition is TRUE.