SELECT DISTINCT department FROM employees;

SELECT DISTINCT department, job_title FROM employees;

SELECT DISTINCT department

FROM employees

ORDER BY department ASC;

SELECT DISTINCT ON (customer_id)

customer_id, order_date, total

FROM orders

ORDER BY customer_id, order_date DESC;

DISTINCT applies to ALL columns in SELECT list

DISTINCT comes immediately after SELECT keyword

NULL values are treated as equal (multiple NULLs = one unique value)

ORDER BY applied after DISTINCT

Cannot use DISTINCT with aggregate functions directly

Core references in this topic include WHERE, =, LIKE. 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.

LIKE

Pattern-matching operator for wildcard string searches.

name LIKE 'Joh%'

EXISTS

Tests whether a correlated or non-correlated subquery returns at least one row.

WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id)

ANY / ALL

Compares one value against every or at least one value from a subquery result.

salary > ALL (SELECT salary FROM interns)

PRIMARY KEY

Uniquely identifies each row and implicitly requires NOT NULL.

customer_id INT PRIMARY KEY

UNIQUE

Prevents duplicate values in a column or column combination.

email VARCHAR(255) UNIQUE

COUNT

Counts rows or non-NULL values depending on the argument.