DISTINCT removes duplicate rows from result set. ALL (default) includes all rows including duplicates. COUNT DISTINCT counts unique values.

**DISTINCT Basics:**

Removes duplicate rows from results.

SELECT DISTINCT column FROM table;

-- Returns only unique values

**ALL (Default):**

Includes all rows including duplicates.

SELECT ALL column FROM table;

-- Same as: SELECT column FROM table;

**COUNT DISTINCT:**

Counts unique values, ignoring duplicates.

SELECT COUNT(DISTINCT customer_id) FROM orders;

-- Counts unique customers, not total orders

**Multiple Columns:**

DISTINCT applies to entire row combination.

SELECT DISTINCT department, job_title FROM employees;

-- Returns unique (department, job_title) pairs

**Performance:**

- DISTINCT requires sorting/hashing (expensive)

- Use indexes on DISTINCT columns

- Consider GROUP BY for complex cases

Fundamental for data analysts and backend engineers. Used in reporting, data cleaning, and ensuring accurate unique counts in analytics.

DISTINCT removes duplicates: count unique customers, list unique products, find distinct categories. Essential for data quality, deduplication, and accurate counting.