ORDER BY sorts query results by one or more columns. Results are returned in specified order - ascending (smallest to largest) or descending (largest to smallest).

**ORDER BY Basics:**

- ASC: Ascending order (default) - 1, 2, 3 or A, B, C

- DESC: Descending order - 3, 2, 1 or C, B, A

- Multiple columns: Sort by first column, then second for ties

**Execution Order:**

ORDER BY happens AFTER SELECT, so you can use column aliases.

**NULL Handling:**

- Default: NULLs sort first (ASC) or last (DESC)

- NULLS FIRST: Force NULLs to beginning

- NULLS LAST: Force NULLs to end

**Performance:**

- Sorting is expensive on large datasets

- Indexes on ORDER BY columns enable fast sorting

- Without index: Full sort in memory (slow)

- With index: Read in sorted order (fast)

**Sort Priority:**

When sorting by multiple columns, first column has highest priority.

Example: ORDER BY department, salary DESC

- First sort by department (A-Z)

- Within each department, sort by salary (high to low)

ORDER BY is essential for user interfaces - users expect sorted lists. Critical for data analysts (reports), backend developers (APIs), and BI tools (dashboards). Sorting large datasets without indexes can cause 10-100x slowdowns. Understanding sort optimization is key to scalable applications.

Every application needs sorted data - product listings by price, customer lists by name, order history by date, leaderboards by score. ORDER BY is used in pagination, top-N queries, and report generation. Understanding sort performance prevents slow queries on large datasets.