DISTINCT, LIMIT, aliases & ordering: the small everyday tools

These four aren't big concepts. They're the small conveniences you'll use in nearly every query. Worth a quick tour so they don't surprise you.

🎯 Explain Like I'm Hired These are your everyday polish tools: DISTINCT removes duplicate rows, LIMIT caps how many rows come back, an alias renames a column or table so the output reads nicely, and ORDER BY can sort by several columns at once. Example: "give me the 5 countries we have customers in" is SELECT DISTINCT country ... LIMIT 5, using distinct to avoid repeating the same country, and limit to stop at five.

DISTINCT: remove duplicate rows

SELECT DISTINCT country FROM customers;   -- each country listed once

DISTINCT collapses repeated rows into one. Useful for "what are all the possible values here?"

LIMIT: cap the number of rows

SELECT * FROM orders ORDER BY placed_at DESC LIMIT 10;   -- 10 most recent

LIMIT 10 returns at most 10 rows. Almost always paired with ORDER BY, otherwise "the first 10" is whatever the database happens to return. OFFSET 10 skips the first 10 (used for paging through results).

Aliases: rename for readability

SELECT SUM(amount) AS total_revenue      -- the column header reads "total_revenue"
FROM orders o                            -- "o" is a short nickname for orders
WHERE o.status = 'delivered';

AS total_revenue gives a computed column a readable name. A table alias (orders o) is a short nickname so you can write o.status instead of orders.status, essential once you're joining several tables.

Ordering by more than one column

SELECT * FROM orders ORDER BY country, placed_at DESC;

This sorts by country first, then within each country sorts newest-first. The database applies the sort columns left to right.

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