3. SELECT: choosing columns

SELECT is how you pick what comes back. Everything in this module builds on it.

Pick specific columns

List the columns you want, separated by commas:

SELECT first_name, last_name, email
FROM customer;

Get every column with *

* means "all columns." Handy for a quick look, but in real code prefer naming the columns you need — it's clearer and doesn't break when someone adds a column later.

SELECT *
FROM category;

Compute new columns

SELECT can do math and build values on the fly. This film table stores a daily rental_rate; here's the price for a 3-day rental:

SELECT title, rental_rate, rental_rate * 3
FROM film;

That third column comes back with an ugly auto-generated name. Give it a friendly one with AS — this is called an alias:

SELECT title, rental_rate * 3 AS three_day_price
FROM film;

AS is optional (rental_rate * 3 three_day_price works too), but writing it makes your intent obvious. Use quotes if you want spaces: AS "Three Day Price".

Remove duplicates with DISTINCT

DISTINCT collapses repeated rows into one. The film table has 1000 rows, but only a handful of ratings — DISTINCT shows each once:

SELECT DISTINCT rating
FROM film;

It applies to the whole row you selected, so SELECT DISTINCT rating, rental_rate gives every unique combination of the two.

Try it

Show each film's title and its replacement cost as a column called cost — but only the distinct replacement_cost values, aliased as cost. (Hint: you only need the one column.)

SELECT replacement_cost
FROM film;

SELECT DISTINCT replacement_cost AS cost
FROM film;