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Lesson 4 / 8

Module 01 · lesson 4

Expressions and calculations in SELECT

Marketing is launching a "−10% on everything" promotion. You need to show the discounted price without changing the actual prices in the database.

You'll need:Column aliases (AS)(opens in a new tab)

SELECT can do math

In SELECT you can not only take ready-made columns but also compute new ones. Ordinary arithmetic is available: +, -, *, /. The general form of a computed column:

SELECT <calculation> AS <column_name>
FROM <table>;

For example, let's compute the value of each product's whole stock (price × quantity in stock):

SELECT name, price * stock AS stock_value
FROM products;

The new column exists only in the DBMS's response — the data itself doesn't change. Without an alias (AS), Postgres names it unreadably (?column?), so always give it a name.

Calculating the discount

A "−10%" discount means that 90% of the price remains. And 90% is the multiplier 0.9. So the discounted price = the price multiplied by 0.9.

Your turn

Output name, price, and the discounted price under the name price_sale (price * 0.9).

Summary

  • SELECT can do math: arithmetic expressions (+ - * /) create new columns.
  • Computed columns exist only in the response — the table doesn't change.
  • Always give a computation a name with AS.