SELECT, columns, aliases and expressions

Choosing columns

A table may have twenty columns, but a screen or a report usually needs three. After SELECT you write the columns you want, separated by commas, and after FROM the table. The result has exactly those columns, in that order.

SELECT name, price FROM products;
-- Basmati Rice 5kg | 650
-- Toor Dal 1kg     | 160
-- ... 8 rows in all

Computed columns

An expression is a small calculation, like price * qty. You can put an expression in the SELECT list, and the database works it out for every row. The table itself does not change; the new column exists only in the result.

SELECT order_id, product_id, qty * price AS line_total FROM order_items;
-- 101 | 1 | 1300
-- 101 | 2 | 480
-- ... 12 rows

GST and ROUND

Many goods carry 18% GST, so the price with tax is price * 1.18. Decimals pile up quickly, so round money to two places with ROUND(value, 2).

SELECT name, price, ROUND(price * 1.18, 2) AS price_with_gst FROM products;
-- Toor Dal 1kg  | 160 | 188.8
-- Cotton Kurta  | 899 | 1060.82
-- Basmati Rice 5kg | 650 | 767

Aliases with AS

An alias is a temporary name for a column (or a table) in one query. Write it after AS. It makes results readable and gives computed columns a proper heading instead of "ROUND(price * 1.18, 2)". If the alias has a space, put it in double quotes: AS "price with GST". The word AS is optional, but writing it makes your intent clear.

SELECT without FROM

SELECT 2499 * 0.9 AS sale_price;   -- 2249.1
SELECT 'Asha' || ' ' || 'Verma';    -- Asha Verma

NULL spreads through calculations

NULL means "no value" or "unknown". Any arithmetic with NULL gives NULL. Sneha Patil has no marks, so marks + 5 for her is NULL, not 5. Module 3 shows how to replace NULL with a value using COALESCE.

Why apps avoid SELECT *

• It reads columns nobody needs, which wastes time and network, especially with big text columns.
• When someone adds a column, the app suddenly gets data it did not expect, and code that reads columns by position breaks.
• It can leak private columns, like a password hash, into places they should never reach.
• SELECT * is fine for exploring a table by hand, as you did in module 1.

Common mistakes

• A missing comma: SELECT name price FROM products does not fail. It returns name under the heading "price", because price is read as an alias. Check your commas.
• A comma before FROM: SELECT name, FROM products gives near "FROM": syntax error. Remove the last comma.
• Integer division giving 3 instead of 3.5. Make one number a decimal: / 2.0.
• Forgetting ROUND on money, so a report shows 1060.8200000001. Round at the end of the calculation.