SELECT Statement

Control exactly which columns appear in your output

Beginner 12 minSELECTcolumnsalias

SELECT: Choosing Your Columns

Every SQL query starts with SELECT — the verb that tells the database which columns to return. You can pick specific fields, create aliases, or compute entirely new columns on the fly. Here's what you can do:

Pick specific columns — SELECT name, price FROM products returns only those two fields. This keeps results focused, improves performance, and avoids accidentally exposing sensitive columns like passwords.

Avoid SELECT * in production — The wildcard * grabs every column. Convenient for quick exploration, but wasteful and risky in real applications where you want tight control over what data gets returned.

Rename with AS (aliasing) — SELECT price AS original_price changes how the column appears in the result without touching the actual table. Useful for creating clear, report-friendly labels in your output.

Compute new columns — SQL evaluates expressions on the fly: SELECT price * 1.1 AS price_with_tax produces a column that doesn't exist in the table. You can use math, string functions, or any valid expression.

Knowledge Canvas

How SELECT Works

The foundation of every SQL query

SELECT picks which columns appear in your output
Column order in SELECT = column order in result
SELECT * returns all columns — exploration only
AS creates aliases that rename output columns

Warnings & Gotchas

Mistakes that catch everyone

SELECT * hides schema changes — broken code waits silently
Aliases don't change the actual table — only the output label
Forgetting commas between column names → syntax error
Column order matters for UNION but not for readability

Power Patterns

Techniques that level up your queries

SELECT DISTINCT removes duplicate rows
Computed columns: price * quantity AS total
String concatenation: first || ' ' || last AS full_name
Conditional output: CASE WHEN ... END AS label

DISTINCTexpressionsCASE

Critical Rules

Memorize these

Always name specific columns in production code
AS aliases are for display — they don't modify data
SELECT runs after FROM, WHERE, GROUP BY, HAVING
Expressions in SELECT can reference any column in scope

Naming & Aliasing

Best practices for readable queries

Use snake_case for aliases: total_revenue, not TotalRevenue
Keep aliases short but descriptive
Alias computed columns always — unnamed columns cause confusion
Wrap aliases in double quotes if they contain spaces or keywords

Interview Angle

What interviewers look for

Can you explain SELECT execution order?
"When would you use SELECT * vs named columns?"
Show awareness of computed columns and CASE
Demonstrate alias discipline in complex queries

Syntax

Syntax Template

1-- Specific columns

2SELECT

3 column1,

4 column2

5FROM table_name;

7-- With alias (renames the output column only)

8SELECT

9 column AS alias_name

10FROM table_name;

12-- All columns + computed expression

13SELECT

14 *,

15 price * 0.9 AS discounted

16FROM products;

SELECT *Selects all columns — convenient for exploration, avoid in production

SELECT col1, col2Selects specific columns by name

AS aliasRenames a column in the output only

ExpressionsYou can use math, string functions, etc. in SELECT

Sample Data

products

10 rows

id	name	category	price	stock
P101	Wireless Mouse	Electronics	24.99	150
P102	Office Desk	Furniture	189	25
P103	LED Desk Lamp	Furniture	45.5	80
P104	USB-C Cable	Electronics	12.99	500
P105	Bluetooth Speaker	Electronics	79	60
P106	Office Chair	Furniture	95	15
P107	Coffee Maker	Appliances	89.99	40
P108	Notebook Set	Stationery	8.5	200
P109	Wireless Headphones	Electronics	159	30
P110	Standing Desk	Furniture	425	8

Worked Example

Show product names and their prices with a 10% discount applied.

SQL

1SELECT

2 name,

3 price AS original_price,

4 ROUND(price * 0.9, 2) AS discounted_price

5FROM products;

We select the product name, show the original price with an alias, and calculate a discounted price using a math expression. ROUND(..., 2) limits the result to 2 decimal places. The discounted_price column does not exist in the table — it is computed on the fly.

Output

5 rows

name	original_price	discounted_price
Wireless Mouse	24.99	22.49
Office Desk	189	170.1
LED Desk Lamp	45.5	40.95
USB-C Cable	12.99	11.69
Bluetooth Speaker	79	71.1

Common Mistakes

❌

Using SELECT * everywhere

SQL

1SELECT

2 *

3FROM products;

Grabbing all columns is tempting but wasteful. It slows queries on large tables, makes results harder to read, and can accidentally expose sensitive data.

Name the columns you actually need. Only use * for quick exploration.

❌

AS is optional, but clarity is not

SQL lets you write SELECT price discounted (without AS). While valid, it is confusing to read and easy to misparse.

Always use AS explicitly: SELECT price AS discounted.

Key Concepts

1SELECT * is for exploration only — name columns in production

2Aliases (AS) affect output labels, never the actual table

3You can SELECT expressions that don't exist as columns

4Column order in SELECT determines column order in the result

Pro Tip

SELECT is the verb of SQL. You cannot retrieve any data without it. Mastering column selection, aliases, and computed expressions gives you complete control over query output.

When to Use

API responses, CSV exports, dashboard feeds, report generation — they all start with choosing the right columns.

Challenge

Solve the problem below

Write a query that shows the name, category, and stock for all products. Rename "stock" to "units_available".

Your Query

Introduction to SQL

WHERE Clause