SQL SELECT & FROM — Reading Data from Tables

Query Execution Flow

How SELECT works (logical view)

You name the source with FROM, then project columns with SELECT. The database engine returns matching rows — it does not change stored data.

Sample Data — users Table

users (reference)json

1| user_id | email               | is_active |
2|---------|---------------------|-----------|
3| 1       | [email protected]   | true      |
4| 2       | [email protected]     | false     |
5| 3       | [email protected]   | true      |

SELECT Specific Columns

Read email and active flagjson

1SELECT email, is_active
2FROM users;

Expected outputjson

1| email | is_active |
2|---|---|
3| [email protected] | true |
4| [email protected] | false |
5| [email protected] | true |

SELECT * — All Columns

Every columnjson

1SELECT * FROM users;

When to use *

Fine for quick exploration in a SQL client. In apps and reports, list columns explicitly for clarity and performance.

Column Aliases (AS)

Rename output columnsjson

1SELECT
2  user_id AS id,
3  email AS contact_email
4FROM users;

Expected outputjson

1| id | contact_email |
2|---|---|
3| 1 | [email protected] |
4| 2 | [email protected] |
5| 3 | [email protected] |

DISTINCT — Remove Duplicates

Unique values onlyjson

1SELECT DISTINCT is_active
2FROM users;

Expected outputjson

1| is_active |
2|---|
3| true |
4| false |

Common Mistakes

Mistake	Problem	Fix
Missing FROM	Invalid syntax	Always specify a table: FROM users
SELECT * everywhere	Over-fetching data	List needed columns
Ambiguous names after JOIN	Error once joins are used	Use table aliases: u.email

Practice Prompts

Write a query that returns only user_id and email from users.
Return unique is_active values using DISTINCT.
Alias email as address in the result.

Try These Tools

SQL Query Explainer

Explain what your SELECT does in plain English

Open tool

SQL Formatter

Format SELECT statements for review

Open tool

SQL Validator

Check SELECT syntax before running

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Continue Learning

Relational Databases & Tables

Schema and keys behind FROM

WHERE — Filtering Rows

Narrow results with conditions

Frequently Asked Questions

What is the most basic SQL query?

SELECT column FROM table; reads one or more columns from a named table. SELECT * FROM table; returns every column.

Should I use SELECT * in production?

Avoid SELECT * in application code when possible. Explicit columns reduce network transfer, make schemas clearer, and prevent breaks when columns are added.

What does DISTINCT do?

DISTINCT removes duplicate rows from the result set based on the selected column combination.

What is a column alias?

AS renames a column in the output (e.g. SELECT email AS user_email). Useful for readable reports and APIs.

Does SELECT modify data?

No. SELECT only reads data. INSERT, UPDATE, and DELETE modify data.