SELECT Queries — Complete Guide
SELECT Queries — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of PostgreSQL Tutorial on Toolliyo Academy.
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PostgreSQL Tutorial · Lesson 11 of 100
SELECT Queries
SQL → Advanced
SQL · 1 — Queries · ~6 min · PostgreSQL — SQL & Queries
What is this?
SELECT retrieves columns from tables or views. You list expressions, optionally alias them, and read result sets row by row. It is read-only unless you add INTO or use SELECT in INSERT.
Why should you care?
Every Flipkart product page, Swiggy ETA screen, and admin dashboard starts as a SELECT against PostgresVerse tables.
See it live — copy this example
Run in pgAdmin or psql.
SELECT
p.name AS product_name,
p.price,
p.specs->>'brand' AS brand
FROM products p
WHERE p.price < 500;What happened?
- The query reads name, price, and a jsonb field from products.
- AS renames columns for the app.
- WHERE filters to affordable items — no write occurs.
Practice next
- Ensure products table exists with sample rows in PostgresVerse.
- Run the SELECT and inspect column headers.
- Add a computed column: price * 1.18 AS price_with_gst.
- Select distinct brands with SELECT DISTINCT specs->>'brand' FROM products;
- Use SELECT COUNT(*) to count rows returned by your filter.
Remember
SELECT projects columns; it does not change data. Aliases improve readability for apps. Always qualify columns when joining tables.
PostgresVerse storefront API
Mobile app calls an endpoint that runs this SELECT for the budget electronics shelf.
Outcome: PM tweaks price filter without redeploying the whole catalog service.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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