Lesson 4 of 5 · 15–25 minutes

SELECT, filtering and sorting

🏠The everyday picture

Asking the register clerk: "Show me names and phone numbers of customers in Patna who joined this year, newest first, just the first ten."

📘What it really is

SELECT name, phone, joined_on
FROM customers
WHERE city = 'Patna'
  AND joined_on >= '2026-01-01'
ORDER BY joined_on DESC
LIMIT 10;

SELECT lists the columns; FROM the table; WHERE filters rows; ORDER BY sorts (ASC or DESC); LIMIT caps the number of rows (with OFFSET for pages, Module 47).
Conditions in WHERE:
comparisons: =, <> (not equal), <, >, <=, >=;
combine with AND, OR, NOT, and brackets to be clear;
IN ('Patna', 'Gaya'): one of a list;
BETWEEN 100 AND 500: a range, including both ends;
LIKE 'As%': text patterns (% means any characters); case rules differ between databases (ILIKE in PostgreSQL ignores case);
IS NULL / IS NOT NULL (Module 52).
Aliases rename columns in the result: SELECT name AS customer_name.
SELECT DISTINCT city FROM customers; lists each city once.

🖱️Try it

Write queries for: customers from Patna or Gaya, sorted by name; customers whose name starts with "R"; customers with no email; the five most recently joined customers; the list of distinct cities.

✅Quick check

What's the difference between WHERE and ORDER BY?

Show the answer

WHERE chooses which rows appear; ORDER BY sorts them.

How do you find customers whose email is missing?

Show the answer

WHERE email IS NULL.