SELECT & WHERE

SELECT retrieves data; WHERE filters rows with conditions.

Operators

• Comparison: =, !=, <, >, <=, >=
• Logical: AND, OR, NOT
• Range: BETWEEN
• List: IN
• Null: IS NULL, IS NOT NULL

Syntax

MYSQL

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SELECT name, email FROM users WHERE age > 25;
SELECT * FROM users WHERE country IN ("USA","UK");
SELECT * FROM users WHERE age BETWEEN 20 AND 30;
SELECT * FROM users WHERE phone IS NOT NULL;
SELECT name AS full_name, price AS product_price FROM products;

SELECT with WHERE

SQL

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-- Select all columns
SELECT * FROM users;

-- Select specific columns
SELECT name, email FROM users;

-- WHERE with comparison
SELECT * FROM users WHERE age >= 18;

-- Multiple conditions
SELECT * FROM users
WHERE city = "Lahore" AND age BETWEEN 20 AND 30;

-- IN and NOT
SELECT * FROM users
WHERE city IN ("Lahore", "Karachi");

SELECT * FROM users
WHERE status != "blocked";

Try It Yourself

Practice

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Exercise

Find users from CA or NY over 18.

Answer

SELECT * FROM users WHERE (state="CA" OR state="NY") AND age > 18;

Quick Quiz

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How to check for NULL?

= NULL

IS NULL

== NULL

Use IS NULL, not = NULL.

Retry Quiz

Interview Questions

WHERE vs HAVING?

WHERE filters before GROUP BY. HAVING filters after aggregation.

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