Today, let's move to the next topic of SQL Roadmap:

Basic SQL Queries: SELECT, WHERE, Filtering 🖥️

What SELECT Does
- SELECT chooses columns
- You decide what data you want to see
- Database returns only those columns

Example Table: customers
customer_id | name | city | signup_date
101 | Rahul | Pune | 2024-01-15
102 | Neha | Mumbai | 2024-02-10
103 | Amit | Delhi | 2024-03-05
104 | Priya | Pune | 2024-04-20

Basic SELECT

SELECT name, city FROM customers;

Output
name | city
Rahul | Pune
Neha | Mumbai
Amit | Delhi
Priya | Pune

What Happens
- Database scans all rows
- Returns name and city columns
- No filtering yet

Why SELECT Matters
- Smaller output
- Faster queries
- Clear analysis

What WHERE Does
- WHERE filters rows
- It answers conditions like who, when, how much

Think Like This
- SELECT decides columns
- WHERE decides rows

Basic WHERE Example

SELECT name, city FROM customers WHERE city = 'Pune';

Output
name | city
Rahul | Pune
Priya | Pune

Common Operators
- = equal
- != not equal
- > greater than
- < less than
- > =, <=

Example

SELECT name, signup_date FROM customers WHERE signup_date >= '2024-03-01';

Output
name | signup_date
Amit | 2024-03-05
Priya | 2024-04-20

Logical Filters
AND
- All conditions must match
Example: SELECT name FROM customers WHERE city = 'Pune' AND signup_date >= '2024-01-01';

Output
name
Rahul
Priya

OR
- Any condition can match
Example: SELECT name FROM customers WHERE city = 'Pune' OR city = 'Mumbai';

Output
name
Rahul
Neha
Priya

IN
- Shortcut for multiple OR conditions
Example: SELECT name FROM customers WHERE city IN ('Pune','Mumbai','Delhi');

Output
name
Rahul
Neha
Amit
Priya

BETWEEN
- Filters within a range
- Inclusive of start and end
Example: SELECT name FROM customers WHERE signup_date BETWEEN '2024-01-01' AND '2024-03-31';

Output
name
Rahul
Neha
Amit

Filtering Numbers
Example table: orders
order_id | customer_id | amount | order_date
1 | 101 | 3000 | 2024-01-16
2 | 102 | 6000 | 2024-02-11
3 | 103 | 4000 | 2024-03-06

SELECT order_id, amount FROM orders WHERE amount > 5000;

Output
order_id | amount
2 | 6000

Filtering Text
- Text values go inside quotes
- Case sensitivity depends on database

Filtering NULL Values
- NULL means missing value
- = NULL does not work
Correct way: SELECT name FROM customers WHERE city IS NULL;

Assume city is NULL for customer_id 103
Output
name
Amit

Exclude NULL: SELECT name FROM customers WHERE city IS NOT NULL;

Output
name
Rahul
Neha
Priya

How SELECT and WHERE Work Together
- FROM picks the table
- WHERE filters rows
- SELECT picks columns
- Result is sent back

Real Business Example
- Manager asks for Pune customers with orders above 5,000
- WHERE applies city and amount filters
- SELECT shows required columns

Assume orders table has customer_id 101 with amount 6000

SELECT name FROM customers WHERE city = 'Pune' AND customer_id IN (SELECT customer_id FROM orders WHERE amount > 5000);

Output
name
Rahul

Common Beginner Mistakes
- Using WHERE before FROM
- Forgetting quotes for text
- Using = NULL
- Writing SELECT * always

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