Day 42: Filtering Data — Fetch Only What Matters!

What Is Filtering?

Open Swiggy and tap the filter button. Veg only. Rating 4.0+. Under 200 rupees. Instantly only matching restaurants appear. You didn't scroll through everything — you set conditions and got exactly what you wanted. SQL WHERE does the exact same thing for your database!

WHERE — Your Database Filter Panel

import sqlite3

conn = sqlite3.connect("rohithbuilds.db")
cursor = conn.cursor()

cursor.execute("SELECT name, score FROM students WHERE score >= 50")
for row in cursor.fetchall():
    print(f"{row[0]} - {row[1]}")

Output: Rohith 87, Sneha 92. Arjun with 45 is blocked by WHERE. The database checked every row, kept only the ones matching your condition, and returned just those. You didn't write a single Python if statement — SQL did all the filtering!

The Filter Operators

WHERE score = 87       -- exact match
WHERE score >= 50      -- greater than or equal
WHERE score != 45      -- not equal
WHERE score > 80       -- greater than
WHERE score < 50       -- less than
WHERE name = 'Rohith'  -- text match needs single quotes

Numbers go bare — no quotes. Strings always need single quotes. Mix them up and SQL either crashes or gives wrong results!

Multiple Filters With AND and OR

-- Both conditions must match
SELECT name, score FROM students WHERE score >= 50 AND score <= 90

-- Either condition can match
SELECT name, score FROM students WHERE score < 50 OR score > 90

AND = both checkboxes must be ticked. OR = either checkbox is enough. Just like Swiggy letting you filter by rating AND price together!

Real World Connection

Zomato showing veg restaurants under 200 rupees — WHERE is_veg = 1 AND price_for_two less than 200. Instagram showing posts from people you follow — WHERE user_id IN your following list. Amazon showing products in stock under 1000 rupees — WHERE price less than 1000 AND in_stock = 1. Every filter panel in every app is a WHERE clause!

Common Mistakes

Mistake 1 — Wrong operator order.

WHERE score => 50   -- WRONG — syntax error!
WHERE score >= 50   -- CORRECT — greater than or equal

Mistake 2 — Missing quotes around text values.

WHERE name = Rohith    -- WRONG — SQL thinks Rohith is a column name!
WHERE name = 'Rohith'  -- CORRECT — single quotes for text always!

Mini Challenge

Quick Quiz

Q: How do you filter for students named Rohith in SQL? A: WHERE name = 'Rohith' — single quotes around text always!

Q: What's the difference between AND and OR in WHERE? A: AND requires both conditions true. OR requires just one to be true!

Q: Is WHERE score => 50 correct? A: No! It's >= not =>. Wrong order causes a syntax error!

Key Takeaways