WHERE

01The problem

Harbourline has thousands of shipments, and nobody wants to scroll through all of them. Kemi's questions are always about a slice of the data:

• "Show me the big bookings: six containers or more."
• "Which shipments were cancelled?"
• "Which customers don't have an account manager yet?"

Each of these keeps some rows and drops the rest. That's what WHERE is for.

02The concept

WHERE comes after FROM and keeps only the rows where a condition is true.

Text values go in single quotes: 'Delivered'. Numbers don't: 6.

03Example

SELECT shipment_id, booking_date, containers, status
FROM shipments
WHERE containers >= 6
  AND status = 'Delivered';

04Walkthrough

• WHERE containers >= 6 keeps shipments of six containers or more.
• AND status = 'Delivered' also requires the shipment to be delivered. With AND, both conditions must be true.
• With OR, either condition is enough.

When you mix AND and OR, use brackets to say what you mean:

SELECT company_name, city
FROM customers
WHERE (city = 'Lagos' OR city = 'Abuja')
  AND industry = 'Pharmaceuticals';

Without the brackets, AND is applied first and you'd get every Lagos customer plus only the pharmaceutical customers in Abuja.

IN is a tidier way to write several ORs on one column: city IN ('Lagos', 'Abuja').

LIKE matches patterns. % stands for "any characters", so '%Foods%' matches any name containing "Foods".

Finally, NULL means "no value". You can't test for it with =, because NULL = NULL isn't true in SQL. Always use IS NULL or IS NOT NULL:

SELECT company_name, city
FROM customers
WHERE account_manager_id IS NULL;

05Practice

06Challenge

07Check your understanding