Why Tables Join: Keys and Relationships

By Ravindra Bagale Last updated: 31 October 2026

Lessons in this chapter:

14.1 Primary Key and Foreign Key in Plain English
14.2 One-to-Many: Customers and Shipments (What One Row Means)
14.3 How to Draw a Tiny Relationship Map Before You Write SQL
14.4 Practice: Match Shipments to Hubs on Paper First

Quiz for this chapter is available on the Learn tracker after deploy.

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Ravi Sir's Cards for this chapter

Primary key and foreign key. Company data is split across tables so each fact is stored once. Keys tell you which rows belong together.

Revision card How do you check whether a column is really a unique key? I compare COUNT(*) with COUNT(DISTINCT key). If the two numbers are equal and the key has no NULLs, ev

How do you check whether a column is really a unique key? I compare COUNT(*) with COUNT(DISTINCT key). If the two numbers are equal and the key has no NULLs, ev

Interview card Orphan key vs missing key. An orphan key has a value that matches no row in the other table. A missing key is NULL, so its current value is unknown.

Orphan key vs missing key. An orphan key has a value that matches no row in the other table. A missing key is NULL, so its current value is unknown.

Comparison card

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