SQL Lesson 5: Keys and relationships

SheetDrill SQL SQL Lesson 5 · Keys and relationships

SQL · Lesson 5

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SQL Lesson 5 done.

You can give a table a primary key, stop duplicates with UNIQUE, link tables with foreign keys, put the foreign key on the right side of a one-to-many, and build a many-to-many with a link table. That's the skeleton of almost every real database.

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What you're learning

A primary key is a row's ID: unique and never empty. A foreign key is a column that holds another table's primary key (customer_id INTEGER REFERENCES customers(id)), and the database refuses rows that point to nothing.

One-to-many (one customer, many cars): the foreign key goes in the "many" table. Many-to-many (jobs and mechanics): neither table can hold the link, so you add a link table with one row per pair and a two-column primary key.

These drills check your design by adding rows: normal data has to fit, and bad data (duplicates, orphans, missing values) has to be refused.