FAQ: Multiple Tables - Primary Key vs Foreign Key


This community-built FAQ covers the “Primary Key vs Foreign Key” exercise from the lesson “Multiple Tables”.

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This exercise can be found in the following Codecademy content:

Web Development
Data Science

Learn SQL

FAQs on the exercise Primary Key vs Foreign Key

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Why I have to use JOIN and not LEFT JOIN?


Do you understand the difference between (inner) join and left join? Knowing the differences can help

left join selects all rows from the left and table and the matching records from the right table
inner join select all rows which match from both tables

Translating to the exercise at hand: left join would give all classes, even the classes with no students, while inner join while only give the classes with students (at least one student has to be in the class for the row to be returned by the query)


It’s inconvenient when you enter a simple query to see what’s on the tables (if they don’t automatically appear in the viewer), follow that with what the instructions request, are returned an error saying that you didn’t follow the instructions, take 30 minutes trying to figure out what you did wrong without using the hint, only to realize that you were correct on the first attempt yet was told you were incorrect because you still had the initial query up.

You’d think that the system wouuld be optimized in a way that acknowledges your correct input while ignoring irrelevant (albeit correctly typed) queries that came before it.