FAQ: Manipulation - Constraints

This community-built FAQ covers the “Constraints” exercise from the lesson “Manipulation”.

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This exercise can be found in the following Codecademy content:

Web Development
Data Science

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FAQs on the exercise Constraints

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In the lesson we deleted rows from the celebs table, but after the final exercise, the database schema still lists four rows. Why? Thank you.

If there is a constraint violation from attempting to insert data into a table that doesn’t fit the constraints, will an error message appear or will that data simply not be entered?

This is important because if the data isn’t entered and there is no error message, the developer may not know that some data wasn’t placed into the table.

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I think we removed all rows that had no information on the Twitter handle. If you
SELECT * FROM celebs;
you’ll see that only 1 row is left. We removed all the others because we only added the Twitter handle for Taylor Swift.

At least that’s how I understand it

This part of MySQL Documentation may answer your question.

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can you add just NULL? If so, does NULL allow no information and NOT NULL not allow no information?

Can constraints be added to an already existing table? Or do they have to be defined at table creation?

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Can somebody help me, I didn’t understand the difference between UNIQUE & PRIMARY KEY, thanks.