Question

I’m trying to normalize a mysql database….

I currently have a table that contains 11 columns for “categories”. The first column is a user_id and the other 10 are category_id_1 – category_id_10. Some rows may only contain a category_id up to category_id_1 and the rest might be NULL.

I then have a table that has 2 columns, user_id and category_id…

What is the best way to transfer all of the data into separate rows in table 2 without adding a row for columns that are NULL in table 1?

thanks!

Answer 1

You can create a single query to do all the work, it just takes a bit of copy and pasting, and adjusting the column name:

INSERT INTO table2
SELECT * FROM (
    SELECT user_id, category_id_1 AS category_id FROM table1
    UNION ALL
    SELECT user_id, category_id_2 FROM table1
    UNION ALL
    SELECT user_id, category_id_3 FROM table1
) AS T
WHERE category_id IS NOT NULL;

Since you only have to do this 10 times, and you can throw the code away when you are finished, I would think that this is the easiest way.