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Adam Sawicki
15,967 PointsMany to Many Relationship
Hi,
Why can't we create composite primary key in each table of many to many relationship, instead of creating extra table which keeps only composite PKs?
BR, Adam
1 Answer
Kevin Korte
28,149 PointsBecause you'd inevitably end up with a bunch of null cells in your table. Because of the nature of a many to many, one row in your table may have a relation with 3 others, while the next row might have a relationship with 7 others. You would need to store the primary keys for each relationship. So your table would have to be as big as the row with the most amount of "others" and you wouldn't easily be able to have more than that many relationships, without expanding your database even larger.
Ultimately, it's not as flexible as a schema, and it's not a sustainable model either.
By moving our many to many relationship to a 3rd table, a couple things happen. One, we avoid null rows since we know that each row in that table is just going to be keys to other rows, and two, we no longer have a limit on how many relationships "many" actually is. If we have a row that had 200 hundred other relationships, it wouldn't matter, we'd just have 200 new entries in our many to many table. It's fully sustainable this way.
Hope that makes sense.
Kevin Gates
15,053 PointsKevin Gates
15,053 PointsBest Answer should be awarded to the above.