my question is what the purpose of using relations between tables , when i can achieve that with joining tables with sql joins ?
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3Foreign keys are there to keep data consistent. Nothing to do with joins. – jarlh Aug 03 '16 at 08:54
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http://stackoverflow.com/questions/10183116/what-are-the-advantages-of-defining-a-foreign-key it shows your answer – PaulShovan Aug 03 '16 at 08:55
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Because you have to. That's why – KaeL Aug 03 '16 at 08:59
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Possible duplicate of [What's wrong with foreign keys?](http://stackoverflow.com/questions/83147/whats-wrong-with-foreign-keys) – KaeL Aug 03 '16 at 08:59
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Foreign Keys give another layer of safety. They provide data integrity on data level. They prevent you from adding a record which does not fulfill the relation.
For example, if you have a table users with three Users (IDs 1, 2 and 3) and another table entries, where entries.uid has a relation to users.id, this means you can't add an entry to entries where you would add an entry.uid of 4, if you don't have an user with ID 4 defined.
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