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Old 03-03-2005, 10:33 PM   #1
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mysql relationships question

Im very new to mysql, and I'm trying to do a small database to link campers to addresses and specific camps and such. I know about setting primary keys and such, but one point eludes me. How does mysql actually link info in different tables properly? I have a name table, an address table, a camp table, and so on, but when I enter data into each of these tables seperately, how does mysql know what camp a person is in? I have the primary key name_id, and in every other table, I have this as a secondary key, as my manual explains to do, but it never explains how mysql will know what data goes together. Sorry this is so vague, I know so little I don't really know what it is I don't know. I've read lots of material on this, but nothing really covers this simple question.
Old 03-03-2005, 11:21 PM   #2
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This url from the MySQL tutorial gives a brief intro to implicit joins

You could search the MySQL for more details about joins

but just quickly something like

SELECT * FROM name LEFT JOIN address ON name.name_id = address.name_id;

might be what your looking for.

the ON clause tells MySQL to associate the data.
Old 03-04-2005, 12:57 AM   #3
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You should look into foreign keys eg:

This is added to the create statement on child tables of the Names table (eg Addresses table). When you add a row to Addresses, the DB will only allow it if a row exists on the Names table with the same vale for the Name column on both tables.
FOREIGN KEY (col_on_this_table) REFERENCES other_table (col_on_other_table)


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