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deleting records with foreign key elements - sql

Posted on 2014-03-17
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Last Modified: 2014-03-18
I have 3 tables that have foreign keys linked to each other.  When I opened the first table to delete a row, it would not let me because I have the keys linked to the other tables.  How do I delete records?
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Question by:al4629740
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by:Juan Ocasio
ID: 39935817
You have to first delete the records in the other tables that are linked to that record. This foreign key is ensuring data integrity in your db and the constraint won't allow you to delete the record if records in other tables are associated with it.
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by:Dale Burrell
Dale Burrell earned 1000 total points
ID: 39935865
Or you can set the relationship to be 'cascade delete'.

http://technet.microsoft.com/en-us/library/ms186973(v=sql.105).aspx
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by:al4629740
ID: 39935881
is that the main purpose of the foreign key?

To me, I think I can control that without needing it through Visual Basic.
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by:Dale Burrell
ID: 39935916
You can always control it in code, but it only takes one person to write some invalid code to trash your database.

I would always use keys, but it is the subject of debate. However there is very little bad about them, a very minor performance hit that will probably never notice. Versus lots of things that could go wrong with bad data, maybe not today, but later when someone else doesn't understand your code :)

However if I ever decided not to use keys I would require all inserts/updates/deletes to be carried out via a database stored procedure to prevent coding mistakes.
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Juan Ocasio earned 1000 total points
ID: 39936490
The main purpose of the foreign key is to prevent records from being stranded in the database.

for example, if you have two tables one named employees and one named employeeaddresses, and your records were as follows:

employees:
id      firstname        lastname
1       John                Doe

employeeaddresses:
id          empid           address
1           1                   123 main street
2           1                   456  johns street

without the foreign key constraint, if I deleted employee 1 in the employee's table, I'd have 2 records that are stranded in the employeeaddresses table.  The foreign key constraint will prevent me from deleting the employee record until I delete the addresses associated with that employee.

BTW:  This is a very simplistic example, but it give you an idea of the importance of keeping that database's integrity.

As mentioned above, this can be controlled by code as you can first delete the records from the employeeaddresses table where empid = 1 before deleting from employees where id = 1, however, typically the dba will set these constraints so that when the programmer codes the logic, if he forgot to delete the data from employeeaddesses, the database would raise an error and alert the programmer of the constraint.
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