Two tables that they have foreign keys pointing to each other

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Two tables that they have foreign keys pointing to each other

EamonSQL

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Points: 2307
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February 1, 2018 at 2:50 pm

#343268

Hi,
I have an unusual problem in that I have to remove records from two tables that they have foreign keys pointing to each other.
Silly I know but that's how it's been designed and I can't drop the constraints in order to delete the records.
If I try deleting records from either table I get a foreign key constraint error message.
If there's any advice out there around removing these records without disabling the constraints I would appreciate it.
If this isn't clearly explained please let me know.
Thanks,
Emaon

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Joe Torre SSChampion Points: 10248 More actions · Answer 1

Try doing both in a single transaction:
begin tran delete tbl1 where ... delete tbl2 where... commit;

drew.allen SSC Guru Points: 76985 More actions · Answer 2

Joe Torre - Thursday, February 1, 2018 3:16 PM
Try doing both in a single transaction:

begin tran
delete tbl1 where ...
delete tbl2 where...
commit;

If this doesn't work and one (or both) of the fields is nullable, update the field to NULL, delete the record from the other table, and then delete the original record.

Drew

J. Drew Allen
Business Intelligence Analyst
Philadelphia, PA

Jeffrey Williams SSC Guru Points: 90303 More actions · Answer 3

If you want to delete the rows that are related - you could modify the tables to use ON DELETE CASCADE. However, if you do not want to delete the related rows you are going to have to update the foreign keys to a different value then delete the rows.

That is - on table 1 update the rows with value 'x' to value of 'y' and delete from table2 the rows that have the value 'x'. Then update the values in table2 with value 'a' to value 'b' - then delete from table1 the rows that have value 'a'.

Before doing any of this - I would recommend that you copy the data in both tables so you can put the data back if something doesn't work right.

Jeffrey Williams
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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 4

Joe Torre - Thursday, February 1, 2018 3:16 PM
Try doing both in a single transaction:
begin tran delete tbl1 where ... delete tbl2 where... commit;

Won't work. Constraints are checked at the statement level. The first statement will violate the constraints and fail. The second will then violate the constraints and fail as well.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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EamonSQL SSCrazy Points: 2307 More actions · Answer 5

Hi everyone,

much belated thanks for all your help on this.

I tried various things but the approach that worked for me was "update the field to NULL, delete the record from the other table, and then delete the original record."
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Cheers,
Eamon

RonKyle SSC-Dedicated Points: 31556 More actions · Answer 6

Just for curiosity sake, are you able and allowed you tell what the tables are. In all my design years, I can't recall something like this. I would just like to see it so I can add it to my bag of design tips. Thanks,

Jonathan AC Roberts SSCoach Points: 18355 More actions · Answer 7

Have you tried updating the FK column values to NULL before deleting the rows?

RonKyle SSC-Dedicated Points: 31556 More actions · Answer 8

Have you tried updating the FK column values to NULL before deleting the rows?

He already stated that this was the approach that worked for him.