Foreign key SQL SET NULL

Question

Foreign key SQL SET NULL

Navigation

#1 by (2 votes)

0

Good people, let's go to one more question, I hope you can help me.

I have the following problem:

When I create a foreign key linking to another field in another table I always define it as CASCADE . But when I enter a field (say in this table I have foreign keys attached to it and one of them can be NULL ) I can not successfully register the table for the fact it does not find in the other referenced table value.

I think it's because it's using the CASCADE type and SET NULL would allow this field to exist with the NULL value.

Defining my foreign key as SET NULL will solve my problem?

php mysql sql

asked by anonymous 08.04.2016 / 03:40

1 answer

Can anyone tell me why they are not loading this image (css)? What is the difference between IDLE Python 3.5 Shell and Python 3.5 (CMD)?

score 2 · Answer 1

Names of oxen

The A table has a foreign key to the B table.

The solution

Set the A field to NULL .

The explanation

The CASCATE foreign keys are triggered when changes occur in the referenced table, not referring table. In this case, a CASCATE SET NULL will make the A field become when is deleted from B .

This is a different case. You want records in A to be born without a referrer in B . This has nothing to do with CASCATE , but rather as the definition itself in A .