For table 1, this INSERT statement will succeed. If you run it 100 times, it will succeed 100 times. insert into referencing_table values (null); The same INSERT statement will fail on table 2. ERROR: null value in column “indexing_table_id” violates not-null constraint DETAIL: Failing row contains (null).
You can’t truncate the table if there is any foreign key referencing it, including disabled constraints. You either need to drop the foreign key constraints or use the DELETE command.
Foreign keys with constraints(in some DB engines) give you data integrity on the low level(level of database). It means you can’t physically create a record that doesn’t fulfill relation. It’s just a way to be more safe.
From the documentation: Multiple ADD, ALTER, DROP, and CHANGE clauses are permitted in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which permits only one of each clause per ALTER TABLE statement. This way you can combine the drop and recreate into one query, and that should … Read more
Check that all your constraints are really spelled out correctly, also check that there’s not any other tables that uses the constraint names FK_userId or FK_blogId
The advantages you have for choosing a character datatype as a primary key field is that you may choose what data it can show. As an example, you could have the email address as the key field for a users table. The eliminates the need for an additional column. Another advantage is if you have … Read more
The first one assigns a user-defined name to the foreign key, the second one will assign a system-generated name to the foreign key. User-defined foreign key names can be useful for subsequent statements like these: ALTER TABLE XTable DROP CONSTRAINT fk_idq; ALTER TABLE XTable ENABLE CONSTRAINT fk_idq; ALTER TABLE XTable DISABLE CONSTRAINT fk_idq; It’s harder … Read more
Yes – you can have a foreign key that references a unique index in another table. CREATE UNIQUE INDEX UX01_YourTable ON dbo.YourTable(SomeUniqueColumn) ALTER TABLE dbo.YourChildTable ADD CONSTRAINT FK_ChildTable_Table FOREIGN KEY(YourFKColumn) REFERENCES dbo.YourTable(SomeUniqueColumn)
In regards to displaying a foreign key field in a form you can use the forms.ModelChoiceField and pass it a queryset. so, forms.py: class DocumentForm(forms.ModelForm): class Meta: model = Document def __init__(self, *args, **kwargs): user = kwargs.pop(‘user’,”) super(DocumentForm, self).__init__(*args, **kwargs) self.fields[‘user_defined_code’]=forms.ModelChoiceField(queryset=UserDefinedCode.objects.filter(owner=user)) views.py: def someview(request): if request.method==’post’: form=DocumentForm(request.POST, user=request.user) if form.is_valid(): selected_user_defined_code = form.cleaned_data.get(‘user_defined_code’) #do stuff … Read more
http://dev.mysql.com/doc/refman/5.0/en/insert-select.html For case1: INSERT INTO TAB_STUDENT(name_student, id_teacher_fk) SELECT ‘Joe The Student’, id_teacher FROM TAB_TEACHER WHERE name_teacher=”Professor Jack” LIMIT 1 For case2 you just have to do 2 separate insert statements