1 foreign key references 2 primary keys

table1
ID (PRIMARY)
 
table2
ID (PRIMARY)
 
table3
IDpar(FOREIGN KEY)

Code (markup):

I have 2 IDs from 2 different tables (table1 and table2) and they are primary keys. In 3. table (table3) I have IDpar which is foreign key and references ID in table1 and table2.

if I do this:

FOREIGN KEY (IDpar) REFERENCES table1(ID)
        ON UPDATE CASCADE
        ON DELETE CASCADE
FOREIGN KEY (IDpar) REFERENCES table2(ID)
        ON UPDATE CASCADE
        ON DELETE CASCADE

Code (markup):

How will database know which value in column IDpar(table3) comes from ID(table1) or ID(table2). ID from both tables can have the same values because they are auto increment field.

ID(table1)
1
2
 
ID(table2)
1
2
 
IDpar(table3)
1 (ID table1)
1 (ID table2)
2 (ID table1)
2 (ID table2)

Code (markup):

Now if I update ID(table1) from 1 to 3, then in table3 IDpar will be updated everywhere from 1 to 3, right?
That should not happen because one value in IDpar comes from table1 and other from table2.

 

Can you explain what you are trying to accomplish? A one-to-one foreign key can only reference a single table. If you think about it, there's no possible way the database would know what to do if a single column in table 1 was relational to a column in 2 separate tables.