WebWhen you create a foreign key, the key that you "point to" in the other table must be a UNIQUE or PRIMARY KEY constraint. You cannot establish a foreign key that points to a column that allow duplicate values. It would be very hard to imagine how the data should "act" if you update one of the duplicate values in the other table (for instance). WebIn your case, a value would be a foreign key that references a primary key elsewhere. A null would indicate the absence of a value, just as it always does. At the next level of abstraction, foreign keys represent relationships. Nullable foreign keys represent optional relationships. No problem. – Walter Mitty Dec 24, 2012 at 16:12 Add a comment
sql - Foreign Key Used in Composite Primary Key - Stack Overflow
WebJun 10, 2012 · Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2024, such a … WebApr 14, 2024 · 290 views, 10 likes, 0 loves, 1 comments, 0 shares, Facebook Watch Videos from Loop PNG: TVWAN News Live 6pm Friday, 14th April 2024 forming byzantium hoi4
sql - can we have a foreign key which is not a primary key in any …
WebAug 31, 2015 · In a table, you can in fact set any column as its primary key. So it could be integer, double, string, etc. Even though nowadays, we mostly use either integers or, more recently, strings as primary key in a table. Since the foreign key is pointing to another table's primary key, this is why you need to specify the foreign key's datatype. WebSep 23, 2010 · Assigning Primary Key And Foreign key to the same column in a Table: create table a1 ( id1 int not null primary key ); insert into a1 values (1), (2), (3), (4); create table a2 ( id1 int not null primary key foreign key references a1 (id1) ); insert into a2 values (1), (2), (3); Share Improve this answer Follow edited Nov 6, 2024 at 0:13 WebThe primary key column value can never be NULL. The foreign key column can accept a NULL value. Count: A table can have only one primary key. A table can have more than one foreign key. Duplication: The primary key is a unique attribute; therefore, it cannot stores duplicate values in relation. We can store duplicate values in the foreign key ... forming bread loaves