Can a foreign key refer to a primary key in the same table?

I just think that the answer is false because the foreign key doesn't have uniqueness property.

But some people said that it can be in case of self joining the table. I am new to SQL. If its true please explain how and why?

Employee table
| e_id | e_name  | e_sala  |  d_id  |
|----  |-------  |-----    |--------|
|  1   |   Tom   |  50K    |    A   |
|  2   | Billy   |  15K    |    A   |
|  3   | Bucky   |  15K    |    B   |


department table
| d_id | d_name  |
|----  |-------  |
|  A   |   XXX   | 
|  B   |   YYY   | 

Now, d_id is foreign key so how it can be a primary key. And explain something about join. What is its use?

I think the question is a bit confusing.

If you mean "can foreign key 'refer' to a primary key in the same table?", the answer is a firm yes as some replied. For example, in an employee table, a row for an employee may have a column for storing manager's employee number where the manager is also an employee and hence will have a row in the table like a row of any other employee.

If you mean "can column(or set of columns) be a primary key as well as a foreign key in the same table?", the answer, in my view, is a no; it seems meaningless. However, the following definition succeeds in SQL Server!

create table t1(c1 int not null primary key foreign key references t1(c1))

But I think it is meaningless to have such a constraint unless somebody comes up with a practical example.

AmanS, in your example d_id in no circumstance can be a primary key in Employee table. A table can have only one primary key. I hope this clears your doubt. d_id is/can be a primary key only in department table.

This may be a good explanation example

CREATE TABLE employees (
id INTEGER NOT NULL PRIMARY KEY,
managerId INTEGER REFERENCES employees(id), 
name VARCHAR(30) NOT NULL
);

INSERT INTO employees(id, managerId, name) VALUES(1, NULL, 'John');
INSERT INTO employees(id, managerId, name) VALUES(2, 1, 'Mike');

-- Explanation: -- In this example. -- John is Mike's manager. Mike does not manage anyone. -- Mike is the only employee who does not manage anyone.

Sure, why not? Let's say you have a Person table, with id, name, age, and parent_id, where parent_id is a foreign key to the same table. You wouldn't need to normalize the Person table to Parent and Child tables, that would be overkill.

Person
| id |  name | age | parent_id |
|----|-------|-----|-----------|
|  1 |   Tom |  50 |      null |
|  2 | Billy |  15 |         1 |

I suppose to maintain consistency, there would need to be at least 1 null value for parent_id, though. The one "alpha male" row.

EDIT: As the comments show, Sam found a good reason not to do this. It seems that in MySQL when you attempt to make edits to the primary key, even if you specify CASCADE ON UPDATE it won’t propagate the edit properly. Although primary keys are (usually) off-limits to editing in production, it is nevertheless a limitation not to be ignored. Thus I change my answer to:- you should probably avoid this practice unless you have pretty tight control over the production system (and can guarantee no one will implement a control that edits the PKs). I haven't tested it outside of MySQL.

Eg: n sub-category level for categories .Below table primary-key id is referred by foreign-key sub_category_id

A good example of using ids of other rows in the same table as foreign keys is nested lists.

Deleting a row that has children (i.e., rows, which refer to parent's id), which also have children (i.e., referencing ids of children) will delete a cascade of rows.

This will save a lot of pain (and a lot of code of what to do with orphans - i.e., rows, that refer to non-existing ids).

Other answers have given clear enough examples of a record referencing another record in the same table.

There are even valid use cases for a record referencing itself in the same table. For example, a point of sale system accepting many tenders may need to know which tender to use for change when the payment is not the exact value of the sale. For many tenders that's the same tender, for others that's domestic cash, for yet other tenders, no form of change is allowed.

All this can be pretty elegantly represented with a single tender attribute which is a foreign key referencing the primary key of the same table, and whose values sometimes match the respective primary key of same record. In this example, the absence of value (also known as NULL value) might be needed to represent an unrelated meaning: this tender can only be used at its full value.

Popular relational database management systems support this use case smoothly.

Take-aways:

1. When inserting a record, the foreign key reference is verified to be present after the insert, rather than before the insert.

2. When inserting multiple records with a single statement, the order in which the records are inserted matters. The constraints are checked for each record separately.

3. Certain other data patterns, such as those involving circular dependences on record level going through two or more tables, cannot be purely inserted at all, or at least not with all the foreign keys enabled, and they have to be established using a combination of inserts and updates (if they are truly necessary).

Adding to the answer by @mysagar the way to do the same in MySQL is demonstrated below -

CREATE TABLE t1 (
    -> c1 INT NOT NULL,
    -> PRIMARY KEY (c1),
    -> CONSTRAINT fk FOREIGN KEY (c1)
    -> REFERENCES t1 (c1)
    -> ON UPDATE RESTRICT
    -> ON DELETE RESTRICT
    -> );

would give error -

ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'fk' in the referenced table 't1'

The correct way to do it is -

CREATE TABLE t1 (
    -> c1 INT NOT NULL,
    -> PRIMARY KEY (c1),
    -> KEY i (c1),
    -> CONSTRAINT fk FOREIGN KEY (c1)
    -> REFERENCES t1 (c1)
    -> ON UPDATE RESTRICT
    -> ON DELETE RESTRICT
    -> );

One practical utility I can think of is a quick-fix to ensure that after a value is entered in the PRIMARY KEY column, it can neither be updated, nor deleted.

For example, over here let's populate table t1 -

INSERT INTO t1 (c1) VALUES
    -> (1),
    -> (2),
    -> (3),
    -> (4),
    -> (5);

SELECT * FROM t1;
+----+
| c1 |
+----+
|  1 |
|  2 |
|  3 |
|  4 |
|  5 |
+----+

Now, let's try updating row1 -

UPDATE t1
    -> SET c1 = 6 WHERE c1 = 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`constraints`.`t1`, CONSTRAINT `fk` FOREIGN KEY (`c1`) REFERENCES `t1` (`c1`) ON DELETE RESTRICT ON UPDATE RESTRICT)

Now, let's try deleting row1 -

DELETE FROM t1
    -> WHERE c1 = 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`constraints`.`t1`, CONSTRAINT `fk` FOREIGN KEY (`c1`) REFERENCES `t1` (`c1`) ON DELETE RESTRICT ON UPDATE RESTRICT)