Can foreign key duplicate?

Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.

Can a foreign key have duplicates?

No two rows can have any identical values for a primary key. A foreign key can contain duplicate values. There is no limitation in inserting the values into the table column. While inserting any value in the foreign key table, ensure that the value is present into a column of a primary key.

Can foreign key be duplicate in SQL Server?

Answers. Yes, you are right.

Is a foreign key unique?

A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you must specify the column names of the key explicitly.

Does foreign key need to have same name?

Since each foreign key value must exactly match the corresponding primary key value, the foreign key must contain the same number and data type of columns as the primary key, and these key columns must be in the same order. A foreign key can also have different column names than the primary key.

Python MySQL Tutorial - Foreign Keys

Can foreign key be primary key?

It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship. If you want the same user record to have the possibility of having more than 1 related profile record, go with a separate primary key, otherwise stick with what you have.

Can foreign key have null or duplicate values?

Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.

Does foreign key improve query performance?

Yes it will improve the performance of you db if you are checking integrity using foreign key instead of running many queries for checking the record is exist in database in your program.

Can foreign key be null?

FOREIGN KEY Constraints and NULL Values

Foreign keys allow key values that are all NULL , even if there are no matching PRIMARY or UNIQUE keys. By default (without any NOT NULL or CHECK clauses), the FOREIGN KEY constraint enforces the match none rule for composite foreign keys in the ANSI/ISO standard.

Can foreign key be repeated more than once in a table?

Any column, or set of columns in a table can be defined as a Foreign Key, as long as it is also the primary key or unique key of some table in the database. And there can be any number of FKs defined in a table, they can even overlap.

Why are foreign keys not redundant data?

Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that data remains consistent and that the database contains no redundant data. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.

Can we insert duplicate values in foreign key column?

Can FOREIGN KEY column contain Duplicate values. YES, FOREIGN KEY column can contain duplicate values. FOREIGN KEY can be used for One to Many Relationship.

What happens when you delete a foreign key?

A foreign key with cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table will automatically be deleted. This is called a cascade delete in SQL Server.

Can a candidate key be null?

The column value of a primary key can never be NULL. The columns in a candidate key can have a NULL value.

Is foreign key automatically indexed?

When you define a foreign key constraint in your database table, an index will not be created automatically on the foreign key columns, as in the PRIMARY KEY constraint situation in which a clustered index will be created automatically when defining it.

What problems do foreign keys introduced?

What is not true about foreign key?

Q 26 - Which of the following is not true about a FOREIGN KEY constraint? A - It is a referential integrity constraint. B - It establishes a relationship between a primary key or a unique key in the same table or a different table. C - A foreign key value cannot be null.

Can Composite Key have duplicate values?

It is allowing duplicates. The only possibility which is not allowed is the duplicates of the composite keys(studentID,classID) as like "C. Champagne" said. So we can have combinations like (1,2)(1,3)(2,3) but not (1,2) or (1,3) again.

Can primary key accept null values?

A primary key defines the set of columns that uniquely identifies rows in a table. When you create a primary key constraint, none of the columns included in the primary key can have NULL constraints; that is, they must not permit NULL values.

What is the difference between primary key and foreign key?

A primary key generally focuses on the uniqueness of the table. It assures the value in the specific column is unique. A foreign key is generally used to build a relationship between the two tables. Table allows only one primary key.

Can a foreign key be a composite key?

A composite foreign key is a foreign key that consists of two or more columns. It is important to note that all the columns in a single foreign key must point to the same table.

Can 2 tables have same primary key?

Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. The primary key of one table may also help to identify records in other tables, and be part of the second table's primary key.

Can a table have only foreign key?

There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a "good" table.

Can a foreign key be updated?

No the foreign key is not updated automatically. You need to update the foreign key in the tables in which it is referenced by yourself else it would result in referential integrity exception. For updating the foreign key automatically you may use TRIGGERS.

Can we delete primary key without deleting foreign key?

If you want the department to remain and to be able to do this, you will need to alter the foreign key to include ON DELETE SET NULL. Otherwise, you will have to drop the constraint, perform the delete, and recreate the constraint.