YES, FOREIGN KEY column can contain duplicate values. FOREIGN KEY can be used for One to Many Relationship. For Example, we have an order table and the order details table.
Can a foreign key be null or duplicate?
Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them contain NULL values. Indexes aren't automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key. Click to see full answer. Also know, can foreign key be null or duplicate?
Can a foreign key have more than one column?
If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.
What is a unique constraint on a foreign key?
When a UNIQUE constraint is defined on the foreign key, only one row in the child table can reference a given parent key value. This model allows nulls in the foreign key. This model establishes a one-to-one relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key.
What is the difference between composite key and foreign key?
Smart code completion, on-the-fly analysis, quick-fixes, refactorings that work in SQL files, and more. A foreign key can be a composite key made up of multiple columns, but the composite key made up of these columns should be regarded as a single value, as it points at a single row in the table the foreign key “points at”.
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.
Does a foreign key have to have unique values?
Although primary key values must be unique, foreign key values are not required to be unique. For example, a single customer at a bank might have multiple accounts.
Can a Key have duplicate values?
You can define keys which allow duplicate values. However, do not allow duplicates on primary keys as the value of a record's primary key must be unique. When you use duplicate keys, be aware that there is a limit on the number of times you can specify the same value for an individual key.
Can a foreign key be non 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.
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 primary key have duplicate records?
A primary key is a column of table which uniquely identifies each tuple (row) in that table. Primary key enforces integrity constraints to the table. Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values.
Which key does not allow duplication of data?
Preventing Duplicates from Occurring in a Table. You can use a PRIMARY KEY or a UNIQUE Index on a table with the appropriate fields to stop duplicate records. Let us take an example – The following table contains no such index or primary key, so it would allow duplicate records for first_name and last_name.
Can a 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.
What is a foreign key in a table?
That is why it is called a foreign key in the other table. This concept is called referential integrity. If a foreign key is declared as a null field it will violate the the very logic of referential integrity.
What does "null" mean in a foreign key?
That is all an FK is by definition. Null by definition is not a value. Null means that we do not yet know what the value is. Let me give you a real life example.
How to allow nulls in FK?
To allow nulls in an FK generally all you have to do is allow nulls on the field that has the FK. The null value is separate from the idea of it being an FK. Whether it is unique or not unique relates to whether the table has a one-one or a one-many relationship to the parent table.
When both UNIQUE and NOT NULL constraints are defined on the foreign key, only one row in the child table
When both UNIQUE and NOT NULL constraints are defined on the foreign key, only one row in the child table can reference a given parent key value, and because NULL values are not allowed in the foreign key, each row in the child table must explicitly reference a value in the parent key. See this:
Can a child table have the same parent key?
Any number of rows in the child table can reference the same parent key value, so this model establishes a one-to-many relationship between the parent and foreign keys. However, each row in the child table must have a reference to a parent key value; the absence of a value (a null) in the foreign key is not allowed.
What is Foreign Key?
FOREIGN KEY is a key that is used to link two tables together. A FOREIGN KEY is a column or collection of columns in one table that refers to the PRIMARY KEY in another table. The table containing the PRIMARY KEY is called the child table and the table that contains the PRIMARY KEY is called the Parent Table.
Can FOREIGN KEY column contain null value
YES, FOREIGN KEY column can contain null values. Null by definition means not a value. Null means that we do not yet know what the value of the column is. For Example, we have three tables Pictures, Videos and Comments in an application which allows comments on pictures and also on videos.
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. For Example, we have an order table and the order details table. If any customer orders five items, he has one order and five order detail records that contain the same orderID as the FK.
