Question: Can Primary Key Be Null?

Can we have multiple NULL values in unique key?

As per the ANSI, UNIQUE constraint allows multiple NULLs.

But in the SQL Server, it allows only one NULL value.

With the UNIQUE constraint, you cannot insert multiple NULLs..

Can a varchar be a primary key?

5 Answers. It totally depends on the data. There are plenty of perfectly legitimate cases where you might use a VARCHAR primary key, but if there’s even the most remote chance that someone might want to update the column in question at some point in the future, don’t use it as a key.

What is the difference between primary key and unique key?

Primary key will not accept NULL values whereas Unique key can accept one NULL value. A table can have only primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.

Can primary key contain duplicate values?

5 Answers. No, it is not possible in SQL Server to insert records that have duplicate values in the Primary Key. It is not possible to have more than one row in the same table with the same primary key (PK).

Can two foreign keys make a primary key?

A primary key can be used as a foreign key, it doesn’t possess them. There’s no limit to how many times they can be used.

What happens if primary key is null?

A primary key must uniquely identify a record – i.e., each record can be expressed in the terms of “the record which has a key that equals X”. Since null is not equal to any value, it cannot be used as a primary key.

Can a unique key be null?

Unique key constraints are used to ensure that data is not duplicated in two rows in the database. One row in the database is allowed to have null for the value of the unique key constraint. Although a table should have a PK, it need not have any additional unique keys.

Can foreign key be null?

A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts. … A table can have many foreign keys.

Can a candidate key be null?

Primary key can’t contain Null value, but Candidate key value can have one NULL value. … If a column (part of candidate keys) contain one null value, then that column will remain as part of candidate key, but not be considered as primary key.

What’s the difference between primary key and foreign key?

Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index. …

Which key accepts null values?

You can only have one primary key per table, but multiple unique keys. Similarly, a primary key column doesn’t accept null values, while unique key columns can contain one null value each. And finally, the primary key column has a unique clustered index while a unique key column has a unique non-clustered index.

Which field Cannot accept null values?

Which field cannot accept null values? Why? Nulls are used when a value is unknown or missing. The primary key cannot accept nulls, because it is supposed to uniquely identify a given row.

Can Super Key have null value?

These keys are also used to create relationship between tables. Super Key and Candidate key both are used to identify records uniquely in a table. Both keys can have null values.

Can a table have no 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 primary key column be null?

Answer: No. We can’t have a Primary Key column with a NULL value. The reason for the same is very simple, primary key purpose is to uniquely identify records. If two records of a single column have a NULL value, the column values are not considered equal.

Can a primary key be null MySQL?

A primary key is a column or a set of columns that uniquely identifies each row in the table. … A primary key column cannot have NULL values. Any attempt to insert or update NULL to primary key columns will result in an error. Note that MySQL implicitly adds a NOT NULL constraint to primary key columns.

Can Unique Key be a foreign key?

It is possible to reference a UNIQUE constraint in a FOREIGN KEY . You could have a Primary key and an Unique key, and you would like to validate both. Yes, you can reference a column (or columns) governed by either a primary key constraint or a unique constraint.

Can foreign key be duplicate?

Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them to 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.

Can you insert null in unique column?

You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it’s possible to insert rows that appear to be duplicates if one of the values is NULL .

Can a table have 2 primary keys?

A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

Why is my foreign key null?

This is entirely independent of whether the column is referenced in a foreign key constraint. The value in the column is NULL because that’s the value that was assigned when the row was inserted. The value was assigned, whether it was explicitly set, or whether it was derived from the default value for the column.