What is unique key in SQL with example?

What is unique key example?

Unique key is a constraint that is used to uniquely identify a tuple in a table. Multiple unique keys can present in a table. NULL values are allowed in case of a unique key.

Difference between Primary Key and Unique Key.

Primary Key Unique Key
present in a table present in a table

What is the use of unique key?

A unique key is used to remove the duplicity of values in a table. However, the usage of a primary key is the same, but there is a difference between both keys. A primary key cannot take a NULL value, but a unique key can have one NULL value as its value.

What is the difference primary key and unique key?

Both Primary key and Unique Key are used to uniquely define of a row in a table. Primary Key creates a clustered index of the column whereas a Unique creates an unclustered index of the column . A Primary Key doesn’t allow NULL value , however a Unique Key does allow one NULL value .

Are all keys unique?

At the most basic definition, “a key is a unique identifier“, so unique key is a pleonasm. Keys that are within their originating entity are unique within that entity. Keys that migrate to another entity may or may not be unique, depending on the design and how they are used in the other table.

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What is meant by unique key?

Unique Key is a column or set of columns that uniquely identify each record in a table. All values will have to be unique in this Key. A unique Key differs from a primary key because it can have only one null value, whereas a primary Key cannot have any null values.

Can foreign key be NULL?

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 we use unique key as primary key?

A primary key column cannot contain NULL values, whereas a unique key can have NULL values, but only one NULL is allowed in a table. A primary key should be unique, but a unique key cannot necessarily be the primary key.

Is a unique key?

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. … There is an automatically defined unique key constraint within a primary key constraint. There may be many unique key constraints for one table, but only one PRIMARY KEY constraint for one table.

Can we use unique key as foreign key?

Yes, why not. 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.

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