What is unique key in SQL?

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. These can also be used as foreign keys for another table.

What is difference between primary 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 .

What is unique key in DB?

In relational database management systems, a unique key is a candidate key that is not the primary key of the relation. All the candidate keys of a relation can uniquely identify the records of the relation, but only one of them is used as the primary key of the relation.

What is unique key in simple words?

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. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.

What is difference between PK and unique key?

Purpose: Primary Key is used to uniquely identify a row but a unique key is used to prevent duplicate values in a column.

What is the Difference between Primary Key and Unique Key - Database Tutorial 14

Can unique key be a primary key?

A primary key should be unique, but a unique key cannot necessarily be the primary key. The primary key by default is a clustered index where data is physically organized in the sequential index.

Can a unique key be NULL?

Key Differences Between Primary key and Unique key:

Primary key will not accept NULL values whereas Unique key can accept NULL values.

Is unique key and candidate key are same?

Candidate key is a set of columns which can uniquely identify the values in a table and can act as a unique key. One of these candidate keys will become the primary key and the rest will become alternate keys. Unique key as the name suggests , is used to uniquely identify a value in a table.

How many unique keys can be defined for a table?

You can define more than one unique key in the table. By default, unique Keys are in non-clustered unique indexes. It constitutes of one or more table columns. The table column can be null, but only one null per column is preferable.

Can primary key have duplicates?

Since both primary key and unique columns do not accept duplicate values, they can be used for uniquely identifying a record in the table. This means that, for each value in the primary or unique key column, only one record will be returned.

How are keys made unique?

Generally keys are created using "codes" where a number represents the height of the peaks on the key. So given say 5 pins with say 10 different height possibilities you're looking at 10^5 unique possibilities. You can then add another dimension of complexity by putting ridges on the sides of the keys.

What is unique data?

Unique data is essentially material that you create rather than data that already exists.

Which is a unique field in table?

Answer: In a relational database, a candidate key uniquely identifies each row of data values in a database table. A candidate key comprises a single column or a set of columns in a single database table.

Can primary key be NULL?

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.

Can we use unique key as foreign key?

No, as unique allow only one null in a column and there cannot be any duplicate data can be inserted in that column which is associated with unique key and on the other hand foreign key allow the duplicate key.

What is unique key in MySQL?

A unique key in MySQL is a single field or combination of fields that ensure all values going to store into the column will be unique. It means a column cannot stores duplicate values.

Can a unique key be composite?

A composite unique key is a unique key made up of a combination of columns. Oracle creates an index on the columns of a unique key, so a composite unique key can contain a maximum of 16 columns.

Can a table have two unique keys?

Yes a table can have n number of unique and foreign keys. 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.

Can a table have 3 primary keys?

A table can only ever have a one primary key. It is not possible to create a table with two different primary keys. You can create a table with two different unique indexes (which are much like a primary key) but only one primary key can exist.

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.

What is difference between unique and distinct in SQL?

The UNIQUE keyword in SQL plays the role of a database constraint; it ensures there are no duplicate values stored in a particular column or a set of columns. On the other hand, the DISTINCT keyword is used in the SELECT statement to fetch distinct rows from a table.

What is a unique attribute?

An attribute for which the value must be unique for each record across the data set. This means that the PDR (Property Description Record) for the primary key attribute must have the mdex-property_IsUnique attribute set to true . For example, for a book, the ISBN number would be a unique attribute.

Is unique key create index?

Provided that the data in each column is unique, you can create both a unique clustered index and multiple unique nonclustered indexes on the same table. Unique indexes ensure the data integrity of the defined columns.

What is unique record?

Unique records are those for which, for the value of a given field, no other records have the same value. Note that in each case we must describe which field(s) we mean when we say unique or duplicate.