You asked: How do you create a unique key in SQL?

How do I change the unique key in SQL?

To modify a UNIQUE constraint using Transact-SQL, you must first delete the existing UNIQUE constraint and then re-create it with the new definition. For more information, see Delete Unique Constraints and Create Unique Constraints.

How do you create a unique index in SQL?

Right-click the table on which you want to create a unique index and select Design. On the Table Designer menu, select Indexes/Keys. In the Indexes/Keys dialog box, click Add. Select the new index in the Selected Primary/Unique Key or Index text box.

What is unique constraint in SQL?

The UNIQUE constraint uniquely identifies each record in a database table. The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

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.

What qualifies as a unique index?

Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values. … When a unique index is defined for a table, uniqueness is enforced whenever keys are added or changed within the index.

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Is a primary key a unique index?

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.

Comparison Chart:

Index Creates clustered index Creates non-clustered index

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 difference between unique and distinct?

The main difference between unique and distinct is that UNIQUE is a constraint that is used on the input of data and ensures data integrity. While DISTINCT keyword is used when we want to query our results or in other words, output the data.

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