A unique key is a set of single or multiple columns of a table that uniquely identifies a record in the table. A Unique Key assures that the value in the column will be unique throughout the table.
Note:
SQL UNIQUE Constraint. The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table. Dec 04, 2015 By random code, I presume you mean a random number or string. Does it have to be unique or just random? A Random Value RAND or RAND( N ) where N is the seed - Generates a random number between 0.0 and 1.0. See this Page on mysql.com A Unique.
In MySQL, we can have multiple âNULLâ values to the UNIQUE key column.
In this article, we will learn what is the UNIQUE KEY Constraint, use of the Unique key, how to create Unique key and what is the difference between Unique key, Primary key, and Foreign key constraint.
Use of Unique key constraint:
How to create a UNIQUE key in MySQL:Create a UNIQUE key while creating a table
We can create a UNIQUE key while creating a table using MySQL UNIQUE constraint.
Syntax:Above MySQL statement will make the âEmployee_IDâ as a UNIQUE key. Example:
W can see the Unique key created under schema panel under Table details in MySQL User interface:
Create a UNIQUE key while altering table
We can create a UNIQUE key while altering the table. However, if the column already contains duplicate data this command will fail.
Syntax:ALTER TABLE Employee Example:How to DROP UNIQUE key constraint in MySQL?
We can drop UNIQUE key constraint using below SQL statement.
Syntax:
Example:Step-1:Step by Step Explanation of creating and dropping UNIQUE key constraint:
Create a table Employee with a UNIQUE key on âEmployee_IDâ column
Insert data into Table Step-2:Since SQL UNIQUE key constraint does not allow a duplicate value, Below insert statement will give an error if we try to insert the same record again.Step-3:
Error message: Error Code: 1062. Duplicate entry â101â for key âEmployee_IDâ
What is the difference between the Primary key and Unique key and Foreign key:
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