SQL UNIQUE Keyword

The SQL UNIQUE keyword is a operator and it is used to check whether the sub-query has any duplicate values in the result. It returns true if the sub-query has no duplicate values, else returns false.

Syntax

The syntax for using UNIQUE operator is given below:

SELECT table1.column
FROM table1
WHERE UNIQUE (
SELECT table2.column
FROM table2
WHERE table1.column = table2.column);

Example:

Consider a database tables called Employee and Contact_Info with the following records:

Table 1: Employee table

Table 2: Contact_Info table

To find all the employees whose contact information is updated in the Contact_Info table, the below mentioned SQL code can be used.

SELECT Employee.EmpID, Employee.Name
FROM Employee
WHERE UNIQUE (
SELECT Contact_Info.column
FROM Contact_Info
WHERE Employee.EmpID = Contact_Info.EmpID);

This will produce the following result:

Please note that, if the sub-query has duplicate values, the query will show an error.