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 containing tables called Employee and Contact_Info with the following records:

Table 1: Employee table

EmpID	Name	City	Age	Salary
1	John	London	25	3000
2	Marry	New York	24	2750
3	Jo	Paris	27	2800
4	Kim	Amsterdam	30	3100
5	Ramesh	New Delhi	28	3000
6	Huang	Beijing	28	2800

Table 2: Contact_Info table

Phone_Number	EmpID	Address	Gender
+1-80XXXXX000	2	XXX, Brooklyn, New York, USA	F
+33-14XXXXX01	3	XXX, Grenelle, Paris, France	M
+31-20XXXXX19	4	XXX, Geuzenveld, Amsterdam, Netherlands	F
+86-10XXXXX458	6	XXX, Yizhuangzhen, Beijing, China	M
+65-67XXXXX4	7	XXX, Yishun, Singapore	M
+81-35XXXXX72	8	XXX, Koto City, Tokyo, Japan	M

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:

EmpID	Name
2	Marry
3	Jo
4	Kim
6	Huang

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

❮ SQL Keywords