Unique constraint in SQL is used to check whether the sub query has duplicate tuples in it’s result. It returns a boolean value indicating the presence/absence of duplicate tuples. Unique construct returns true only if the sub query has no duplicate tuples, else it return false.

Important Points:

• Evaluates to true on an empty sub query.
• Returns true only if there are unique tuples present as the output of the sub query (two tuples are unique if the value of any attribute of the two tuples differ).
• Returns true if the sub query has two duplicate rows with at least one attribute as NULL.

Syntax:

SELECT table.ID
FROM  table
WHERE UNIQUE (SELECT table2.ID
              FROM table2
              WHERE table.ID = table2.ID);

Note: During the execution, first the outer query is evaluated to obtain table.ID. Following this, the inner sub query is processed which produces a new relation that contains the output of inner query such that table.ID == table2.ID. If every row in the new relation is unique then unique returns true and the corresponding table.ID is added as a tuple in the output relation produced. However, if every row in the new relation is not unique then unique evaluates to false and the corresponding table.ID is not add to the output relation.

Note: The SQL statement without UNIQUE clause can also be written as:

SELECT table.ID
FROM  table
WHERE 1 <= (SELECT count(table2.ID)
              FROM table2
              WHERE table.ID = table2.ID);

Queries

Example 1: Find all the instructors that taught at most one course in the year 2017.
Instructor relation:

SQL Query:

SELECT I.EMPLOYEEID, I.NAME
FROM Instructor as I
WHERE UNIQUE (SELECT Inst.EMPLOYEEID
              FROM Instructor as Inst
              WHERE I.EMPLOYEEID = Inst.EMPLOYEEID
                          and Inst.YEAR = 2017);

Output:

Explanation: In the Instructor relation, only instructors Will and Smith teach a single course during the year 2017. The sub query corresponding to these instructors contains only a single tuple and therefore the unique clause corresponding to these instructors evaluates to true thereby producing these two instructors in the output relation.

Example 2: Find all the courses in Computer Science department that has only a single instructor allotted to that course.
Course relation:

SQL Query:

SELECT C.COURSEID, C.NAME
FROM Course as C
WHERE UNIQUE (SELECT T.INSTRUCTORID
              FROM Course as T
              WHERE T.COURSEID = C.COURSEID 
                          and C.DEPARTMENT = 'Computer Science');

Output: