How can we use MySQL UNION operator on datasets?

Basically, MySQL UNION operator is used to combine the result sets of 2 or more SELECT statements. It removes duplicate rows between the various SELECT statements. Each SELECT statement within the UNION operator must have the same number of fields in the result sets within similar data types. Its syntax would be as follows −

Syntax

SELECT expression1, expression2, … expression_n
FROM table
[WHERE conditions]
UNION [DISTINCT]
SELECT expression1, expression2, … expression_n
FROM table
[WHERE conditions]

Here, expression1, expression2, … expression_n are the columns that we wish to retrieve.

Table, are the tables from which we want to retrieve the records.

WHERE condition, it is optional that must be met for the records to be selected.

DISTINCT, it is also optional that removes duplicates from the result set, but the inclusion of the DISTINCT modifier has no impact on the result set of the UNION operator because by default UNION operator already removes duplicates.

Example

In this example, we are two tables namely Student_detail and Student_info having the following data −

mysql> Select * from Student_detail;
+-----------+---------+------------+------------+
| studentid | Name    | Address    | Subject    |
+-----------+---------+------------+------------+
|    101    | YashPal | Amritsar   | History    |
|    105    | Gaurav  | Chandigarh | Literature |
|    130    | Ram     | Jhansi     | Computers  |
|    132    | Shyam   | Chandigarh | Economics  |
|    133    | Mohan   | Delhi      | Computers  |
|    150    | Rajesh  | Jaipur     | Yoga       | 
|    160    | Pradeep | Kochi      | Hindi      |
+-----------+---------+------------+------------+
7 rows in set (0.00 sec)

mysql> Select * from Student_info;
+-----------+-----------+------------+-------------+
| studentid | Name      | Address    | Subject     |
+-----------+-----------+------------+-------------+
|    101    | YashPal   | Amritsar   | History     |
|    105    | Gaurav    | Chandigarh | Literature  |
|    130    | Ram       | Jhansi     | Computers   |
|    132    | Shyam     | Chandigarh | Economics   |
|    133    | Mohan     | Delhi      | Computers   |
|    165    | Abhimanyu | Calcutta   | Electronics |
+-----------+-----------+------------+-------------+
6 rows in set (0.00 sec)

Now, the following query using UNION operator returns all the ‘studentid’ values both the tables.

mysql> Select Studentid FROM student_detail UNION SELECT Studentid FROM student_info;
+-----------+
| Studentid |
+-----------+
|    101    |
|    105    |
|    130    |
|    132    |
|    133    |
|    150    |
|    160    |
|    165    |
+-----------+
8 rows in set (0.00 sec)