UNION in Oracle

ORACLE UNION
To combine the output sets of two or more Oracle SELECT statements, the Oracle UNION operator is used. During the combining process, the UNION operator removes the duplicate rows between the SELECT statements’ results.

There are however two mandatory conditions for using the UNION operator in Oracle.

• Each SELECT statement must have the same number of expressions.
• Each corresponding expression in the different SELECT statement should be of the same data type.

Syntax:

SELECT expr_1, expr_2, ... expr_n  
FROM table1  
WHERE conditions  
UNION  
SELECT expr_1, expr_2, ... expr_n  
FROM table2  
WHERE conditions;   

Parameters:
expr_1, expr_2, … expr_n: It is used to specify the columns of the table which needs to be retrieved.
table1, table2: It is used to specify the name of the tables from which the records need to be retrieved.
conditions: It is used to specify the conditions to be strictly followed for selection.

Example 1: Fetching single field from two tables.
Students Table:

Teachers Table:

Query:

SELECT student_id  as ID
FROM students
UNION  
SELECT teacher_id  as ID
FROM teachers;

Output:

ID
1
2
3
4
5
101
102
103

Explanation:
The ‘students’ and the ‘teachers’ are already existing tables. After the UNION, a combination of student_id and teacher_id would appear, both being the columns of the same data type but of different tables. During this process, the duplicate sets will be removed.

Example 2: Fetching multiple fields from two tables.
Students Table:

Teachers Table:

Query:

SELECT student_id as ID, student_age as AGE
FROM students
UNION  
SELECT teacher_id as ID, teacher_age as AGE
FROM teachers;

Output:

Explanation:
The ‘students’ and the ‘teachers’ are already existing tables. After the UNION, a group of the combination of student_id and teacher_id would appear along with another group of the combination of student_age and teacher_age. The corresponding column of the two tables is of the same data type. During this process, the duplicate sets will be removed from the corresponding columns’ result.

Example 3: Fetching multiple fields from two tables with conditions.
Students Table:

Teachers Table:

Query:

SELECT student_id as ID, student_age as AGE
FROM students
WHERE student_id > 2  
UNION  
SELECT teacher_id as ID, teacher_age as AGE
FROM teachers
WHERE teacher_age >= 30;

Output:

Explanation:
The ‘students’ and the ‘teachers’ are already existing tables. After the UNION, a group of the combination of student_id and teacher_id would appear along with another group of the combination of student_age and teacher_age. The corresponding column of the two tables is of the same data type. During this process, the duplicate sets will be removed from the corresponding columns’ result. Here, the Union of the teacher_id column will be done only for the rows of the table ‘students’ with student_id greater than 2 and the Union of the student_age column will be done only for the rows of the table ‘teachers’ with teacher_age greater than 30.

Example 4: Fetching multiple fields from two tables with conditions and using ORDER BY clause.
Students Table:

Teachers Table:

Query:

SELECT student_name as NAME, age
FROM students 
UNION  
SELECT teacher_name as NAME, age
FROM teachers
WHERE age >= 30 
ORDER BY age;

Output:

Explanation:
The ‘students’ and the ‘teachers’ are already existing tables. After the UNION, a group of the combination of student_id and teacher_id would appear along with another group of the combination of student_age and teacher_age. The corresponding column of the two tables is of the same data type. During this process, the duplicate sets will be removed from the corresponding columns’ result. Here, the Union of the student_age column will be done only for the rows of the table ‘teachers’ with age greater than 30, while there is no condition for the union of teacher_name and student_name. The result will be finally available in the sorted order in the ascending sequence of the ages.