# 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:

 STUDENT_ID STUDENT_NAME STUDENT_AGE 1 Joy 10 2 Smiley 13 3 Happy 11 4 James 13 5 Bond 10

Teachers Table:

 TEACHER_ID TEACHER_NAME TEACHER_AGE 101 James 30 102 Bond 25 103 Smith 40

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:

 STUDENT_ID STUDENT_NAME STUDENT_AGE 1 Joy 10 2 Smiley 13 3 Happy 11 4 James 13 5 Bond 10

Teachers Table:

 TEACHER_ID TEACHER_NAME TEACHER_AGE 101 James 30 102 Bond 25 103 Smith 40

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:

 ID AGE 1 10 2 13 3 11 4 13 5 10 101 30 102 25 103 40

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:

 STUDENT_ID STUDENT_NAME STUDENT_AGE 1 Joy 10 2 Smiley 13 3 Happy 11 4 James 13 5 Bond 10

Teachers Table:

 TEACHER_ID TEACHER_NAME TEACHER_AGE 101 James 30 102 Bond 25 103 Smith 40

Query:

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

Output:

 ID AGE 3 11 4 13 5 10 101 30 103 40

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:

 STUDENT_ID STUDENT_NAME AGE 1 Joy 10 2 Smiley 13 3 Happy 11 4 James 13 5 Bond 10

Teachers Table:

 TEACHER_ID TEACHER_NAME AGE 101 James 30 102 Bond 25 103 Smith 40

Query:

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

Output:

 NAME AGE Joy 10 Bond 10 Happy 11 James 13 Smiley 13 Tom 30

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.