you know about set operators which is used mathematically. We implement database with set Operators.
UNION
In Oracle, UNION operator is used to combine the result sets of two or more Oracle SELECT statements.
Syntax:
(SELECT expression1, expression2, ... expression_n
FROM table1
WHERE conditions )
UNION
(SELECT expression1, expression2, ... expression_n
FROM table2
WHERE conditions);
Example:
SELECT student_id
FROM students
UNION
SELECT admission_id
FROM admission; 
Note: If you don't want to remove duplicates, use Oracle UNION ALL operator.UNION ALL Operator
In Oracle, the UNION ALL operator is used to combine the result sets of 2 or more SELECT statements.
Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types.
Syntax
SELECT expression1, expression2, ... expression_n
FROM table1
WHERE conditions
UNION ALL
SELECT expression1, expression2, ... expression_n
FROM table2
WHERE conditions; Example:
SELECT student_id
FROM students
UNION ALL
SELECT admission_id
FROM admission; 
INTERSECT
In Oracle, INTERSECT Operator is used to return the results of 2 or more SELECT statement. It picks the common or intersecting records from compound SELECT queries.
Syntax
SELECT expression1, expression2, ... expression_n
FROM table1
WHERE conditions
INTERSECT
SELECT expression1, expression2, ... expression_n
FROM table2
WHERE conditions; Example:
SELECT student_id
FROM students
INTERSECT
SELECT student_id
FROM admission;
MINUS
In Oracle, MINUS operator is used to return all rows in the first SELECT statement that are not returned by the second SELECT statement.
Each SELECT statement has a dataset and the MINUS operator returns all documents from the first dataset and then removes all documents from the second dataset.
Syntax
SELECT expression1, expression2, ... expression_n
FROM table1
WHERE conditions
MINUS
SELECT expression1, expression2, ... expression_n
FROM table2
WHERE conditions; Example:
SELECT dept_id
FROM department
MINUS
SELECT dept_id
FROM admission; 
Thank you for reading. If any Query, Feel free to comment down below.