In Oracle, UNION operator is used to combine the result sets of two or more Oracle SELECT statements. It combines the both SELECT statement and removes duplicate rows between them

Each SELECT statement within the UNION operator 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  
SELECT expression1, expression2, ... expression_n  
FROM table2  
WHERE conditions;   

Parameters

1) expression1, expression2, ... expression_n: It specifies the columns that you want to retrieve

2) table1, table2: it specifies the tables from where you retrieve the records

3) conditions: it specifies the conditions that must be fulfilled for the records to be selected

Note: The number of expressions must be same in both of the SELECT statements

Oracle UNION Example: (Fetch single field)

SELECT supplier_id  
FROM suppliers  
UNION  
SELECT supplier_id  
FROM order_details  

output

In this example, supplier_id is defined in both of the table "suppliers" and "order_details". After the UNION, it would appear once in the result set because Oracle UNION operator removes duplicate sets

Note: If you don't want to remove duplicates, use Oracle UNION ALL operator

Oracle UNION Example: (Using ORDER BY)

The Oracle UNION operator can be used with ORDER BY clause to orders the results of the query

SELECT supplier_id, supplier_name  
FROM suppliers  
WHERE supplier_id <= 20  
UNION  
SELECT s_id, s_name  
FROM shopkeepers  
WHERE s_name = 'dhirubhai'  
ORDER BY 1;  

Output

In the above example, result is sorted by supplier_name/s_name in ascending order, as denoted by ORDER BY 1