In Oracle, the UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It is different from UNION operator in a way that it does not remove duplicate rows between the various SELECT statements. It returns all of the rows

Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types

Difference between UNION and UNION ALL operators

UNION operator removes duplicate rows while UNION ALL operator does not remove duplicate rows

Syntax

SELECT expression1, expression2, ... expression_n  
FROM table1  
WHERE conditions  
UNION ALL  
SELECT expression1, expression2, ... expression_n  
FROM table2  
WHERE conditions;   

Parameters

1- expression1, expression2, expression_n: It specifies the columns that you want to retrieve. Number of expressions must be same in both SELECT statements

2- table1, table2: It specifies the table name that you want to retrieve records from

3- conditions: It specifies the conditions that must be fulfilled for the records to be selected

Oracle UNION ALL Operator Example

SELECT supplier_id  
FROM suppliers  
UNION ALL  
SELECT supplier_id  
FROM order_details;  

The above example will return the supplier_id multiple times in the result set if the same value appeared in both the supplier_id and order_details table

Output