UNION ALL in Oracle SQL
Key Concepts
UNION ALL in Oracle SQL is used to combine the result sets of two or more SELECT statements. Unlike UNION, which removes duplicate rows, UNION ALL includes all rows from each query, even if they are duplicates. Understanding the following key concepts is essential for effectively using UNION ALL:
1. Combining Result Sets
UNION ALL combines the result sets of multiple SELECT statements into a single result set. Each SELECT statement must have the same number of columns, and the corresponding columns must have compatible data types.
2. Including Duplicates
Unlike UNION, which removes duplicate rows, UNION ALL includes all rows from each query, including duplicates. This can be useful when you need to retain all data, even if there are overlapping rows.
3. Performance Considerations
UNION ALL is generally faster than UNION because it does not require the additional step of removing duplicates. However, it is important to ensure that the combined result set is not excessively large, as this can impact performance.
Detailed Explanation
1. Combining Result Sets
When using UNION ALL, each SELECT statement must have the same number of columns, and the corresponding columns must have compatible data types. For example, to combine the results of two queries from different tables:
SELECT EmployeeID, FirstName, LastName FROM Employees
UNION ALL
SELECT CustomerID, FirstName, LastName FROM Customers;
2. Including Duplicates
UNION ALL includes all rows from each query, even if they are duplicates. For example, if both queries return the same row, that row will appear twice in the combined result set:
SELECT OrderID, ProductID FROM Orders WHERE OrderDate = '2023-10-01'
UNION ALL
SELECT OrderID, ProductID FROM Orders WHERE OrderDate = '2023-10-01';
3. Performance Considerations
UNION ALL is generally faster than UNION because it does not require the additional step of removing duplicates. However, it is important to ensure that the combined result set is not excessively large, as this can impact performance. For example, combining large tables without proper indexing can lead to slow query execution:
SELECT * FROM LargeTable1
UNION ALL
SELECT * FROM LargeTable2;
Examples and Analogies
Example 1: Combining Sales Data
Imagine you have sales data from two different regions and you want to combine them into a single report. Using UNION ALL will ensure that all sales records are included, even if there are duplicates:
SELECT Region, SalesAmount FROM Sales WHERE Region = 'North'
UNION ALL
SELECT Region, SalesAmount FROM Sales WHERE Region = 'South';
Example 2: Merging Customer Lists
Suppose you have two customer lists from different sources and you want to merge them into a single list. UNION ALL will include all customers, even if they appear in both lists:
SELECT CustomerID, FirstName, LastName FROM Customers1
UNION ALL
SELECT CustomerID, FirstName, LastName FROM Customers2;
Example 3: Combining Inventory Records
If you have inventory records from two different warehouses and you want to combine them into a single inventory report, UNION ALL will ensure that all inventory records are included:
SELECT ProductID, Quantity FROM Inventory WHERE Warehouse = 'A'
UNION ALL
SELECT ProductID, Quantity FROM Inventory WHERE Warehouse = 'B';