NATURAL JOIN

A NATURAL JOIN in Oracle is a type of join operation that combines rows from two or more tables based on columns that have the same names and compatible data types in both tables. The key characteristic of a NATURAL JOIN is that it automatically matches columns with the same name and performs the join without explicitly specifying the condition (like in an INNER JOIN or LEFT JOIN).

Here are the key points and details about NATURAL JOIN in Oracle:

 

1. Definition of NATURAL JOIN:

• A NATURAL JOIN automatically finds all columns with the same name in both tables and joins the tables based on those columns.
• It performs an equi-join based on the common columns.

 

2. Syntax:

The basic syntax of the NATURAL JOIN is:

SELECT column1, column2, ...

FROM table1

NATURAL JOIN table2;

• table1 and table2 are the names of the two tables you are joining.
• The NATURAL JOIN will match all columns that exist in both tables with the same name and data type, and then return a result set where these columns are used as the join condition.

3. How NATURAL JOIN works:

• The join automatically detects all columns with the same name in both tables.
• For each pair of columns with the same name, the query will include only one instance of that column in the result set.

For example:

SELECT *

FROM employees

NATURAL JOIN departments;

If both the employees and departments tables have a column named department_id, the NATURAL JOIN will automatically join the two tables based on department_id without needing to specify it explicitly.

4. Key Points:

a. Implicit Join Condition:

• The NATURAL JOIN performs an implicit join based on column names and data types.
• It is not necessary to specify which columns to join on. This can save time, but it also has the risk of joining on unintended columns if there are multiple columns with the same name across the tables.

b. Duplicate Columns:

• A NATURAL JOIN eliminates duplicates from the result set. If the same column appears in both tables, the result will only show one instance of that column, even if it exists in both tables.

c. Column Name and Data Type Compatibility:

• For the NATURAL JOIN to work, the columns must have the same name and compatible data types. If the columns have the same name but different data types, Oracle will throw an error.

d. Automatic Join on Common Columns:

• The columns with the same name are implicitly used in the ON clause, meaning you do not need to specify the join condition manually. The join condition is determined automatically.

e. No Explicit Join Condition:

• Unlike other join types (like INNER JOIN, LEFT JOIN, or RIGHT JOIN), you do not need to provide an explicit ON clause with a NATURAL JOIN. The join is performed using all matching columns.

 

5. Example:

Let’s say you have the following two tables:

employees:

employee_id	employee_name	department_id
1	John Doe	101
2	Jane Smith	102

departments:

department_id	department_name
101	HR
102	IT

Query:

SELECT *

FROM employees

NATURAL JOIN departments;

Result:

employee_id	employee_name	department_id	department_name
1	John Doe	101	HR
2	Jane Smith	102	IT

Here, NATURAL JOIN uses the common column department_id to perform the join.

6. Performance Considerations:

• NATURAL JOIN can sometimes be less explicit and more error-prone if the tables contain multiple columns with the same name.
• It can be harder to maintain if you’re unaware of all the columns that exist in the tables.
• Inner Join (with explicit conditions) is often preferred in large-scale or complex databases to ensure you’re joining on specific columns and reducing potential errors.

7. Differences Between NATURAL JOIN and Other Joins:

a. INNER JOIN:

An INNER JOIN requires explicit specification of the columns to join on. It does not automatically determine which columns to use for the join.

Example:

SELECT *

FROM employees e

INNER JOIN departments d

ON e.department_id = d.department_id;

b. OUTER JOIN:

An outer join (LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN) also requires explicit specification of the join condition, and it includes rows with no match in one or both of the tables.

8. Limitations:

·        No Control over Join Columns: Since a NATURAL JOIN automatically uses all columns with the same name for the join, it could lead to unintended results if the tables have additional columns with the same name but different meanings.

·        Complexity with Large Tables: In larger databases with many tables and common column names, NATURAL JOIN might cause confusion, as you might not be fully aware of which columns are being used to perform the join.

9. Best Practices:

• Use NATURAL JOIN only when you are certain that the tables share common columns with the same name and meaning.
• For more control and clarity, it is often better to use explicit join conditions like INNER JOIN, LEFT JOIN, or RIGHT JOIN where you specify which columns to join.

10. Summary:

The NATURAL JOIN in Oracle simplifies join operations by automatically matching columns with the same name and data type. While it can save time and reduce query length, it requires caution, especially when working with complex or large tables. It is generally recommended to use it when the structure of your tables is well-defined and predictable.