NATURAL JOIN FAQS

1. What is a NATURAL JOIN in Oracle?

A NATURAL JOIN in Oracle is a type of join that automatically joins two tables based on columns with the same name and compatible data types. It eliminates the need for explicitly specifying the join condition in the ON clause.

 

2. How does a NATURAL JOIN work?

• It automatically finds columns with the same name in both tables.
• It performs an equi-join based on those columns and returns the rows that match in both tables.
• It eliminates duplicate columns in the result set, keeping only one instance of each common column.

 

3. What is the syntax for NATURAL JOIN?

The syntax for a NATURAL JOIN is:

SELECT column1, column2, ...

FROM table1

NATURAL JOIN table2;

Where table1 and table2 are the tables being joined, and Oracle will automatically match columns with the same name.

 

4. Can NATURAL JOIN handle multiple matching columns?

Yes! If there are multiple columns with the same name in both tables, the NATURAL JOIN will use all those columns as join keys automatically. However, it will only return one instance of each matching column in the result.

 

5. What happens if the tables have different column names?

If the tables have no columns with the same name, the NATURAL JOIN will return an empty result set because there are no join conditions.

 

6. Can I specify the join condition manually with NATURAL JOIN?

No, you cannot specify the join condition manually in a NATURAL JOIN. The join condition is automatically determined by matching columns with the same name. If you need more control, you should use an INNER JOIN or other join types with explicit conditions.

 

7. What are the advantages of using NATURAL JOIN?

• Simplicity: It simplifies the query, as you don't need to explicitly mention the columns to join on.
• Less Boilerplate: Saves you from writing multiple ON conditions when the tables have several common columns.

 

8. What are the disadvantages of using NATURAL JOIN?

• Lack of Control: Since it automatically joins based on columns with the same name, you have no control over which columns are used in the join condition.
• Potential for Errors: If tables have columns with the same name that are unrelated, it may lead to incorrect results.
• Unpredictability: It's harder to troubleshoot or understand the query if you don’t know which columns are being used in the join.

 

9. Can I use NATURAL JOIN with OUTER JOINs?

Yes, you can combine NATURAL JOIN with OUTER JOIN types, such as LEFT OUTER JOIN or RIGHT OUTER JOIN. The syntax would look like this:

SELECT *

FROM table1

NATURAL LEFT JOIN table2;

However, this still uses the same principle of matching columns by name and data type.

 

10. What happens if the columns have different data types in NATURAL JOIN?

The columns with the same name must have compatible data types for a NATURAL JOIN to work. If the data types are incompatible, Oracle will throw an error.

 

11. Is NATURAL JOIN always the best option?

No, NATURAL JOIN is not always the best choice. It's ideal for simple joins where the tables have a small number of common columns with the same meaning. However, for more complex queries or when you want explicit control over which columns to join, using INNER JOIN or LEFT JOIN with explicit join conditions is recommended.

 

12. Can NATURAL JOIN be used on more than two tables?

Yes, you can perform a NATURAL JOIN on more than two tables. Oracle will join all the tables based on columns that have the same name in each pair of tables involved in the join.

SELECT *

FROM table1

NATURAL JOIN table2

NATURAL JOIN table3;

 

13. What is the difference between INNER JOIN and NATURAL JOIN?

• INNER JOIN requires you to explicitly specify the columns to join on.
• NATURAL JOIN automatically joins based on all columns with the same name in the tables, without you needing to specify them.

Example of INNER JOIN:

SELECT *

FROM table1

INNER JOIN table2

ON table1.column1 = table2.column1;

 

14. Does NATURAL JOIN return duplicate columns?

No, NATURAL JOIN removes duplicate columns. If both tables have a column with the same name, the result will contain only one instance of that column.

 

15. How does NATURAL JOIN differ from using USING clause?

While both NATURAL JOIN and USING join use the same columns for the join, the USING clause allows you to specify the columns explicitly.

For example, with the USING clause:

SELECT *

FROM table1

JOIN table2

USING (common_column);

Here, you explicitly define the join condition, unlike NATURAL JOIN, which automatically determines the columns to use for the join.

 

16. Can I perform a NATURAL JOIN with SELECT DISTINCT?

Yes, you can combine NATURAL JOIN with SELECT DISTINCT to remove duplicate rows from the result set. The DISTINCT keyword will filter out any duplicate rows in the result.

SELECT DISTINCT *

FROM table1

NATURAL JOIN table2;