SQL Interview Question and Answers | Natural Join in SQL

A natural join is a type of SQL join operation that combines rows from two tables based on columns with the same name and data type. Instead of explicitly specifying the columns to join on using an `ON` clause, a natural join automatically matches columns with identical names in both tables.

The general syntax for a natural join is:

```sql
SELECT *
FROM table1
NATURAL JOIN table2;
```

Here's an example of how a natural join works:

Consider two tables, "employees" and "departments," with the following structures:

**employees:**
```
employee_id | first_name | last_name | department_id
1 | John | Smith | 101
2 | Jane | Doe | 102
3 | Bob | Johnson | 101
4 | Alice | Brown | 103
```

**departments:**
```
department_id | department_name
101 | HR
102 | Sales
103 | IT
```

If you perform a natural join between the "employees" and "departments" tables, the database will automatically match rows where the "department_id" column has the same name in both tables, resulting in a merged result set:

```sql
SELECT *
FROM employees
NATURAL JOIN departments;
```

The result of this natural join would look like this:

```
employee_id | first_name | last_name | department_id | department_name
1 | John | Smith | 101 | HR
2 | Jane | Doe | 102 | Sales
3 | Bob | Johnson | 101 | HR
4 | Alice | Brown | 103 | IT
```

**When to Use a Natural Join:**

Natural joins are not commonly used in practice for several reasons:

1. **Implicit Matching:** While they automatically match columns with the same name, natural joins can lead to unexpected and potentially incorrect results if column names change over time, or if columns with the same name exist in the tables but should not be used for joining.

2. **Lack of Clarity:** Natural joins can make queries less clear and self-explanatory. It's often a good practice to explicitly specify the join conditions using an `ON` clause for better readability and to avoid ambiguity.

3. **Compatibility:** Not all database systems support natural joins, or they may behave differently. This can lead to portability issues when moving SQL code between different database systems.

4. **Maintenance:** Natural joins may become problematic when dealing with large and complex database schemas, as they rely on column names rather than explicitly defined relationships.

Instead of using natural joins, it's typically recommended to use explicit `INNER JOIN`, `LEFT JOIN`, `RIGHT JOIN`, or `FULL JOIN` operations with an `ON` clause to define the specific join conditions. This provides more control, clarity, and maintainability in your SQL queries and avoids potential issues related to natural joins.

#sql