Left Join - SQL

In SQL, a left join, also known as a left outer join, is a type of join operation used to combine rows from two tables based on a related column, returning all rows from the left table (the first table specified in the JOIN clause) and the matched rows from the right table (the second table specified in the JOIN clause). If there is no match found in the right table, NULL values are included for the columns from the right table.

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column = table2.column;

Where

• columns: Represents the columns to be retrieved from the tables.
• table1 and table2: Specifies the tables to be joined.
• LEFT JOIN: Indicates that all rows from the left table (table1) will be returned, along with matching rows from the right table (table2).
• ON: Specifies the join condition, which establishes the relationship between the tables based on the specified columns.

Consider two tables, "employees" and "departments," where you want to retrieve all employees and their corresponding departments. Some employees may not be assigned to any department.

SELECT employees.name, departments.department_name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;

In this query, the LEFT JOIN ensures that all rows from the "employees" table are included in the result set, even if there is no matching row in the "departments" table. If an employee is not associated with any department, the department-related columns will contain NULL values.

Conclusion

In SQL, a left join is a powerful tool for combining data from multiple tables based on a related column while ensuring that all rows from the primary table are included in the result set. By understanding its syntax and behavior, SQL developers can effectively utilize left joins to handle various data retrieval and analysis tasks.