For an instructor lead, in-depth look at learning SQL click below.
Introduction
In the world of SQL (Structured Query Language), JOINs are powerful tools for retrieving data from multiple tables based on some related columns between them. Among the several types of joins available in SQL, the LEFT JOIN holds significant importance. In this blog post, we will delve into the fundamentals of the LEFT JOIN clause and learn some practical examples of its applications.
What is LEFT JOIN in SQL?
LEFT JOIN, also known as LEFT OUTER JOIN, is a SQL clause that returns all records from the left table (table1), and the matched records from the right table (table2). When there is no match, the result is NULL on the right side.
|
1 2 3 4 5 6 7 |
-- Syntax of LEFT JOIN SELECT table1.column1, table2.column2... FROM table1 LEFT JOIN table2 ON table1.matching_column = table2.matching_column; |
A Simple Illustration of LEFT JOIN
Consider two tables – Employees and Department, where the ‘Employees’ table lists all employees and their department ID, and the ‘Department’ table lists all departments and their IDs. Let’s find all employees and their corresponding department names.
|
1 2 3 4 |
-- Employees Table SELECT * FROM Employees; |
|
1 2 3 4 |
-- Department Table SELECT * FROM Department; |
We can use the LEFT JOIN clause as follows:
|
1 2 3 4 5 6 |
SELECT Employees.Name, Department.DepartmentName FROM Employees LEFT JOIN Department ON Employees.DepartmentID = Department.DepartmentID; |
This statement returns a table that includes a list of all employees, even if they are not associated with a department (with NULL in the DepartmentName column).
Use Cases of LEFT JOIN
LEFT JOIN in SQL becomes crucial in instances where we:
– Require all records from the left table irrespective of any matching records in the right table.
– Need to combine rows from two or more tables based on a related column between them.
– Want to add additional information from one table to another based on a shared attribute.
Conclusion
The power of JOINs in database querying is undeniable, and learning to use them effectively is a critical skill for anyone working with SQL. Through LEFT JOIN, we can retrieve all records from the main table and matched ones from the joined tables – a capability that adds immense flexibility to our data querying tasks.
|
1 |