SQL JOINS Tutorial for beginners | Practice SQL Queries using JOINS - Part 2

Introduction

This tutorial is designed for beginners to understand various types of SQL JOINs, including FULL OUTER JOIN, CROSS JOIN, NATURAL JOIN, and SELF JOIN. Joins are essential in SQL for combining records from two or more tables based on related columns. Having a solid grasp of these concepts will help you write effective SQL queries.

Step 1: Understanding Joins

Before diving into the new types of joins, it’s important to understand the basic types we covered previously:

• INNER JOIN

  • Returns only the rows that have matching values in both tables.

• LEFT JOIN

  • Returns all rows from the left table and the matched rows from the right table. If no match, NULL values are returned for columns from the right table.

• RIGHT JOIN

  • Returns all rows from the right table and the matched rows from the left table. If no match, NULL values are returned for columns from the left table.

Practical Advice

• Use INNER JOIN when you need only the records that exist in both tables.
• LEFT JOIN is useful when you need all records from the left table regardless of matches in the right table.
• RIGHT JOIN is the opposite of LEFT JOIN; use it when you need all records from the right table.

Step 2: FULL OUTER JOIN

The FULL OUTER JOIN returns all records when there is a match in either left or right table records. If there is no match, NULL values are returned for non-matching rows.

Example SQL Query

SELECT *
FROM table1
FULL OUTER JOIN table2
ON table1.id = table2.id;

Practical Advice

• Use FULL OUTER JOIN to get a comprehensive view of data from both tables, including unmatched records.

Step 3: CROSS JOIN

CROSS JOIN produces the Cartesian product of the two tables, meaning it combines all rows from the first table with all rows from the second table.

Example Scenario

• Use CROSS JOIN when you need to generate all possible combinations of records from two tables.

Example SQL Query

SELECT *
FROM table1
CROSS JOIN table2;

Practical Advice

• Be cautious with CROSS JOIN, as it can create a very large result set if both tables have many records.

Step 4: NATURAL JOIN

NATURAL JOIN automatically joins tables based on columns with the same name and data type.

Key Differences from INNER JOIN

• Unlike INNER JOIN, NATURAL JOIN does not require you to specify the join condition. However, it can lead to ambiguity if there are multiple columns with the same name.

Practical Advice

• It is generally recommended to avoid using NATURAL JOIN due to the potential for unexpected results. Always specify join conditions explicitly with INNER JOIN.

Step 5: SELF JOIN

SELF JOIN is used to join a table to itself to compare rows within the same table.

Real-World Example

• Imagine an employee table where you want to find employees and their managers. You can join the table on the manager ID.

Example SQL Query

SELECT a.name AS Employee, b.name AS Manager
FROM employees a
JOIN employees b ON a.manager_id = b.id;

Practical Advice

• Use SELF JOIN when you need to compare or relate rows within the same table.

Conclusion

Understanding these JOIN types is crucial for effective SQL querying. Here's a quick recap:

• FULL OUTER JOIN returns all records with matched and unmatched rows.
• CROSS JOIN creates all possible combinations of rows from two tables.
• NATURAL JOIN simplifies joining tables based on common columns, but should be used cautiously.
• SELF JOIN allows you to relate records within the same table.

Next Steps:

• Practice writing SQL queries using these joins.
• Explore additional resources for more complex SQL concepts.