SQL Joins are one of the most important concepts in database management. A SQL Join allows you to combine data from two or more tables based on related columns. In real-world applications, data is rarely stored on a single table.
For example, an employees table may store personal information, while a departments table holds department details. Using SQL Joins, we can bring this data together for analysis and reporting.
Why Use SQL Joins?
- To avoid data duplication and follow normalization.
- To retrieve meaningful results by combining multiple tables.
- To query relationships like employees and departments, sales and clients, etc.
- To answer business questions that require data from more than one source.
Types of Joins
INNER JOIN in SQL
The INNER JOIN returns only matching rows between two tables based on a condition.
This query will only return employees who are assigned to a department.
Practice: Write a query to return only the projects which are assigned to an employee using INNER JOIN.
LEFT JOIN in SQL
The LEFT JOIN returns all rows from the left table, and matching rows from the right table. If there is no match, NULL is returned.
This query will return all employees and their department (even if not assigned).
Practice: Write a query to return all the projects and their employee using LEFT JOIN.
RIGHT JOIN in SQL
The RIGHT JOIN is the opposite of LEFT JOIN. It returns all rows from the right table and matching rows from the left table.
This query will return all departments and their employees.
Practice: Write a query to return all the employees and their projects using RIGHT JOIN.
FULL JOIN in SQL
The FULL JOIN returns all records when there is a match in either left or right table. This can be also simulated with UNION.
This query will show all employees and all departments (with or without matches).
Practice: Write a query to return all the employees and all the projects using FULL JOIN.
SELF JOIN in SQL
A SELF JOIN is when a table joins with itself. This is often used for hierarchical relationships, like employees and managers.
This query will get employees along with their manager’s name.
Common Mistakes with SQL Joins
- Forgetting the ON condition, leading to a Cartesian product (every row matched with every row).
- Using the wrong join type (e.g., INNER JOIN instead of LEFT JOIN).
- Not handling NULL values correctly in joins.
- Missing aliases, which makes queries hard to read.
FAQs
1. What is the difference between JOIN and UNION in SQL?
JOIN combines columns from two or more tables based on a relationship.
UNION combines rows from multiple SELECT queries into a single result set.
If you want to merge related information side by side, use JOIN. If you want to stack results on top of each other, use UNION.
2. Which SQL Join is the fastest?
In most cases, INNER JOIN is the fastest because it only returns matching rows.
LEFT, RIGHT, and FULL JOINS may be slower as they include unmatched rows and NULL handling. The actual performance also depends on indexes and query optimization.
3. Why do I get duplicate rows when using JOIN in SQL?
Duplicate rows often appear when there are multiple matches in the joined tables. To fix this:
Use DISTINCT to remove duplicates.
Check if the join condition is correct (e.g., avoid joining on non-unique keys).
4. How do NULL values affect SQL Joins?
In INNER JOIN, rows with NULL in the join column are excluded.
In LEFT or RIGHT JOIN, NULL values appear when no match exists in the other table.
FULL JOIN will show NULLs on both sides when no match is found.
5. Can I join more than two tables in SQL?
You are not limited to just joining two tables. SQL allows you to chain multiple JOIN clauses together to combine data from three or more tables in a single query. This is very common in real-world databases where information is normalized across different tables.