SQL joins are the foundation of database management systems, enabling the combination of data from multiple tables based on relationships between columns. Joins allow efficient data retrieval, which is essential for generating meaningful observations and solving complex business queries.
Understanding SQL join types, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, and NATURAL JOIN, is critical for working with relational databases.
In this article, we will cover the different types of SQL joins, including INNER JOIN, LEFT OUTER JOIN, RIGHT JOIN, FULL JOIN, and NATURAL JOIN. Each join type will be explained with examples, syntax, and practical use cases to help us understand when and how to use these joins effectively.
What is SQL Join?
SQL JOIN clause is used to query and access data from multiple tables by establishing logical relationships between them. It can access data from multiple tables simultaneously using common key values shared across different tables. We can use SQL JOIN with multiple tables. It can also be paired with other clauses, the most popular use will be using JOIN with WHERE clause to filter data retrieval.
Example of SQL JOINS
Consider the two tables, Student and StudentCourse, which share a common column ROLL_NO. Using SQL JOINS, we can combine data from these tables based on their relationship, allowing us to retrieve meaningful information like student details along with their enrolled courses.
Student Table
StudentCourse Table
Both these tables are connected by one common key (column) i.e ROLL_NO. We can perform a JOIN operation using the given SQL query:
Query:
SELECT s.roll_no, s.name, s.address, s.phone, s.age, sc.course_idFROM Student sJOIN StudentCourse sc ON s.roll_no = sc.roll_no;Output
Types of JOIN in SQL
There are many types of Joins in SQL. Depending on the use case, we can use different type of SQL JOIN clause. Below, we explain the most commonly used join types with syntax and examples:
Key Terms
table1: First table.
table2: Second table
matching_column: Column common to both the tables.
Note: We can also use RIGHT OUTER JOIN instead of RIGHT JOIN, both are the same.
RIGHT JOIN Example
In this example, the RIGHT JOIN retrieves all rows from the StudentCourse table and the matching rows from the Student table based on the ROLL_NO column.
Query:
SELECT Student.NAME,StudentCourse.COURSE_ID FROM StudentRIGHT JOIN StudentCourse ON StudentCourse.ROLL_NO = Student.ROLL_NO;
OUTPUT
4. SQL FULL JOIN
FULL JOIN creates the result-set by combining results of both LEFT JOIN and RIGHT JOIN. The result-set will contain all the rows from both tables. For the rows for which there is no matching, the result-set will contain NULL values.
SYNTAX
SELECT table1.column1,table1.column2,table2.column1,....FROM table1 FULL JOIN table2ON table1.matching_column = table2.matching_column;Key Terms
table1: First table.
table2: Second table
matching_column: Column common to both the tables.
FULL JOIN Example
This example demonstrates the use of a FULL JOIN, which combines the results of both LEFT JOIN and RIGHT JOIN. The query retrieves all rows from the Student and StudentCourse tables. If a record in one table does not have a matching record in the other table, the result set will include that record with NULL values for the missing fields
Query:
SELECT Student.NAME,StudentCourse.COURSE_ID FROM StudentFULL JOIN StudentCourse ON StudentCourse.ROLL_NO = Student.ROLL_NO;
5. SQL Natural Join (?)
Natural join can join tables based on the common columns in the tables being joined. A natural join returns all rows by matching values in common columns having same name and data type of columns and that column should be present in both tables.
Both table must have at least one common column with same column name and same data type.
The two table are joined using Cross join.
DBMS will look for a common column with same name and data type. Tuples having exactly same values in common columns are kept in result.
Natural join Example
Look at the two tables below- Employee and Department
Problem: Find all Employees and their respective departments.
Solution Query: (Employee) ? (Department)
Conclusion
SQL joins are essential tools for anyone working with relational databases. Understanding the different types of joins in SQL, like INNER JOIN, LEFT OUTER JOIN, RIGHT JOIN, and FULL JOIN, allows us to combine and query data effectively. With the examples and syntax covered here, we should feel confident applying these SQL join types to our data to retrieve meaningful observations and manage complex queries with ease. Use these SQL join techniques to streamline our data handling and enhance our SQL skills.
FAQs
What are the 4 types of join SQL?
In SQL, the four main types of joins are: INNER JOINLEFT JOINRIGHT JOINFULL JOIN
What is a join in SQL?
A join in SQL is a relational database operation used to combine rows from two or more tables based on a related column between them.
What is the difference between INNER JOIN and LEFT JOIN?
The main difference between INNER JOIN and LEFT JOIN lies in how they handle unmatched rows. INNER JOIN focuses on matched rows only, while LEFT JOIN includes all rows from the left table, with NULLs where there is no match in the right table.
