SQL Inner Join on More Than 2 Tables and Aggregate Function

As a technical blogger, I have seen numerous questions from developers who are struggling with complex database queries, particularly when dealing with inner joins and aggregate functions. In this article, we will explore how to perform an inner join on more than two tables and use aggregate functions to group data.

Background

Before diving into the solution, let’s briefly discuss the basics of SQL and inner joins.

SQL (Structured Query Language) is a programming language designed for managing and manipulating data stored in relational database management systems. An inner join is a type of join that returns records that have matching values in both tables.

Inner Join Syntax

The basic syntax for an inner join is as follows:

SELECT column1, column2, ...
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;

In this example, table1 and table2 are the two tables being joined, and column_name is the common column between the two tables.

The Problem

The problem presented in the question is as follows:

• We have three tables: department, course, and student_grade.
• Each table has a primary key (e.g., departmentid, courseid, enrollmentid) that uniquely identifies each record.
• We want to perform an inner join on these tables based on the following conditions:
  • Join department with course on the departmentid column.
  • Then, join the resulting table with student_grade on the courseid column.
• We also want to calculate the distinct count of personids for each department.

The Solution

To solve this problem, we need to use the GROUP BY clause in conjunction with aggregate functions. Here’s how:

SELECT d.departmentid, d.name,
       COUNT(DISTINCT sg.personid) AS person_count
FROM department d
INNER JOIN course c ON c.departmentid = d.departmentid
INNER JOIN student_grade sg ON sg.courseid = c.courseid
GROUP BY d.departmentid, d.name;

In this solution:

• We use the INNER JOIN clause to join department with course, and then join the resulting table with student_grade.
• We select only two columns: d.departmentid and d.name.
• Inside the SELECT clause, we calculate the distinct count of personids using the COUNT(DISTINCT) function.
• Finally, we use the GROUP BY clause to group the results by departmentid and name.

Explanation

The key to solving this problem is understanding how to apply aggregate functions in conjunction with grouping. When you use an aggregate function like COUNT(DISTINCT), it automatically groups the data for you.

In the above solution, the COUNT(DISTINCT) function counts each unique value of personid and returns a single count per group (i.e., per department). By grouping by both departmentid and name, we ensure that each department is included in the results with its corresponding student count.

Conclusion

Performing an inner join on multiple tables and using aggregate functions to calculate distinct counts can be challenging, but it’s a common requirement in many database applications. By following the steps outlined above, you should be able to write effective SQL queries to solve similar problems.

Last modified on 2024-11-06