Difference Between On Clause And Where Clause When Used With Left Join in SQL Server

Introduction

This article aims to show the difference between the On clause and Where clause when used with a left join in SQL Server.

Background

I have been working with SQL Server databases for over 5 years, and I was unaware of the difference between the On clause and Where clause when used with left join. When I asked our DBA about this, he said there was a difference, but he could not explain the difference. Then I started exploring this topic and got some interesting observations of these two clauses when used with left join. So today, I decided to share my knowledge so that everyone could benefit from my work.

Using the code

Today I will explain the difference between the On and Where clauses when used with left join in SQL Server. When the On clause is used in an outer join, the On clause will have no effect on the outer table, all rows from the outer table will be returned, and the On clause determines which rows of the subordinate table join to the outer table.

Rows of the outer table that do not meet the condition specified in the On clause in the join are extended with null values for subordinate columns (columns of the subordinate table), whereas the Where clause filters the rows that were returned to the final output. It's difficult to understand this from the above definition, so let's try to understand this difference with an example. Suppose we have two tables, Departments (deptId, deptName) and Employees (empID, DeptID, EmpName, Salary), and deptiD is the foreign key of the department table.

An employee can have only one department, whereas a department can have many employees.

We have this sample data in the Department table:

DeptId	DeptName
1	HR
2	Payroll
3	Admin
4	Marketing
5	HR & Accounts

And here is the sample data for the Employees table:

EmpId	EmpName	DeptId	EmpSalary
1	John	1	5000.00
2	Albert	1	4500.00
3	Crain	2	6000.00
4	Micheal	2	5000.00
5	David	NULL	34.00
6	Kelly	NULL	457.00
7	Rudy	1	879.00
8	Smith	2	7878.00
9	Karsen	5	878.00
10	Stringer	5	345.00
11	Cherly	NULL	NULL

Case 1

If we are asked to show all the employees and their relevant departments, we will write a query like this.

select * from employees e 
<p>left join departments d on e.deptid = d.deptid

And the result of this query is:

EmpId	EmpName	DeptId	EmpSalary	DeptId	DeptName
1	John	1	5000.00	1	HR
2	Albert	1	4500.00	1	HR
3	Crain	2	6000.00	2	Payroll
4	Michael	2	5000.00	2	Payroll
5	David	NULL	34.00	NULL	NULL
6	Kelly	NULL	457.00	NULL	NULL
7	Rudy	1	879.00	1	HR
8	Smith	2	7878.00	2	Payroll
9	Karsen	5	878.00	5	HR & Accounts
10	Stringer	5	345.00	5	HR & Accounts
11	Cheryl	NULL	NULL	NULL	NULL

Here you can see that all rows from the Employees table are listed along with their department if they have a department; otherwise, deptid and deptname are null.

Case 2

Let's say we are asked to show the list of all employees and the department name of only those employees with an "HR" or "HR & Accounts" department; then, we would write a query like this.

select * from employees e
left join departments d on e.deptid = d.deptid 
and ( d.deptname = 'HR' or d.deptname = 'HR & Accounts')

and the result of this query is

EmpId	EmpName	DeptId	EmpSalary	DeptId	DeptName
1	John	1	5000.00	1	HR
2	Albert	1	4500.00	1	HR
3	Crain	2	6000.00	NULL	NULL
4	Michael	2	5000.00	NULL	NULL
5	David	NULL	34.00	NULL	NULL
6	Kelly	NULL	457.00	NULL	NULL
7	Rudy	1	879.00	1	HR
8	Smith	2	7878.00	NULL	NULL
9	Karsen	5	878.00	5	HR & Accounts
10	Stringer	5	345.00	5	HR & Accounts
11	Cheryl	NULL	NULL	NULL	NULL

You can notice here that only the same number of rows are returned as in query 1, but here the department of only those employees listed whose department name is "HR" or "HR & Accounts." As you can see, "Crain"," Micheal", and "Smith" have department "Payroll" (see table 1), but it's not listed in the above result set because here we applied the filter in the "On clause"; hence only those rows become part of the join that has department name "HR" or "HR & Employee", and as a result, all other employees who don't have department name "HR" and "HR & Accounts" show their department name as null. This is an example of the On clause in a left join.

Case 3

What if we are asked to show only employees with their department name "HR" or "HR & Accounts"? We would write a query like this.

select * from employees e
left join departments d on e.deptid = d.deptid
where ( d.deptname = 'HR' or d.deptname = 'HR & Accounts')

And the result of this query is.

EmpId	EmpName	DeptId	EmpSalary	DeptId	DeptName
1	John	1	5000.00	1	HR
2	Albert	1	4500.00	1	HR
3	Rudy	1	879.00	1	HR
4	Karsen	5	878.00	5	HR & Accounts
5	Stringer	5	345.00	5	HR & Accounts

This is an example of the Where clause in the left join. You can see here that only those listed employees have their department "HR" or "HR & Accounts," so we did the same query as query 1. You can check the result of query 1 in table 1; we just applied a filter in the Where clause to filter the rows and to return only those employees with their department name as "HR" or "HR & Accounts".

Now you have seen all three cases and can see how these differ from each other. This was an example of a (1:1) one-to-one table, as one employee can have only one department. Let's execute the above 3 cases with (1:M), a one-to-many table relationship where one department can have multiple employees (or more than one employee can be in one department).

Case 4

Suppose we require all departments and the employees listed in these departments; then we would write a query like this.

select * from departments d
left join employees e on e.deptId = d.deptId

You can see in the selectSelect result that all departments are listed, even those without employees, as "Admin" and "Marketing."

The result will look like this.

DeptId	DeptName	EmpId	EmpName	DeptId	EmpSalary
1	HR	1	John	1	5000.00
1	HR	2	Albert	1	4500.00
1	HR	7	Rudy	1	879.00
2	Payroll	3	Crain	2	6000.00
2	Payroll	4	Michael	2	5000.00
2	Payroll	8	Smith	2	7878.00
3	Admin	NULL	NULL	NULL	NULL
4	Marketing	NULL	NULL	NULL	NULL
5	HR & Accounts	9	Karsen	5	878.00
5	HR & Accounts	10	Stringer	5	345.00

You can see in the result that all departments are listed, even those without employees, as "Admin" and "Marketing."

Case 5

But what if we are asked to show all departments and only those employees with the department name "HR " or "HR & Accounting"? Then we would write a query like.

select * from departments d
left join employees e on e.deptId = d.deptId 
and ( d.deptname = 'HR' or d.deptname = 'HR & Accounts')

It will show the following records.

DeptId	DeptName	EmpId	EmpName	DeptId	EmpSalary
1	HR	1	John	1	5000.00
1	HR	2	Albert	1	4500.00
1	HR	7	Rudy	1	879.00
2	Payroll	NULL	NULL	NULL	NULL
3	admin	NULL	NULL	NULL	NULL
4	Marketing	NULL	NULL	NULL	NULL
5	HR & Accounts	9	Karsen	5	878.00
5	HR & Accounts	10	Stringer	5	345.00

You can see in the result this is the On clause in a left join; it just returns all rows from the Department table and only those rows from the Employee table that have the department "HR" or "HR & Account.". We have employees in the Payroll department (Table 4 as a result of query 4,) but it's not listed in the above result set as we just put an extra condition in the "On the clause" that the department name should be "HR" or "HR & Accounts".

That's why only those rows were part of this, join with department names "HR" and "HR & Accounts." Hence all rows from the Department table were returned,d and only those matching rows from the Employees table with their department as "HR" or "HR & Account.".

Case 6

What if we are asked to show only the "HR" and "HR & Accounts" departments and their relevant employees? We would then write a query like this.

select * from departments d
left join employees e on e.deptId = d.deptId
where ( d.deptname = 'HR' or d.deptname  = 'HR & Accounts')

And the result of the above query would be:

DeptId	DeptName	EmpId	EmpName	DeptId	EmpSalary
1	HR	1	John	1	5000.00
1	HR	2	Albert	1	4500.00
1	HR	7	Rudy	1	879.00
5	HR & Accounts	9	Karsen	5	878.00
5	HR & Accounts	10	Stringer	5	345.00

So what happened here? We just did a left join as in query 4, and you can see the result in table 4. And the,n we apply a filter on the result set to return only those rows with deptNmae as "HR" or "HR&Account.". This is a Where clause in the left join. Now you can see how it differs from the query result, 5 where we put this condition in the "On claus.".

There is no difference between the Where and On clauses when used with the inner join.

Summary

I hope you enjoyed reading the article. I have shown you six different cases to explain the concept. I have attached a script that can be used to create the tables and sample data, and then you can practice all by yourself. I will be waiting for your feedback.

Reference

Joins in SQL Server