One common question that often arises when working with SQL Server is the difference between an INNER JOIN and a WHERE clause. While the result set may not differ significantly, there are important distinctions to consider.

When it comes to performance, the SQL Server engine is quite intelligent and will automatically optimize your query in most cases. Therefore, there is usually no significant difference in performance between using an INNER JOIN or a WHERE clause.

However, from a syntax perspective, it is recommended to use INNER JOIN when your query involves more than one table. This is because INNER JOIN is the ANSI valid syntax and promotes better readability and maintainability of your code.

It’s important to note that when working with OUTER JOINs, the question of JOIN vs WHERE becomes irrelevant, as the results can vary significantly. OUTER JOINs introduce a different logic and should be used when the business requirements call for it.

Let’s take a look at a quick example to illustrate the difference between INNER JOIN and WHERE clause.

USE TempDB

-- Creating Sales Rep Table
CREATE TABLE SalesRep (
    ID INT,
    SalesRep VARCHAR(10)
)

INSERT INTO SalesRep (ID, SalesRep)
VALUES (1, 'Pinal'),
       (2, 'Vinod'),
       (3, 'Balmukund'),
       (4, 'Chandra'),
       (5, 'DJ')

-- Creating Sales Orders Table
CREATE TABLE Orders (
    ID INT,
    Amount INT
)

INSERT INTO Orders (ID, Amount)
VALUES (1, 100),
       (1, 200),
       (2, 500),
       (2, 300),
       (3, 600)

-- Select Data
SELECT * FROM SalesRep
SELECT * FROM Orders

-- INNER JOIN
SELECT s.SalesRep, o.Amount
FROM SalesRep s
INNER JOIN Orders o ON s.ID = o.ID

-- WHERE
SELECT s.SalesRep, o.Amount
FROM SalesRep s, Orders o
WHERE s.ID = o.ID

In the above example, both the INNER JOIN and WHERE clause queries return the exact same result set. This is because the JOIN condition and WHERE clause have almost the same condition.

However, it’s important to understand that OUTER JOINs can produce completely different results compared to WHERE clause queries. The logic behind OUTER JOINs is fundamentally different, and they should be used when the business requirements dictate it.

-- OUTER JOIN
SELECT s.SalesRep, o.Amount
FROM SalesRep s
LEFT OUTER JOIN Orders o ON s.ID = o.ID

-- WHERE
SELECT s.SalesRep, o.Amount
FROM SalesRep s, Orders o
WHERE s.ID = o.ID

In the above example, the OUTER JOIN query returns different results compared to the WHERE clause query. This highlights the importance of understanding the business logic and using the appropriate join type.

In conclusion, while the result set may not differ significantly, it is recommended to use INNER JOIN when working with multiple tables for better syntax and readability. Additionally, it’s crucial to understand the differences between INNER JOIN and WHERE clause when working with OUTER JOINs to ensure accurate and meaningful results.

-- Clean up
DROP TABLE SalesRep
DROP TABLE Orders

By following these best practices and understanding the nuances of INNER JOIN and WHERE clause, you can write more efficient and maintainable SQL queries in SQL Server.