In the world of SQL Server, there are multiple ways to retrieve data from multiple tables. One common question that arises is whether to use a subquery or a join. In this blog post, we will explore the different options and discuss the best approach.

Recently, a reader named Paulo R. Pereira left a comment on one of our previous articles, highlighting the intelligence of the SQL Server Engine in determining the best plan for a query. Paulo mentioned that using the IN or EXISTS operators is often the best choice, as they can handle different types of relationships efficiently.

Let’s take a look at an example to understand this concept better:

USE AdventureWorks;

-- Use of SOME
SELECT * 
FROM HumanResources.Employee E 
WHERE E.EmployeeID = SOME ( 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeAddress EA 
    UNION ALL 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeDepartmentHistory EA 
);

-- Use of IN
SELECT * 
FROM HumanResources.Employee E 
WHERE E.EmployeeID IN ( 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeAddress EA 
    UNION ALL 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeDepartmentHistory EA 
);

-- Use of EXISTS
SELECT * 
FROM HumanResources.Employee E 
WHERE EXISTS ( 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeAddress EA 
    UNION ALL 
    SELECT EA.EmployeeID 
    FROM HumanResources.EmployeeDepartmentHistory EA 
);

When we analyze the execution plans of these queries, we can see that SQL Server Engine generates different plans for each query. This is because the engine is smart enough to determine the best plan based on the query structure and the relationships between the tables.

Paulo’s comment highlights the fact that the IN or EXISTS operators are often the best choice when dealing with a 1:0..N relationship. These operators can handle such relationships efficiently, ensuring that the query returns the desired results.

On the other hand, if we use a JOIN (Inner Join), the query may return more results than expected if the relationship is 1:0..N. Additionally, using NOT IN or NOT EXISTS operators can result in different query plans compared to a LEFT JOIN (Left Outer Join) due to the use of Left Anti Semi Join or Left Outer Join + Filter.

In conclusion, when deciding between a subquery or a join, it is important to consider the relationship between the tables and the desired results. The SQL Server Engine is intelligent enough to determine the best plan for each query, but understanding the different options can help optimize your queries and improve performance.

Thank you, Paulo, for your valuable contribution to this discussion.