Difference Between Nested Subquery, Correlated Subquery and Join Operation

Last Updated : 19 Sep, 2024

Nested subqueries, correlated subqueries, and join operations are common methods for querying data, but they all have different behaviors and serve various purposes. Joins are used to combine two or more different tables based on a common field between them. We can easily retrieve data from multiple tables using joins.

Similarly, Nested and correlated subqueries are a combination of two queries (inner and outer queries) but serve different purposes. All these help in optimizing performance and ensuring accurate data retrieval. So, in this article, we are going to discuss the JOIN Operation and Subquery(nested and correlated) in detail.

JOIN Operation

A join operation is a binary operation used to combine data or rows from two or more tables based on a common field between them. It is often spread across multiple tables in a relational database. INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN are different types of Joins.

Example:

Orders (OrderID , OrderDate);
Customers (CustomerID, CustomerName, ContactName, Country);

Find details of customers who have ordered.

Query

SELECT * from Customers JOIN Orders 
ON Orders.OrderID=Customers.CustomerID;

Output

output

Advantages of Join Operations

Performance: Joins are faster than subqueries and efficient as well especially with large datasets.
Flexibility: it is easier to combine and work with data with multiple tables in one query, allowing us to handle complex data relationships.
Readability: understanding the complex multi-table queries will be easy.

Disadvantages of Join Operations

Complex large queries: it is difficult to read and understand easily if the query is very large with multiple joins in it.
Duplication of Data: duplication of rows may occur sometimes which need to manages carefully.

Subquery

The provided SQL script creates two tables, Customers and Orders, and populates them with sample data. However, the Orders table lacks a foreign key to associate orders with customers. This can lead to difficulties in linking orders to specific customers and querying related information. To resolve this, you should add a CustomerID column to the Orders table and establish a foreign key relationship with the Customers table to ensure data integrity and enable meaningful queries across these tables.

CREATE TABLE Customers(
  CustomerID INT PRIMARY KEY NOT NULL,
  CustomerName VARCHAR(50),
  ContactName VARCHAR(50),
  Country VARCHAR(50);

CREATE TABLE Orders(
OrderID INT ,
OrderDate DATE);

INSERT INTO Orders(OrderID,OrderDate)
VALUES(1,'11/12/2020'),
(2,'14/12/2014'),
(3,'17/1/2019'),
(4,'18/2/2020');

INSERT INTO Customers(CustomerID,CustomerName, ContactName,Country)
VALUES (1,'John','David','USA'),
       (2,'Leo','David','UK'),
       (3,'David','John','USA'),
       (4,'Betty','Leo','UK'),
       (5,'Peter','Peter','UAE');

Customers table:

Customer Table

Orders table:

Order Table

Nested Subquery

A Nested subquery also known as inner query is placed inside another SQL query called as outer query and the Inner query will runs first, and executed only once. Outer query is executed with the result from Inner query. Hence, the Inner query is used in execution of the Outer query. Often used with WHERE , FROM or SELECT clause.

Example:

Orders (OrderID, CustomerID, OrderDate);
Customers (CustomerID, CustomerName, ContactName, Country);

Find details of customers who have ordered.

Query

SELECT * FROM Customers WHERE 
CustomerID IN (SELECT CustomerID FROM Orders);

Output

output

Advantages of Nested Subqueries

Modularity: Nested subqueries allows queries to manage in parts means the queries are broken down into smaller parts and which is easier to manage.
Readability: Breaking a query into smaller parts makes it easier to read and understand, better for simple use cases.
Simplifies Logic: Nested queries sometimes allows to avoid complex joins and filtering conditions.
Independent execution: subquery is executed independently from the outer query, which makes it reusable in different scenarios.

Disadvantages of Nested Queries

Performance: They can be slower, especially in those cases when an inner query runs multiple times for each row in the outer query. This creates the issue in performance.
Limited Optimization: because of complex nested queries many databases struggle to optimize them and which leads to inefficient execution plans.
Complexity in Multi-Level Nesting: As if there are multilevel or deeply nested queries is present then it is harder to manage and debug them.
Larger Data can be less efficient: If there is larger datasets than nested queries can become inefficient, as they may require more resources and time to process than join operations.

Correlated Subquery

In Correlated Query, Outer query executes first and for every Outer query row Inner query is executed. Hence, the Inner query uses values from the Outer query.

Example:

Orders (OrderID , OrderDate);
Customers (CustomerID, CustomerName, ContactName, Country);

Find details of customers who have ordered.

Query

SELECT * FROM Customers where 
EXISTS (SELECT CustomerID FROM Orders 
WHERE Orders.OrderID=Customers.CustomerID);

Output

output

Advantages of Correlated Queries

Row-by-Row Evolution: for making them useful for filtering or matching data between related tables it allows to perform comparisons and checks on a row-by-row basis.
Dynamic Querying: correlated queries allows for dynamic and flexible data retrieval as the subquery is dependent on each row from the outer queries.
Complex Conditions can be Simplified: across related data these subquery can simplified the logic in cases where you need to apply complex conditions.

Disadvantages of Correlated Queries

Performance Issues: If the dataset is large and the subquery is executed for every row processed by the outer query, then the performance gets slow.
Complexity: for beginners to write and maintain them can be complex and difficult task , also to read and understand them easily.
Resource-intensive: These subqueries consumes more processing power and memory as they are complex sometimes and require multiple executions.

Application of Join Operation and Subquery

To understand the difference between Nested Subquery, Correlated Subquery, and Join Operation firstly we have to understand where we use subqueries and where to use joins.

When we want to get data from multiple tables we use join operation. Example: Let’s consider two relations as:

Employee (eId, eName, eSalary, dId);
Department (dId, dName, dLocation);

Now, we have to find employee names and Department name working at London Location. Here, we have to display eName from employee table and dName from Department table. Hence we have to use Join Operation.

SELECT e.eName, d.dName from Employee e, 
Department d 
where e.dId=d.dId and d.dLocation="London";

When we want to get data from one table and the condition is based on another table we can either use Join or Subquery. Now, we have to find employee names working at London Location. Here, we have to display only eName from the employee table hence we can use either Join Operation or Subquery Using Join Operation:

SELECT e.eName from Employee e, Department d 
where e.dId=d.dId and d.dLocation="London";

Using Subquery:

SELECT eName from Employee 
where dId=(SELECT dId from Department where dLocation="London");

After understanding the basic difference between Join and Subqueries, Now we will understand the difference between Nested Subquery, Correlated Subquery, and Join Operation.

Difference between Nested Query, Correlated Query and Join Operation

Parameters	Nested Query	Correlated Query	Join Operation
Definition	In Nested query, a query is written inside another query and the result of the inner query is used in the execution of the outer query.	In Correlated query, a query is nested inside another query and an inner query uses values from the outer query.	The join operation is used to combine data or rows from two or more tables based on a common field between them. INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN are different types of Joins.
Approach	Bottom-up approach i.e. Inner query runs first, and only once. The outer query is executed with result from the Inner query.	Top to Down Approach i.e. Outer query executes first and for every Outer query row Inner query is executed.	It is basically cross product satisfying a condition.
Dependency	Inner query execution is not dependent on Outer query.	Inner query is dependent on the Outer query.	There is no Inner Query or Outer Query. Hence, no dependency is there.
Performance	Performs better than Correlated Query but is slower than Join Operation.	Performs slower than both Nested Query and Join operations as for every outer query inner query is executed.	By using joins we maximize the calculation burden on the database but joins are better optimized by the server so the retrieval time of the query using joins will almost always be faster than that of a subquery.

Conclusion

In conclusion, nested queries are best for simpler tasks where results from one query are used in another, but they can be slower for complex operations. Correlated queries offer dynamic referencing of outer query values, but they are often less efficient due to repeated executions. Join operations are generally the most efficient for combining data from multiple tables, making them ideal for complex and large-scale data retrieval.

Difference between Where and Having Clause in SQL

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Article Tags :

Difference Between Nested Subquery, Correlated Subquery and Join Operation

JOIN Operation

Advantages of Join Operations

Disadvantages of Join Operations

Subquery

Nested Subquery

Advantages of Nested Subqueries

Disadvantages of Nested Queries

Correlated Subquery

Advantages of Correlated Queries

Disadvantages of Correlated Queries

Application of Join Operation and Subquery

Difference between Nested Query, Correlated Query and Join Operation

Conclusion

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