Difference between Nested Subquery, Correlated Subquery and Join Operation

Join Operation :
Join operation is a binary operation used to combine data or rows from two or more tables based on a common field between them. INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN are different types of Joins.

Example –

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

Find details of customers who have ordered.

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

Subquery
When a query is included inside another query, the Outer query is known as Main Query, and Inner query is known as Subquery.

• Nested Query –
  In Nested Query,  Inner query runs first, and only once. Outer query is executed with result from Inner query.Hence, Inner query is used in execution of Outer query.
  Example –

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

  Find details of customers who have ordered.

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

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

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

  Find details of customers who have ordered.

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

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 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 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 :