How to perform subqueries using the IN and EXISTS clauses in SQL?

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Aryan Kumar · Answer

Performing subqueries using the IN and EXISTS clauses in SQL allows you to create more complex and dynamic queries by embedding one query within another. These subqueries are also known as inner queries or nested queries. Let's explore how to use both the IN and EXISTS clauses:
Using the IN Clause:
The IN clause is used to compare a value against a set of values returned by a subquery. It checks whether a value exists in the result set of the subquery. Here's the basic syntax:
SELECT column1
FROM table1
WHERE column2 IN (subquery);
column1: The column you want to retrieve from the main query.table1: The table from which you're selecting data.column2: The column you want to compare with the result of the subquery.subquery: The subquery that returns a list of values.
Example using IN:
Suppose you have a table called Customers and you want to find all customers who have placed orders:
SELECT CustomerName
FROM Customers
WHERE CustomerID IN (SELECT CustomerID FROM Orders);
In this example, the subquery (SELECT CustomerID FROM Orders) retrieves a list of CustomerIDs from the
Orders table. The main query then finds customer names where the CustomerID is in that list.
Using the EXISTS Clause:
The EXISTS clause is used to check whether a subquery returns any rows. It returns
TRUE if the subquery returns at least one row and FALSE otherwise. Here's the basic syntax:
SELECT column1
FROM table1
WHERE EXISTS (subquery);
column1: The column you want to retrieve from the main query.table1: The table from which you're selecting data.subquery: The subquery that is checked for existence.
Example using EXISTS:
Suppose you want to find all customers who have placed orders using the 
EXISTS clause:
SELECT CustomerName
FROM Customers C
WHERE EXISTS (SELECT 1 FROM Orders O WHERE O.CustomerID = C.CustomerID);
In this example, the subquery (SELECT 1 FROM Orders O WHERE O.CustomerID = C.CustomerID) checks for the existence of at least one order for each customer. The main query selects customer names where the
EXISTS condition is TRUE.
Both the IN and EXISTS clauses are powerful tools for performing subqueries in SQL, and the choice between them depends on your specific query requirements. Generally, use
IN when you want to compare a single value against a set of values returned by the subquery and use
EXISTS when you want to check for the existence of rows in the subquery's result set.

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How to perform subqueries using the IN and EXISTS clauses in SQL?

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Using the IN Clause:

Using the EXISTS Clause:

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