What is the use of EXISTS and not EXISTS in SQL?

EXISTS is a logical operator that is used to check the existence, it is a logical operator that returns boolean result types as true or false only.

It will return TRUE if the result of that subquery does not contain any rows otherwise FALSE will be returning as result. We can use it within IF conditions or Sub Queries.

The SQL NOT EXISTS Operator will act quite opposite to EXISTS Operator. It is used to restrict the number of rows returned by the SELECT Statement.

The NOT EXISTS in SQL Server will check the Subquery for rows existence, and if there are no rows then it will return TRUE, otherwise FALSE. Or we can simply say, SQL Server Not Exists operator will return the results exactly opposite to the result returned by the Subquery.

Before going into this example, I suggest you to refer the SQL Subquery article to understand the subquery designing and query parsing.

SQL NOT EXISTS Syntax

The basic syntax of the NOT EXISTS in SQL Server can be written as:

SELECT [Column Names]
FROM [Source]
WHERE NOT EXISTS (Write Subquery to Check)

• Columns: It allows us to choose the number of columns from the tables. It may be One or more.
• Source: One or more tables present in the Database. SQL JOINS are used to join multiple tables.
• Subquery: Here we have to provide the Subquery. If the subquery returns true then it will return the records otherwise, it doesn’t return any records.

In this article, we will show you, How to use the SQL Server NOT EXISTS Operator with examples. For this, We are going to use the below-shown data

Using NOT EXISTS in SQL

The NOT EXISTS condition in SQL Server is made up of two logical operators: EXISTS (which was previously discussed) and NOT (which is used to negate a Boolean input).

What is the difference between the SQL terms EXISTS and NOT EXISTS?

NOT EXISTS, unlike EXISTS, returns TRUE if the subquery's result contains no records. If a single entry in a table fits the subquery, the NOT EXISTS function returns FALSE, and the subquery's execution is halted. NOT EXISTS, in plain English, allows you to find records that don't match the subquery.

SYNTAX

SELECT
      column_name
FROM Table_Name
WHERE NOT EXISTS (SELECT
		          column_name
                  FROM Table_Name
                  WHERE condition);

NOT EXISTS is used in the WHERE clause with a subquery to see if the result of the subquery is TRUE or FALSE. The outer select statement's rows are subsequently narrowed down using the Boolean value.

To put it another way, the NOT EXISTS subquery checks every entry from the outer query, returns TRUE or FALSE, and then passes the value to the outer query to use. In plain English, SQL NOT EXISTS returns all entries that do not fulfill the EXISTS condition.

SQL Exists and Not Exists Example

We'll query the Customer table to locate entries where the CustomerID doesn't exist in the Order table to show how NOT EXISTS works in SQL Server. As a result, we'll obtain a list of clients who haven't placed any orders yet if we use NOT EXISTS.

SELECT
      *
FROM Sales.Customer c
WHERE NOT EXISTS IN (SELECT
                    *
              FROM Sales.[Order] o
              WHERE o.CustomerId = o.OrderId)

Result

That's all about how to use EXISTS and NOT EXISTS clause in SQL. Because SQL is a logical language, everything works fairly simply. Its logical operators examine the situation and return a Boolean value. In some situations, these logical operators can be used interchangeably, and the user can choose which one to employ. We gave a complete review and comparison of SQL EXISTS, NOT EXISTS, IN in this post so that you may make the best decisions possible in your work.

When to use exists and not exists?

What is the use of not exists in SQL?

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