How To Use The SQL NOT EXISTS and EXISTS Operator ?

Understanding SQL EXISTS and NOT EXISTS Operators

Even though SQL EXISTS and NOT EXISTS operators have been around since the beginning, many find them confusing. This is because they leverage subqueries, which are powerful tools for filtering data based on conditions in another table.

What they do:

• EXISTS: Checks if at least one record exists based on the subquery's condition. If the subquery returns any rows (one or more), EXISTS evaluates to TRUE.
• NOT EXISTS: The opposite of EXISTS. It's satisfied only if the subquery returns zero rows. In other words, if there are no matching records in the subquery, NOT EXISTS evaluates to TRUE.

Benefits of using them:

• Flexibility: They allow for complex filtering logic based on data in different tables.
• Performance: In some cases, they can optimize queries by stopping the subquery execution as soon as a matching record is found (EXISTS) or no records are found (NOT EXISTS).

This article will cover:

• The purpose of EXISTS and NOT EXISTS operators
• Subqueries explained
• Basic syntax for using these operators
• Examples to illustrate their functionality
• Performance considerations when using them

Let's dive deeper and explore these concepts in detail!

Understanding SQL EXISTS and NOT EXISTS Operators

SQL offers operators like EXISTS and NOT EXISTS to check for the existence of records based on a subquery. A subquery is essentially a mini-query nested within another query. It retrieves data used to filter the main query's results.

What they do:

• EXISTS: This operator checks if the subquery returns at least one record. If the subquery finds any rows, EXISTS evaluates to TRUE.
• NOT EXISTS: The opposite of EXISTS. It's satisfied only if the subquery returns zero rows. In other words, if there are no matching records in the subquery, NOT EXISTS evaluates to TRUE.

Similarities and Differences with IN Operator:

The EXISTS operator can achieve similar results to the IN operator in some cases, but their syntax differs. EXISTS uses a subquery, while IN takes a list of values for comparison.

Using EXISTS and NOT EXISTS:

These operators can be used with SELECT, UPDATE, or DELETE statements to filter data based on subquery results.

Key Points:

• EXISTS helps find rows where a related record exists based on the subquery.
• NOT EXISTS helps find rows where there are no related records based on the subquery.
• They offer flexibility in filtering data across tables.

Next Steps:

This article will provide examples to illustrate how EXISTS and NOT EXISTS work in practice. We'll also explore considerations for using them effectively in your queries.

Note: The text about the "WITH" clause and CTEs is not directly related to EXISTS and NOT EXISTS and can be omitted for a more focused explanation.

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A subquery is also named as the inner query or the nested query that is frequently used within other queries. The SQL subquery can be nested with multiple statements like SELECT, INSERT, UPDATE, or DELETE statements, etc. Mostly, we use a subquery in SQL with Where and EXISTS clauses.

 Here are some basic rules for using subqueries in SQL:

• Each subquery should be enclosed within parentheses.
• We cannot use Order By clause within the subquery until we don’t use the TOP clause.
• Each subquery should include the regular SELECT statement.
• The Having, Where, Group By clauses can be used optionally within a subquery.
• You cannot use the Compute clause within a subquery.
• A subquery is generally nested inside WHERE and Has clause or outer statements like SELECT, INSERT, UPDATE, or DELETE, etc.
• SQL server allows you to nest subqueries up to 32 levels. It may vary based on requirements.

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Now let’s understand the basic syntax of the SQL NOT EXISTS and EXISTS operator.

SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name(s)
FROM table_name
WHERE condition);

• Here, you have to give the column name of the table.
• Expressions can be a single constant, variable, scalar function, the column name, or the pieces of a SQL query that compares values and perform arithmetic calculations when required.
• The name should be added in the second line of the code.
• The last line of code checks the existence of one or more rows in a subquery. When a single row satisfies the condition, it returns the Boolean value True. If there are no matching rows then it returns the Boolean value False otherwise you can use the “NOT EXISTS” operator too.

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SELECT
*
FROM
table_name
WHERE
NOT EXISTS (subquery);

• You can choose the number of columns from the table.
• The SQL JOINS are used to join multiple tables.
• If the subquery returns true then it will return the records true, else it won’t return any records.
• Read our comprehensive blog to understand the difference between View & Table In SQL.

Let’s check out some examples of SQL EXISTS Operator.

Below is an example to help you better understand how to use SQL NOT EXIST and EXIST Operator.

Image source: MySQL

The last example is a double-nested NOT EXISTS query. Which means, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers the question “does a city exist with a store that is not in Stores”? But it is easier to say that a nested NOT EXISTS answers the question “is x TRUE for all y?”

Let’s know how you can use EXISTS condition with SELECT, NOT, DELETE,  and UPDATE statements.

The IN operator can also be used to get the same output. But the question is, which operator is more suitable for different situations? If the subquery returns a very large record-set, the EXISTS operator works great here. However, if the record-set is smaller then you must use the IN operator.

Count operator is used to check the total number of rows within a table while EXISTS operator is used to check the existence of rows when a particular condition is satisfied. In simple words, “EXISTS” operator may short-circuit after having found the first matching row while “Count” is not allowed to show such type of behavior.

The SQL database works slightly better when using the EXISTS operator instead of the Count operator because of a much better cardinality estimate in the middle of an execution plan. Every time you want to check a row for existence, you should always use the EXISTS operator in that case. Also, the associated costs with the EXISTS operator are almost 30 times less than the Count operator.

“It is obvious to check the existence faster rather than to count all rows together if you are really doing something for the existence only.”

Every SQL operator makes the difference to improve the performance of a query. An SQL operator works with data items and returns the result. Oracle uses operators under different conditions that can be used with queries to filter the result set. In the same EXISTS operator ensure the best performance when the query contains voluminous data to evaluate. 

The reason is that the EXISTS operator follows the “at least found” principle in queries. It is set to TRUE if even a single row satisfies the condition. In some cases, it stops the execution at the same place where the result is set to TRUE. However, you should use comparison operators with EXISTS to continue the table scanning ahead.

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• Most DBAs prefer to use the NOT EXISTS clause between NOT EXISTS and NOT IN operator.
• When SQL includes a NOT IN clause, a subquery is generally used.
•   With NOT EXISTS, a correlated subquery is used. 
• In many cases a NOT IN will produce the same execution plan as a NOT EXISTS query or a not equal query (!=).
• In some cases a correlated NOT EXISTS subquery can be re-written with a standard outer join with a NOT NULL test.
• Some NOT EXISTS subqueries can be tuned using the MINUS operator to tune a NOT EXISTS.

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With this discussion, we come to the end of the blog. Here, you know the basics of SQL EXISTS operator, all about SQL not existing and how to use it with a SQL query. We have discussed one detailed example too for your reference to understand the concept in depth. But one example is not enough to practice this operator. 

To learn multiple SQL NOT EXIST and EXISTS examples, you should join the online SQL certification training right away and start practicing different operators quickly. It will help you learn SQL practically and become a SQL professional with attractive salary packages. 

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Q1) . Where NOT EXISTS SQL is used?

Ans:- NOT EXISTS SQL means nothing returned by the subquery. It is used to restrict the number of rows returned by the SELECT statement. In the server, it checks the Subquery for row existence, and if there are no browns then it will return true otherwise false. 

Q2). How do you avoid NOT EXISTS in SQL?

Ans:- To resolve the SQL server NOT EXISTS issue, make sure you are searching on indexed. Make sure, there should be no manipulation of the data within those columns. columns. Now, replace the No EXISTS with a left outer join, it works better in the large data sats. 

Q3). How do you check if record NOT EXISTS in SQL?

Ans:- There are different ways to check if record NOT EXIST in SQL, here are few:

• With the use of the EXISTS clause in the IF statement. 
• By using EXISTS  clause in the case statement to check
• “And the last through EXISTS clause in the where clause to check

Q4). What is NOT in SQL?

Ans:- Not in SQL operator is used when the user is looking to retrieve a column, which has no entries in the table or referencing table. A customer table contains the records of all the customers and the transaction table tracks the transaction between the store and the customer. 

Q5). How does SQL NOT EXISTS work?

Ans:- The SQL NOT EXISTS command is used to check the existence of particular values in the given subquery. The subquery does not return any data, it just returns True or False values depending on the subquery values existence check. 

Q6). How would you use it and exist in SQL?

Ans:- With the SQL Not in command, you can specify numerous values in the Where clause. While the SQL NOT EXISTS command is helpful in checking for the existence of specific values in the given subquery. 

Q7).  How do you create table if NOT EXISTS in SQL?

Ans:- To create if NOT EXISTS SQL table, first, specify the name of the table that you are looking to create after the Create Table Keywords. Second, use the if NOT EXISTS SQL option to create a new table if it does not exist. Now,  if you want to, you can specify the schema name to which the new table belongs and at last, specify the column list of the table. 

Q8). Why do we use if EXISTS in SQL?

Ans:- SQL if EXISTS is used to test for the existence of any record in a subquery. The EXISTS operator returns true if the subquery returns one or more records. 

Q9). What to use instead of SQL NOT EXISTS?

Ans:- A good alternative of In and EXISTS is an Inner Join, which is a Left Outer Join with Where clause checking for Null values can be used as an alternative for Not In and NOT EXISTS. 

Q10). How do you use EXISTS in SQL instead of in?

Ans:- The EXISTS keyword is responsible for checking true or false while, In keyword does comparison of all values in the corresponding sub query column.