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Finding it difficult to understand the use of SQL NOT EXISTS and EXISTS operators? You have landed at the right place.
SQL NOT EXISTS and EXISTS operators have been available since SQL:86, the very first edition of the SQL Standard, still many fail to understand that SQL subquery expressions are powerful when it comes to filtering a given table based on a condition evaluated on a different table.
Nothing to worry, in this article we will be discussing about
Let’s get started with the basics first.
The purpose of the SQL “EXISTS” and “NOT EXISTS” operator is to check the existence of records in a subquery. One more similar operator is the “with” clause that was introduced in 1999 to support the CTE (Common Table Expressions) features. These operators are predefined in the SQL and used together with the UPDATE, DELETE, or SELECT statement. The SQL EXISTS and NOT EXISTS operators must be used together because they are not independent by themselves.
When SQL EXISTS is used along with the Where clause, it tests the existence of rows in a subquery.
The SQL NOT EXISTS Operator will perform quite opposite to the EXISTS Operator. It is helpful in restricting the number of rows returned by the SELECT Statement.
The subquery word has been used multiple times till time. Do you actually know the meaning of subquery in SQL? If not, don’t worry. We will discuss in brief what is a SQL subquery before going deeper.
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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:
Now let’s know the basic syntax of the SQL NOT EXISTS and EXISTS operator.
Let’s see some examples of SQL EXISTS Operator to understand it better.
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.
To get the first and last name of the customers who placed at least one order.
To get the last and first name of the customers who have not placed any order.
To delete the record of all the customers from the Order Table whose last name is ‘xyz’’.
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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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:
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.
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