![]() The general rule of thumb is that if the subquery contains a large volume of data, the EXISTS operator provides better performance. On the other hands, when the IN operator is combined with a subquery, MySQL must process the subquery first and then uses the result of the subquery to process the whole query. The EXISTS stops scanning the table when a matching row found. The reason is that the EXISTS operator works based on the “at least found” principle. The query that uses the EXISTS operator is much faster than the one that uses the IN operator. Now, check the performance of the query that uses the IN operator. Orders.customernumber = customers.customernumber) Code language: SQL (Structured Query Language) ( sql ) Let’s compare the query that uses the IN operator with the one that uses the EXISTS operator by using the EXPLAIN statement. Orders) Code language: SQL (Structured Query Language) ( sql ) To find the customer who has placed at least one order, you can use the IN operator as shown in the following query: SELECT To do this, you use the EXISTS operator in WHERE clause of the DELETE statement as follows: DELETE FROM customersĪ.customernumber = customers.customerNumber) Code language: SQL (Structured Query Language) ( sql ) MySQL EXISTS operator vs. One final task in archiving the customer data is to delete the customers that exist in the customers_archive table from the customers table. SELECT * FROM customers_archive Code language: SQL (Structured Query Language) ( sql ) ![]() Third, query data from the customers_archive table to verify the insert operation. Second, insert customers who do not have any sales order into the customers_archive table using the following INSERT statement. LIKE customers Code language: SQL (Structured Query Language) ( sql ) To do this, you use these steps:įirst, create a new table for archiving the customers by copying the structure from the customers table: CREATE TABLE customers_archive Suppose that you want to archive customers who don’t have any sales order in a separate table. Second, the CONCAT() function concatenate the phone extension with the number 1.First, the EXISTS operator in the WHERE clause gets only employees who works at the office in San Fransisco.This example adds the number 1 to the phone extension of employees who work at the office in San Francisco: UPDATE employeesĬity = 'San Francisco' AND offices.officeCode = employees.officeCode) Code language: SQL (Structured Query Language) ( sql ) Offices.officeCode = employees.officeCode) Code language: SQL (Structured Query Language) ( sql ) The following statement finds employees who work at the office in San Franciso: SELECT Suppose that you have to update the phone’s extensions of the employees who work at the office in San Francisco. ) Code language: SQL (Structured Query Language) ( sql ) Orders.customernumber = customers.customernumber ![]() The following example uses the NOT EXISTS operator to find customers who do not have any orders: SELECT Otherwise, the subquery returns no row and the EXISTS operator returns false. As a result, the EXISTS operator returns true and stops examining the orders table. If the customerNumber, which appears in the customers table, exists in the orders table, the subquery returns the first matching row. In this example, for each row in the customers table, the query checks the customerNumber in the orders table. = customers.customernumber) Code language: SQL (Structured Query Language) ( sql ) The following statement uses the EXISTS operator to find the customer who has at least one order: SELECT MySQL SELECT EXISTS examplesĬonsider the following customers and orders tables in the sample database. Let’s take some examples of using the EXISTS operator to understand how it works. The results are the same because MySQL ignores the select list appeared in the SELECT clause. Note that you can use SELECT *, SELECT column, SELECT a_constant, or anything in the subquery. In other words, the NOT EXISTS returns true if the subquery returns no row, otherwise it returns false. The NOT operator negates the EXISTS operator. In addition, the EXISTS operator terminates further processing immediately once it finds a matching row, which can help improve the performance of the query. If the subquery returns at least one row, the EXISTS operator returns true, otherwise, it returns false. ![]() EXISTS(subquery) Code language: SQL (Structured Query Language) ( sql ) The following illustrates the basic syntax of the EXISTS operator: SELECT The EXISTS operator is often used to test for the existence of rows returned by the subquery. The EXISTS operator is a Boolean operator that returns either true or false. Summary: in this tutorial, you will learn how to use the MySQL EXISTS operator and when to use it to improve the performance of the queries.
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