how to use not exists in sql

Introduce

In this article, we will see how the SQL EXISTS operator works and when you should use it. application developers don’t realize how powerful SQL subquery expressions really are when filtering a certain table based on a condition that is evaluated on another table.

Database table model

Suppose we have the following two tables in our database, which form a one-to-many table relationship. Student table is parent table and student table is child table because it has Foreign Key column student_id which refer to Primary Key id column in student table.Student table contains the following two records: | id | first_name | last_name | access point | | – | – | – | – | | 1 | Alice | Smith | 8.95 | | 2 | Bob | Johnson | 8.75 | And, the student_grade table stores the grades received by the student: | id | class_name | class | student_id | | – | – | – | – | | 1 | Mathematics | 10 | 1 | | 2 | Mathematics | 9.5 | 1 | | 3 | Mathematics | 9.75 | 1 | | 4 | Science | 9.5 | 1 | | 5 | Science | 9 | 1 | | 6 | Science | 9.25 | 1 | | 7 | Mathematics | 8.5 | 2 | | 8 | Mathematics | 9.5 | 2 | | 9 | Mathematics | 9 | 2 | | 10 | Science | 10 | 2 | | 11 | Science | 9.4 | 2 |

SQL exists

Let’s say we want to get all students who score 10 in Math Read more: how to scan on hp officejet 4630 If we are only interested in student identifiers, then we can run a query like this: SELECT topqa. infoent_id FROM student_grade WHERE topqa.infoe = 10 AND topqa.infos_name = ‘Math’ COMMAND BY topqa.infoent_idBuy However, the application is interested in displaying a student’s full name, not just an identifier, so we i need information from student table like To filter student records with 10 points in Math we can use SQL EXISTS operator, like this: SELECT id, first_name, last_name FROM student WHERE EXISTS (SELECT 1 FROM student_grade WHERE topqa.infoent_id = topqa). info AND topqa.infoe = 10 AND topqa.infos_name = ‘Math’) ORDER BY id When we run the above query we can see that only the Alice row is selected: | id | first_name | last_name | | – | – | – | | 1 | Alice | Smith | The outer query selects the student row columns that we want to return to the client. However, the WHERE clause is using the EXISTS operator with a linked inner subquery. The EXISTS operator returns true if the subquery returns at least one record and false if no rows are selected. The database engine doesn’t have to run the subquery entirely. If a record is matched, the EXISTS operator returns true and the other associated query row is selected. The inner subquery is correlated because the student_id column of the student_grade table is matched with the id column of the outer student table.

SQL DOESN’T EXIST

Let’s consider we want to select all students with no score lower than 9. For this we can use NOT EXISTS, which negates the logic of the EXISTS operator. Read more: how to fix unaccelerated pixel format in windows 10 operators return true if below subquery returns no records. However, if a record is matched by the inner subquery, the NOT EXISTS operator will return false and subquery execution may be stopped. the following SQL query: SELECT id, first_name, last_name FROM student DOESN’T EXIST (SELECT 1 FROM student_grade WHERE topqa.infoent_id = topqa.info AND topqa.infoe < 9) ORDER BY id When we run the above query we have can see that only Alice records are matched: | id | first_name | last_name | | - | - | - | | 1 | Alice | Smith | Great, isn't it?

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Inference

The advantage of using the SQL operators LIVE and NOT EXIST is that the execution of the inner subquery can stop as long as a matching record is found. matched records can significantly speed up overall query response time.Read more: how to get samsung premium themes for free | Top Q&A

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