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Problem :

I have written the typical MyPersons table and I have also written the MyOrders table and it is defined in a particular way so that I can do the JOIN query like the below one to return the MyOrders for all of the MyPersons.

SELECT MyPersons.LastName, MyPersons.FirstName, MyOrders.OrderNo
FROM MyPersons
ON MyPersons.id=MyOrders.Person_id

Now my question is, how should I write the query that must return me all MyPersons with NO MyOrders?

I must write this using Mysql.

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2 Answers

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Solution :

To get the desired output you can use LEFT JOIN and IS NULL as shown in the below query:

SELECT     MyPersons.LastName, MyPersons.FirstName
FROM       MyPersons
LEFT JOIN  MyOrders ON MyPersons.id = MyOrders.Person_id
WHERE      MyOrders.Person_id IS NULL;

Now you will get the result of the left join which will always contains all of the records from the "left" table (MyPersons), even if your join-condition does not find any matching record in your "right" table (MyOrders). And when there is no match, the columns of your "right" table will be NULL in your result set.


You can try the below query it should work... there is always  more than one way to do it.

select * from MyPersons where MyPersons.id not in (select MyPersons_id from MyOrders)


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You may want to employ LEFT JOIN and IS NULL:

SELECT     Persons.LastName, Persons.FirstName
FROM       Persons
LEFT JOIN  Orders ON Persons.id = Orders.Person_id
WHERE      Orders.Person_id IS NULL;

The result of a left join constantly holds all records of the "left" table (Persons), even in case the join-condition does not trace any matching record in the "right" table (Orders). At the time there is no match, the columns of the "right" table will NULL in the result set.

This must work. theres more than one method to do it.

select * from persons where person.id not in (select person_id from orders)

Only for fulfillment, here is the not exists version:

select * from persons p 
where not exists
(select null from orders o where o.person_id = p.id)

Approach: Employing sub-query and NOT IN clause


In case we have a list of customers who have ever ordered, it will be facile to know who never ordered.

We can employ the following code to obtain such list.

select customerid from orders;

Then, we can employ NOT IN to query the customers who are not in this list.


select customers.name as 'Customers'
from customers
where customers.id not in
    select customerid from orders

One of the most usual solutions to this issue is by employing the SQL JOIN clause. You can employ the LEFT OUTER JOIN to solve this issue, as displayed below:

SELECT C.Name FROM Customers C LEFT JOIN Orders O ON C.Id = O.CustomerId WHERE O.CustomerId is NULL

Read more: https://javarevisited.blogspot.com/2016/01/sql-exists-example-customers-who-never-ordered.html#ixzz6O2CbX1wD

At the time you link two tables in SQL employing a LEFT OUTER JOIN, then a big table will be made with NULL values in the column which don't subsists in another table.

For example, the big table will have four columns C.Id, C.Name, O.Id, and O.CustomerId, for Customers who have never ordered anything, the O.CustomerId will be NULL.

Many programmers practice the mistake of employing != in JOIN condition to resolve this problem, with the assumption that if = returns matching rows, then != will return those ids which are not subsistent in another table. 

In case you struggle to perceive join, then I point out you take a look at Jose Portilla's fantastic SQL course The Complete SQL Bootcamp on Udemy. His teaching method is amazing and you will perceive joins in no time.

Anyhow, this issue is really a fantastic example of how and when to employ EXISTS clause:


Read more: https://javarevisited.blogspot.com/2016/01/sql-exists-example-customers-who-never-ordered.html#ixzz6O2DCsNKN

This is a correlated subquery, where the inner query will execute for every row of the outer query, and just those customers will be returned who have not ordered anything.

However, the most easy solution is by using NOT IN Clause.

SELECT A.Name FROM Customers A WHERE A.Id NOT IN (SELECT B.CustomerId FROMs Orders B)

Read more: https://javarevisited.blogspot.com/2016/01/sql-exists-example-customers-who-never-ordered.html#ixzz6O2DQWRKz


10 6 4
31,120 points

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