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General Tech Bugs & Fixes 2 years ago
Posted on 16 Aug 2022, this text provides information on Bugs & Fixes related to General Tech. Please note that while accuracy is prioritized, the data presented might not be entirely correct or up-to-date. This information is offered for general knowledge and informational purposes only, and should not be considered as a substitute for professional advice.
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I want to display the number of purchases each customer has made. If they've made 0 purchases, I want to display 0.
Desired Output:
------------------------------------- | customer_name | number_of_purchases | ------------------------------------- | Marg | 0 | | Ben | 1 | | Phil | 4 | | Steve | 0 | -------------------------------------
Customer Table:
----------------------------- | customer_id | customer_name | ----------------------------- | 1 | Marg | | 2 | Ben | | 3 | Phil | | 4 | Steve | -----------------------------
Purchases Table:
-------------------------------------------------- | purchase_id | customer_id | purchase_description | -------------------------------------------------- | 1 | 2 | 500 Reams | | 2 | 3 | 6 Toners | | 3 | 3 | 20 Staplers | | 4 | 3 | 2 Copiers | | 5 | 3 | 9 Name Plaques | --------------------------------------------------
My current query is as follows:
SELECT customer_name, COUNT(*) AS number_of_purchaes FROM customer LEFT JOIN purchases ON customer.customer_id = purchases.customer_id GROUP BY customer.customer_id
However, since it's a LEFT JOIN, the query results in rows for customers with no purchases, which makes them part of the COUNT(*). In other words, customers who've made 0 purchases are displayed as having made 1 purchase, like so:
LEFT JOIN
COUNT(*)
LEFT JOIN Output:
------------------------------------- | customer_name | number_of_purchases | ------------------------------------- | Marg | 1 | | Ben | 1 | | Phil | 4 | | Steve | 1 | -------------------------------------
I've also tried an INNER JOIN, but that results in customers with 0 purchases not showing at all:
INNER JOIN
INNER JOIN Output:
------------------------------------- | customer_name | number_of_purchases | ------------------------------------- | Ben | 1 | | Phil | 4 | -------------------------------------
How could I achieve my REPLY 0 views 0 likes 0 shares Facebook Twitter Linked In WhatsApp
SUM(ISNULL(purchases.customer_id)) AS number_of_purcahses
You can try like this:
Sample Data:
create table customer(customer_id integer, customer_name varchar(20)); create table purchaser(purchaser_id varchar(20), customer_id integer, description varchar(20)); insert into customer values(1, 'Marg'); insert into customer values(2, 'Ben'); insert into customer values(3, 'Phil'); insert into customer values(4, 'Steve'); insert into purchaser values(1, 2, '500 Reams'); insert into purchaser values(2, 3, '6 toners'); insert into purchaser values(3, 3, '20 Staplers'); insert into purchaser values(4, 3, '20 Staplers'); insert into purchaser values(5, 3, '20 Staplers'); SELECT c.customer_id, c.customer_name, COUNT(p.purchaser_id) AS number_of_purchaes FROM customer c LEFT JOIN purchaser p ON c.customer_id = p.customer_id GROUP BY c.customer_id;
SQL fiddle: http://sqlfiddle.com/#!9/32ff0a/2
customer_name is not a part of group by. won't work
customer_name
group by
5.7+ will allow this, even in set sql_mode=ONLY_FULL_GROUP_BY, provided customer_id is a PK or unique not null secondary key. When the group by field is unique, other rows from the same table can't be an invalid aggregation as their is only one value possible.
set sql_mode=ONLY_FULL_GROUP_BY
Instead of count(*) use count(purchase_id)
count(*)
count(purchase_id)
SELECT customer_name, COUNT(purchase_id) AS number_of_purchaes FROM customer LEFT JOIN purchases ON customer.customer_id = purchases.customer_id GROUP BY customer_name
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