sql How to choose results using HAVING correctly - mysql

Question

I have the following 2 tables (showing only sum of the columns here):
CUSTOMER:id, fname, lname.
SALES_ORDER: id, cust_id, sales_rep.
And the id from CUSTOMER connects with the cust_id from SALES_ORDER.
What I need to do is: Return customer details + number of orders made by that customer, for customers that have had at least one order handled by employee number '129'. This is the query I tried:
SELECT customer.id, fname, lname, count(*)
FROM customer,
sales_order
WHERE customer.id = sales_order.cust_id
GROUP BY customer.id
HAVING sales_rep = 129;
This does indeed return customers that answer the above + the number of order they had...but upon browsing the DB I see there are other clients that weren't fetched by the query. What am I doing wrong?
Thanks!

Answer 1

What about this:
SELECT customer.id, fname, lname, count(*)
FROM customer,
sales_order
WHERE customer.id = sales_order.cust_id
GROUP BY customer.id
HAVING MAX(IIF(sales_rep = 129, 1, 0)) = 1;
The idea is to calculated the count, but show only these records, where at least on of the sales_rep is 129. We are using IIF (you can use CASE WHEN for older SQL Server versions) to check if a particular record is for the specific sales_rep and MAX to check for at least one such record.
If you do not want to use HAVING you can filter the rows using additional filtering criteria:
SELECT customer.id, fname, lname, count(*)
FROM customer
INNER JOIN sales_order
ON customer.id = sales_order.cust_id
WHERE customer.id IN
(
SELECT DISTINCT customer.id
FROM sales_order
WHERE sales_rep = 129
)
GROUP BY customer.id
The idea is to calculated the count for each customer and then using a WHERE clause to display only users you have at least one sales_rep = 129.