MySQL - How to limit one result per ID? - mysql

Question

I have the following query which creates a view table showing the highest salesperson in a store with few other details:
CREATE OR REPLACE VIEW sales_data AS
SELECT s.storename AS "Store",
e.employee_name AS "Employee",
e1.employee_name AS "Manager",
SUM(p.total_sale_value) AS "Sales Value"
FROM fss_Shop s
JOIN Employee e ON e.storeid = s.storeid
JOIN Payment p ON p.employee_number = e.employee_number
JOIN Employee e1 ON e1.employee_number = e.manager_number
WHERE s.storeid=1
GROUP BY e.employee_name
ORDER BY SUM(p.total_sale_value) DESC LIMIT 1;
The above query will only show the sales data for a single store and the reason being as I have stated WHERE s.storeid=1. I have 20 stores in my table. How can I change the above query so that it gives me sales data for 20 stores (so that's 20 rows).

Answer 1

CREATE OR REPLACE VIEW employee_sales_totals AS
SELECT
e.*,
SUM(p.total_sale_value) AS total_sale_value
FROM
Employee e
INNER JOIN
Payment p
ON p.employee_number = e.employee_number
GROUP BY
e.id -- This should be the Primary Key / Surrogate Key of the employee table
;
CREATE OR REPLACE VIEW shop_top_employee_by_sales_value AS
SELECT
s.storename AS "Store",
e.employee_name AS "Employee",
m.employee_name AS "Manager",
p.total_sale_value AS "Sales Value"
FROM
(
SELECT storeid, MAX(total_sale_value) AS total_sale_value
FROM employee_sales_totals
GROUP BY storeid
)
p
INNER JOIN
employee_sales_totals e
ON e.storeid = p.storeid
AND e.total_sale_value = p.total_sale_value
INNER JOIN
fss_Shop s
ON s.storeid = e.storeid
INNER JOIN
Employee m
ON m.employee_number = e.manager_number
;
As per the answer to your previous question, if multiple employees are tied for the same total sales amount in the same store, all such employees would be returned.

Answer 2

Consider using a ranking variable by SalesValue for each employee per store and then choose the RANK=1 in outer query:
SELECT main.Store, main.Employee, main.Manager, main.SalesValue
FROM
(SELECT agg.*,
#store:=agg.Store AS CURR_STORE,
#rank:=CASE WHEN #val > agg.SalesValue THEN #rank+1 ELSE 1 END AS RANK,
#val:=CASE WHEN #store <> agg.Store THEN #val ELSE agg.SalesValue END AS CURR_VAL
FROM
(SELECT s.storename AS "Store",
e.employee_name AS "Employee",
e1.employee_name AS "Manager",
SUM(p.total_sale_value) AS "SalesValue"
FROM fss_Shop s
INNER JOIN Employee e ON e.storeid = s.storeid
INNER JOIN Payment p ON p.employee_number = e.employee_number
INNER JOIN Employee e1 ON e1.employee_number = e.manager_number
GROUP BY s.storename,
e.employee_name,
e1.employee_name
) As agg
CROSS JOIN (SELECT #rank:= 0) AS r1
CROSS JOIN (SELECT #val:= 0) AS r2
CROSS JOIN (SELECT #store:= 0) AS r3
ORDER BY agg.Store, agg.SalesValue DESC
) As main
WHERE main.RANK = 1;
DEMO
Rextester (using random data with only one Sales table)
Alternatively, if variables cannot be used, consider creating two views where latter references the former: 1) initial aggregate query, 2) correlated subquery to retrieve top employee per store
CREATE OR REPLACE VIEW sales_data AS
SELECT s.storename AS "Store",
e.employee_name AS "Employee",
e1.employee_name AS "Manager",
SUM(p.total_sale_value) AS "SalesValue"
FROM fss_Shop s
INNER JOIN Employee e ON e.storeid = s.storeid
INNER JOIN Payment p ON p.employee_number = e.employee_number
INNER JOIN Employee e1 ON e1.employee_number = e.manager_number
GROUP BY s.storename,
e.employee_name,
e1.employee_name;
CREATE OR REPLACE VIEW top_sales_data AS
SELECT s.*
FROM sales_data s
WHERE (SELECT Count(*) FROM sales_data sub
WHERE sub.SalesValue > s.SalesValue
AND sub.Store = s.Store) = 0;