MySQL - How to limit one result per ID?

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 
SE         


        

Answer 1

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)

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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;