Views that store values

It can be useful to store a particular set of values within a SELECT statement or within a procedure. For example, suppose a company prefers to analyze the results of its sales staff by thirds of a year, instead of by quarter. Since there is no built-in date part for thirds, as there is for quarters, it is necessary to store the dates within the procedure. 

WITH thirds ( q_name, q_start, q_end ) AS
( SELECT 'T1', '2000-01-01', '2000-04-30' UNION
  SELECT 'T2', '2000-05-01', '2000-08-31' UNION
  SELECT 'T3', '2000-09-01', '2000-12-31' )
SELECT q_name,
       SalesRepresentative,
       count(*) AS num_orders,
       SUM( p.UnitPrice * i.Quantity ) AS total_sales
FROM thirds LEFT OUTER JOIN SalesOrders AS o
    ON OrderDate BETWEEN q_start and q_end
                   KEY JOIN SalesOrderItems AS I
                   KEY JOIN Products AS p
 GROUP BY q_name, SalesRepresentative
 ORDER BY q_name, SalesRepresentative;

This method should be used with care, as the values may need periodic maintenance. For example, the above statement must be modified if it is to be used for any other year.

You can also apply this method within procedures. The following example declares a procedure that takes the year in question as an argument. 

CREATE PROCEDURE sales_by_third ( IN y INTEGER )
BEGIN
  WITH thirds ( q_name, q_start, q_end ) AS
  ( SELECT 'T1', YMD( y, 01, 01), YMD( y, 04, 30 ) UNION
    SELECT 'T2', YMD( y, 05, 01), YMD( y, 08, 31 ) UNION
    SELECT 'T3', YMD( y, 09, 01), YMD( y, 12, 31 ) )
  SELECT q_name,
         SalesRepresentative,
         count(*) AS num_orders,
         SUM( p.UnitPrice * i.Quantity ) AS total_sales
  FROM thirds LEFT OUTER JOIN SalesOrders AS o
    ON OrderDate BETWEEN q_start and q_end
            KEY JOIN SalesOrderItems AS I
            KEY JOIN Products AS p
  GROUP BY q_name, SalesRepresentative
  ORDER BY q_name, SalesRepresentative;
END;

The following statement calls the previous procedure.

CALL sales_by_third (2000);
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