MySQL mystery error condition

I'm working with rate plans for modems, trying to find the best rate plan given the table's values and actual usage.

I may not be going about this the most efficient way, but with only 5K records I'm not currently worried about performance.
That doesn't mean I'm adverse to a wiser way though!

intlplan_rate_charge and intlplan_overage_rate_per_MB are DECIMAL(10,4) ;
intlplan_limit_MB is SMALLINT UNSIGNED ;

Stepping up to calculate the best rate AND its associated code and wanting to set a field using SIGN() function to indicate:
-1 for plan too high, need to downgrade
0  for plan just right, and
+1 for plan too low, need to upgrade

Here's what I have:

  SELECT invoice_id INTO @var_latest_inv
  FROM   Invoice
  WHERE  date_charges_import_completed = (SELECT MAX(date_charges_import_completed) FROM Invoice );
  
  # --------------------------------------
  CREATE  TABLE `Modem_Usages` AS
  SELECT modem_config_id
       , intl_rate_plan_id
       , intl_data_usage
  FROM   ModemConfig mc
  JOIN   InvoicedModem im
  USING  (wireless_no)
  WHERE  im.invoice_id = @var_latest_inv ;
  
  # --------------------------------------
  CREATE  TABLE `Calcd_Charge` AS
  SELECT mu.modem_config_id
       , mu.intl_rate_plan_id
       , mu.intl_data_usage
       , irp.intlplan_code
       , irp.intlplan_limit_MB AS `current_threshold`
       , CASE WHEN mu.intl_data_usage/1000 <= intlplan_limit_MB THEN intlplan_rate_charge 
                                                        ELSE intlplan_rate_charge + (intl_data_usage/1000 - intlplan_limit_MB) * intlplan_overage_rate_per_MB END AS `calcd_charge`
  FROM IntlRatePlan   irp
  JOIN `Modem_Usages` mu
  USING (intl_rate_plan_id)  ;
  
  # --------------------------------------
  #REATE  TABLE `Enable_Best_Calc` AS
  SELECT  mu.modem_config_id
       ,  mu.intl_rate_plan_id
       , irp.intlplan_code
       ,   i.intlplan_limit_MB  AS 'OtherMBs'
       ,   i.intlplan_code      AS 'AvailPlans'
       ,  CASE WHEN mu.intl_data_usage/1000 <= i.intlplan_limit_MB THEN i.intlplan_rate_charge 
                                                        ELSE i.intlplan_rate_charge + (mu.intl_data_usage/1000 - i.intlplan_limit_MB) * i.intlplan_overage_rate_per_MB END AS `calcd_charge`
  FROM   `Modem_Usages`  mu
  JOIN   `IntlRatePlan` irp
  USING  (intl_rate_plan_id)
  JOIN   `IntlRatePlan` i
  WHERE  (i.intlplan_limit_MB <> irp.intlplan_limit_MB AND SUBSTRING(i.intlplan_code,1,4) = SUBSTRING(irp.intlplan_code,1,4)
    OR    i.intlplan_limit_MB =  irp.intlplan_limit_MB AND i.intlplan_code = irp.intlplan_code)
   AND   i.date_no_more_irp_contracts > CURRENT_DATE() 
   AND   i.intlplan_code NOT IN ('Unassigned', 'Suspended', 'NoRatePlan')
   ORDER BY 1, 4;
  
  # --------------------------------------
  SELECT modem_config_id,  MIN(ROUND(calcd_charge,4)) AS `LowestCharge` FROM `Enable_Best_Calc` GROUP BY 1  limit 6;

Select allOpen in new window

I intentionally did a cross join to get all available plans and best charge for each.

Now to get the associated Codes of current and best, and to generate that tristate value, I join this on itself 3 times.
Piecemeal shown first, then what SHOULD WORK:

  SELECT modem_config_id,  MIN(ROUND(calcd_charge,4)) AS `LowestCharge` FROM `Enable_Best_Calc` GROUP BY 1  limit 6;
  
  SELECT  a.*
        , ' ' AS 'Separator'
        , b.LowestCharge
  FROM    `Enable_Best_Calc` a
  JOIN    (SELECT modem_config_id,  MIN(ROUND(calcd_charge,4)) AS `LowestCharge` FROM `Enable_Best_Calc` GROUP BY 1) b
    ON    a.modem_config_id = b.modem_config_id 
  WHERE   a.intlplan_code = a.AvailPlans limit 9;
  
  
  SELECT  a.*
        , ' ' AS 'Separator'
        , b.LowestCharge
        , ' ' AS 'Separator'
        , c.AvailPlans        AS preferred_plan_code
        , c.OtherMBs          AS preferred_limit
        , SIGN(c.OtherMBs - a.OtherMBs) as preferred_plan_tristate
  FROM    `Enable_Best_Calc` a
  JOIN    (SELECT modem_config_id,  MIN(ROUND(calcd_charge,4)) AS `LowestCharge` FROM `Enable_Best_Calc` GROUP BY 1) b
    ON    a.modem_config_id = b.modem_config_id 
  JOIN    `Enable_Best_Calc` c
    ON    a.modem_config_id = c.modem_config_id  AND ROUND(c.calcd_charge,4) = ROUND(b.LowestCharge,4)
  WHERE   a.intlplan_code = a.AvailPlans ;

Select allOpen in new window

Output of the middle of the three looks like:The larger / 3rd query merges tables a with b to show all of a.* with b's lowest charge,
then table c provides me the code and limit associated with the lowest charge.

This works fine AS LONG AS I LEAVE OUT THE SIGN((c.OtherMBs - a.OtherMBs).

I've tried ROUND(,0) on each of these arguments, but I still get:
ERROR 1690 (22003): BIGINT UNSIGNED value is out of range in '(ROUND(`c`.`OtherMBs`,0) - ROUND(`a`.`OtherMBs`,0))'

I don't get it because 'Other_MBs' is from intl_limit_MB, which is a SMALLINT UNSIGNED.
I'm not assigning the value, just subtracting them.

Must I CAST them to SIGNED?  I'm guessing so...

Well, that worked.  CAST(zyx AS SIGNED) for each argument worked.
Now to clean up to just what I need.

Still, if you can suggest a significantly smarter way to do this I'm all ears!  Love to learn it.