MySQL计数与联接的情况

Question

I'm trying to create a simple select statement and can't seem to get it right. I have three tables. Here's the schema.

CREATE TABLE IF NOT EXISTS `SalesRepDealer` (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `DealerId` varchar(20) NOT NULL,
  `DealerName` varchar(50) NOT NULL,
  `SalesRepId` varchar(30) NOT NULL,
  `SalesRepName` varchar(30) NOT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=629 ;

CREATE TABLE IF NOT EXISTS `PriorYearBoatOrders` (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `DealerId` varchar(20) NOT NULL,
  `PriorYearSalesTotal` int(11) NOT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=2234 ;

CREATE TABLE IF NOT EXISTS `BoatOrdersCurrentYear` (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `DealerId` varchar(40) COLLATE utf8_bin NOT NULL,
  `DealerName` varchar(50) COLLATE utf8_bin NOT NULL,
  `OrderNum` varchar(10) COLLATE utf8_bin NOT NULL,
  `BoatDescription` varchar(50) COLLATE utf8_bin NOT NULL,
  `SerialNum` varchar(50) COLLATE utf8_bin NOT NULL,
  `CompleteDate` varchar(15) COLLATE utf8_bin DEFAULT NULL,
  `ShipDate` varchar(15) COLLATE utf8_bin DEFAULT NULL,
  `ShipBy` varchar(15) COLLATE utf8_bin DEFAULT NULL,
  `Invoice` varchar(15) COLLATE utf8_bin DEFAULT NULL,
  `DocUnitPrice` int(11) DEFAULT NULL,
  `Customer` varchar(50) COLLATE utf8_bin DEFAULT NULL,
  `Po` varchar(30) COLLATE utf8_bin NOT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=8192 ;

Here's my initial query that counts the open orders and shipped orders and filters the list by sales rep id. This works perfectly, and gives me this result.

SELECT DealerId, DealerName, sum(case when ShipDate = '' then 1 else 0 end)  AS NumOpenOrders, sum(case when ShipDate != '' then 1 else 0 end)  AS NumShipped FROM BoatOrdersCurrentYear WHERE DealerId IN (SELECT DealerID FROM SalesRepDealer WHERE SalesRepId = 'bba') Group By DealerName

DealerId    DealerName  NumOpenOrders   NumShipped
BACA082017  Antler's Resort & Marina    0   5
CDGWAZ001   Got Water Marine    7   31
CDORHI014   HILLYERS SANTIAM MOTOR SPORTS   10  9
CDCAJM317   JOHNSON MARINE SUPPLIES INC 4   5
CDCABA0117  KDME, INC   4   0
CDIAMM015   Mark's Marine, Inc. 0   16
CDCANE001   Newport Pontoons, Inc.  0   1
CDIDPR043   Prestige Water Sports   15  4

Now I just want to add in the prior year totals. I tried joins and other combinations of selects but the counts screw up and start getting wonky. I just need to take the Dealer Id from above query and match it up to the PriorYearBoatOrders and return PriorYearSalesTotal. But here's the query I've been trying and the results.

SELECT BoatOrdersCurrentYear.DealerId, BoatOrdersCurrentYear.DealerName, sum(case when BoatOrdersCurrentYear.ShipDate = '' then 1 else 0 end)  AS NumOpenOrders, sum(case when BoatOrdersCurrentYear.ShipDate != '' then 1 else 0 end)  AS NumShipped, PriorYearBoatOrders.PriorYearSalesTotal FROM BoatOrdersCurrentYear, PriorYearBoatOrders  WHERE BoatOrdersCurrentYear.DealerId IN (SELECT DealerId FROM SalesRepDealer WHERE SalesRepId = 'bba')Group By DealerName

DealerId    DealerName  NumOpenOrders   NumShipped  PriorYearSalesTotal
BACA082017  Antler's Resort & Marina    0   11165   5
CDGWAZ001   Got Water Marine    15631   69223   5
CDORHI014   HILLYERS SANTIAM MOTOR SPORTS   22330   20097   5
CDCAJM317   JOHNSON MARINE SUPPLIES INC 8932    11165   5
CDCABA0117  KDME, INC   8932    0   5
CDIAMM015   Mark's Marine, Inc. 0   35728   5
CDCANE001   Newport Pontoons, Inc.  0   2233    5
CDIDPR043   Prestige Water Sports   33495   8932    5

I'm not sure what's happening and why the totals are so far off. But i'm pretty new to query writing. Any help is appreciated, sorry for the poorly formatted schema and queries.

Answer 1

For every dealer, you're joining the data from this year with every dealer's data from last year - giving you a cross join; you need to add a join condition matching on DealerId.

But you also need to start with a list of DealeIds, otherwise even with the condition on DealerId you'll be cross joining the orders.

SELECT
    d.DealerId,
    cy.DealerName,
    sum(case when cy.ShipDate = '' then 1 else 0 end) AS NumOpenOrders,
    sum(case when cy.ShipDate != '' then 1 else 0 end) AS NumShipped,
    py.PriorYearSalesTotal
FROM (SELECT distinct DealerId FROM SalesRepDealer WHERE SalesRepId = 'bba') d
JOIN BoatOrdersCurrentYear cy on cy.DealerId = d.DealerId
LEFT JOIN PriorYearBoatOrder py on py.DealerId = d.DealerId
group by 1,2

Note the use of:

• aliases to improve readability
• "modern" (since 1992) join syntax
• subquery to generate the list of dealerIds
• left join to return null is there were no sales last year