Question

我正在尝试选择多个表，同时还有空列。例如我的表：

calendar:  //filled with dates from 2011-01-01 to 2015-12-31
+-----------+
| datefield |
+-----------+
| 2012-1-1  |
| 2012-1-2  |
| ...       |
| 2012-2-3  |
| 2012-2-4  |
+-----------+

car: 
+--------+---------+
| car_id |  name   |
+--------+---------+
|      1 | Ford    |
|      2 | Peugeot |
|      3 | Fiat    |
+--------+---------+

carsales: 
+-------------+-----------+--------+-------------+
| car_sale_id | sell_time | car_id | customer_id |
+-------------+-----------+--------+-------------+
|           1 | 2012-1-2  |      1 |           1 |
|           1 | 2012-1-2  |      2 |           1 |
|           2 | 2012-1-3  |      3 |           2 |
+-------------+-----------+--------+-------------+

customer: 
+-------------+---------+
| customer_id | country |
+-------------+---------+
|           1 | NL      |
|           2 | EN      |
+-------------+---------+

现在我想要一个列表，列出客户从'NL'，按车名，以及2012-1-1和2012-2-1（按周）销售的汽车数量。日期应始终存在。

例如：

+----------+----------+-------+
|   Week   | Car name | Sales |
+----------+----------+-------+
| 1        | Ford     |     0 |
| 1        | Peugeot  |     0 |
| 1        | Fiat     |     0 |
| 2        | Ford     |     1 |
| 2        | Peugeot  |     1 |
| 2        | Fiat     |     0 |
| 3        | Ford     |     0 |
| 3        | Peugeot  |     0 |
| 3        | Fiat     |     0 |
| etc      | ...      |   ... |
+----------+----------+-------+

我想出了这个：

SELECT WEEKOFYEAR(calendar.datefield) as 'Week', car.name, COUNT(carsales.car_id)
FROM car, customer, calendar
LEFT JOIN carsales ON DATE(calendar.datefield) = DATE(carsales.sell_time)
WHERE calendar.datefield > '2012-01-01' AND calendar.datefield < '2012-02-01'
AND car.id = carsales.car_id
AND customer.country = 'NL'
AND customer.customer_id = carsales.customer_id
GROUP BY 'Week', car.name
ORDER BY 'Week', car.name;

我错过了什么？

Answer 1

你正在以正确的方式思考......

SELECT WEEKOFYEAR(carsales.sell_time) as 'Week', car.name, COUNT(carsales.car_sale_id) 
FROM carsales 
LEFT JOIN car ON car.id=carsales.car_id 
LEFT JOIN customer ON customer.customer_id=carsales.customer_id 
WHERE carsales.sell_time BETWEEN '2012-01-01' AND '2012-02-01' 
AND customer.country = 'NL' 
GROUP BY 'Week', car.name 
ORDER BY 'Week', car.name;

Answer 2

您需要先创建结果集，以便在您期望的时间段内显示所有可能的“周”。这是基于不完全了解您的日历表的内容...它是否代表每一天？或者只是给定周的第一天。如果是这样，我将不得不调整一下。然后，针对汽车类型获得笛卡尔结果，以便每辆汽车每周都会显示。最后，我们可以LEFT JOIN（所以我们不会放弃周或汽车）到销售数据。根据您对特定国家/地区的资格，也可以向客户加入。

SUM（IF（））是为了确保您只计算有汽车销售的客户和客户，并且客户在“NL”国家/地区内。任何其他销售都将被忽略。

SELECT 
      AllWeeksAllCars.WeekNumber,
      AllWeeksAllCars.Name,
      SUM( IF( Customer.Customer_ID > 0, 1, 0 ) ) as CarSales
   FROM 
      ( select AllWeeks.*,
               Car.car_id,
               Car.Name
           from 
              ( select 
                      WEEKOFYEAR( Calendar.DateField ) as WeekNumber,
                      MIN( Calendar.DateField ) as FirstDate,
                      MAX( Calendar.DateField ) as LastDate
                   from
                      Calendar
                   where
                          Calendar.dateField > '2012-01-01'
                      AND Calendar.dateField < '2012-01-01' 
                   group by
                      `WeekNumber` ) AllWeeks,
              car
           order by
              AllWeeks.WeekNumber,
              Car.Name  ) AllWeeksAllCars

         LEFT JOIN CarSales
            on CarSales.Sell_Time between AllWeeksAllCars.FirstDate and  AllWeeksAllCars.LastDate
           AND CarSales.Car_ID =  AllWeeksAllCars.Car_ID

             LEFT JOIN Customer
                on CarSales.Customer_ID = Customer.Customer_ID
                AND Customer.Country = 'NL'
   GROUP BY
      AllWeeksAllCars.WeekNumber,
      AllWeeksAllCars.Name

Answer 3

您要查找的信息位于carsales表中。我会从那里开始并留下内部加入汽车和日历。

SQL左边连接多个表

3 个答案: