带有条件的多个表的 mySQL Left Join

Question

我有一个问题。我有 4 张 table :

Invoice_Payment、发票、客户和日历

基本上，我有以下查询，效果很好，除了没有 date_due 的月份不会返回。即仅返回包含 date_due 的月份。

注意:日历表仅列出一年中的每一天。它包含一个 col 调用 date_field

数据库:http://oberto.co.nz/jInvoice.txt

预期输出:我当前的查询(如下)将返回类似以下内容的内容:

month       total
August      5
September   0
October  196
November  205
December  214
January  229

注意到九月没有返回吗？这是因为表Invoice_Payment没有date_due记录

我想我必须使用左连接并使用以下内容连接日历表:LEFT JOIN 日历 ON Invoice_Payments.date_paid = Calendar.date_field。但我运气不好

SELECT MONTHNAME(Invoice_Payments.date_paid) as month, SUM(Invoice_Payments.paid_amount) AS total
FROM Invoice, Client, Invoice_Payments
WHERE Client.registered_id = 1
AND Client.id = Invoice.client_id
And Invoice.id = Invoice_Payments.invoice_id
AND date_paid IS NOT NULL
GROUP BY YEAR(Invoice_Payments.date_paid), MONTH(Invoice_Payments.date_paid)

感谢任何帮助。

Answer 1

听起来您正在尝试查找某个范围内所有日期的值，无论是否有值。假设我们有一个日历表，其结构如下:

Create Table Calendar
(
    [Date] not null Primary Key
)

您的查询可能如下所示(其中 X 和 Y 代表您正在调查的范围的开始日期和结束日期):

Select Year(C.Date), MonthName(C.Date) As Month
    , Coalesce(Sum(IP.paid_amount),0) As Total
From Calendar As C
    Left Join (Invoice As I
        Join Client As C1
            On C1.id = I.client_id
                And C.registered_id = 1
        Join Invoice_Payments As IP
            On IP.Invoice_id = I.Id)
        On IP.date_paid = C.date
Where C.Date Between X and Y
Group By Year(C.Date), MonthName(C.Date)

从技术上讲，上面的查询应该可以解决问题。但是，另一种选择是使用您在评论中查询的派生表:

Select Year(C.Date), MonthName(C.Date) As Month
    , Coalesce(Sum(Z.paid_amount),0) As Total
From Calendar As C
    Left Join   (
                Select IP.date_paid, IP.paid_amount
                From Invoice As I
                        Join Client As C1
                            On C1.id = I.client_id
                                And C.registered_id = 1
                        Join Invoice_Payments As IP
                            On IP.Invoice_id = I.Id
                ) As Z
        On Z.date_paid = C.date
Where C.Date Between X and Y
Group By Year(C.Date), MonthName(C.Date)