带有条件的多个表上的 mySQL 左连接

Question

我有个问题。我有4张桌子：

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。IE 只会返回带有 date_due 的月份。

注意：日历表只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：下面我当前的查询将返回如下内容：

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

请注意九月没有返回？这是因为表 Invoice_Payment 没有 date_due 记录

我想我必须使用左联接并加入日历表，例如：LEFT JOIN Calendar ON Invoice_Payments.date_paid = Calendar.date_field。但我没有运气

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)

任何帮助表示赞赏。

Answer 1

听起来您正在尝试查找范围内所有日期的值，而不管是否有值。假设我们有一个结构如下的日历表：

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

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

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)