在mySQL查询中使用条件

我需要在mySQL查询中根据查询结果设置一个条件

SELECT

返回我

IVC\U接收人

，我想用这个数据来设定条件。但它不起作用

问题是：

SELECT 
    KDX_Id, 
    KDX_Name, 
    KDX_FirstName, 
    KDX_Company, 
    BOO_Id, 
    DEB_Date, 
    IFNULL(SUM(IVC_Sum + IVC_Taxes), 0) AS DEB_Total,
    DATEDIFF('2018-07-10', DEB_Date) AS DEB_Nb_Days,
    IVC_Recipient 
FROM ___Debtors 
JOIN ___Invoices 
    ON ___Invoices.IVC_Id = ___Debtors.DEB_InvoiceId 
JOIN ___Bookings 
    ON ___Bookings.BOO_Id = ___Invoices.IVC_BookingId


IF(IVC_Recipient = 'guest') {
    JOIN ___Kardex 
        ON ___Kardex.KDX_Id = ___Bookings.BOO_GuestId
}

IF(IVC_Recipient = 'company') {
    JOIN ___Kardex 
        ON ___Kardex.KDX_Id = ___Bookings.BOO_CompanyId
}    

请问我这里缺什么

---

谢谢。

您可以使用

CASE

表达式：

SELECT 
    KDX_Id, 
    KDX_Name, 
    KDX_FirstName, 
    KDX_Company, 
    BOO_Id, 
    DEB_Date, 
    IFNULL(SUM(IVC_Sum + IVC_Taxes), 0) AS DEB_Total,  -- there is no GROUP BY
    DATEDIFF('2018-07-10', DEB_Date) AS DEB_Nb_Days,
    IVC_Recipient 
FROM ___Debtors 
JOIN ___Invoices 
    ON ___Invoices.IVC_Id = ___Debtors.DEB_InvoiceId 
JOIN ___Bookings 
    ON ___Bookings.BOO_Id = ___Invoices.IVC_BookingId
JOIN ___Kardex 
        ON ___Kardex.KDX_Id = (CASE 
             WHEN IVC_Recipient = 'guest' THEN  ___Bookings.BOO_GuestId
             WHEN IVC_Recipient = 'company' THEN ___Bookings.BOO_CompanyId
             END)

在这些情况下，几乎任何“如果”都可以表示为和：

JOIN ___Kardex 
    ON (IVC_Recipient = 'guest' AND ___Kardex.KDX_Id = ___Bookings.BOO_GuestId)
    OR (IVC_Recipient = 'company' AND ___Kardex.KDX_Id = ___Bookings.BOO_CompanyId)

…但是，根据索引使用的工作方式，有时甚至可以更简单地连接并在WHERE条件下对结果进行排序（尽管这是我最后的手段“我们可以通过这种方式获得更快的速度”尝试）

JOIN ___Kardex 
    ON  ___Kardex.KDX_Id IN (___Bookings.BOO_GuestId, ___Bookings.BOO_CompanyId)
WHERE (IVC_Recipient = 'guest' AND ___Kardex.KDX_Id = ___Bookings.BOO_GuestId)
    OR (IVC_Recipient = 'company' AND ___Kardex.KDX_Id = ___Bookings.BOO_CompanyId)