MySQL 选择列（如果存在）

Question

我进行了以下查询，但是当我尝试再选择一列时，我没有得到任何结果。我只是不知道如何选择该列（if 语句是有问题的）。

问题是cash_payment_detail 表中的sub_acc 列也可以是null。任何帮助都意义重大。请注意，我还在学习中。

SELECT
     LPAD(cash_payments_detail.`control_acc`,5,0) AS Control_Account,
     CAccountDescription As Control_Account_Name,
     LPAD(cash_payments_detail.`sub_acc`,4,0) AS Sub_Account,
     **if (coasub.`SAccountNo` = cash_payments_detail.`sub_acc`, coasub.`SAccountDescription`," ") As Sub_Account_Name**
FROM
    coasub, coacontrol, `cash_payments_detail` cash_payments_detail INNER JOIN `cash_payments` cash_payments ON cash_payments_detail.`cpvno` = cash_payments.`cpvno`
WHERE
control_acc = coacontrol.`CAccountNo`

Answer 1

您的问题在于您是否必须在编写此类查询之前很好地学习连接，并注意您编写的连接在以下情况下不会返回记录 cash_payment_detail.sub_acc 为空 左连接将返回空值；） 使用 left Join 而不是 inner join... 试试这个查询：

SELECT
cash_payments.cpvno AS Voucher_Number, date_format(cash_payments.vdate,'%d %M %Y') AS Voucher_Date,
sec_users.name AS Entered_By, cash_payments.payto AS Payment_To, cash_payments.payref AS On_Account_Of, LPAD(cash_payments.branch,5,0) AS Cash_Control_Account, branches.cash_control_name AS Cash_Control_Name,
branches.name AS Branch_Name,
LPAD(cash_payments_detail.control_acc,5,0) AS Control_Account, CAccountDescription As Control_Account_Name,
LPAD(cash_payments_detail.sub_acc,4,0) AS Sub_Account, if (SAccountNo = sub_acc, SAccountDescription," ") As Sub_Account_Name,
cash_payments_detail.job_no AS Job, cash_payments_detail.narration AS Narration, cash_payments_detail.debit AS Debit
FROM
coasub, coacontrol, branches, sec_users, cash_payments_detail cash_payments_detail 
LEFT JOIN cash_payments cash_payments ON cash_payments_detail.cpvno = cash_payments.cpvno 
WHERE
control_acc = coacontrol.CAccountNo AND cashcontrol = cash_payments.branch AND cash_payments.login = sec_users.login