sql - 使用 Coalesce 时 '.' 附近的语法不正确

Question

我正在尝试连接三个表，我成功连接了两个。

这有效(两个表)

SELECT  ImportExportBadFile.ID, ImportExportSettingID,
    ImportExportBadFile.UserID,
    Coalesce(UserName,'') UserName  
    FROM
    ImportExportBadFile 
    LEFT OUTER JOIN Users HGSQLUsers ON ImportExportBadFile.UserID = HGSQLUsers.ID
    ORDER BY  ImportExportBadFile.DateTimeStamp DESC

• UserName属于Users表

现在，当我尝试添加另一个表时:

SELECT  ImportExportBadFile.ID, ImportExportSettingID,
    SETT.Name,
    ImportExportBadFile.UserID,
    Coalesce(UserName,'') HGSQLUsers.UserName  
    FROM
    ImportExportBadFile 
    LEFT OUTER JOIN Users HGSQLUsers ON ImportExportBadFile.UserID = HGSQLUsers.ID
    INNER JOIN ImportExportSettings SETT ON ImportExportBadFile.ImportExportSettingID = SETT.ID  
    ORDER BY  ImportExportBadFile.DateTimeStamp DESC

It gives Error : "Incorrect syntax near '.'." at line : "Coalesce(UserName,'') HGSQLUsers.UserName"

如果我只写 UserName 它给出了

"Ambiguous column name 'UserName'."

帮助。

Answer 1

可能 UserName 列存在于两个表中，您需要指定要从哪个表中获取 UserName，如下所示

SELECT  ImportExportBadFile.ID, ImportExportSettingID,
    SETT.Name,
    ImportExportBadFile.UserID,
    Coalesce( HGSQLUsers.UserName  ,'') as "HGSQLUsers.UserName"
    FROM
    ImportExportBadFile 
    LEFT OUTER JOIN Users HGSQLUsers ON ImportExportBadFile.UserID = HGSQLUsers.ID
    INNER JOIN ImportExportSettings SETT ON ImportExportBadFile.ImportExportSettingID = SETT.ID
ORDER BY  ImportExportBadFile.DateTimeStamp DESC

错误 “'.' 附近的语法不正确。”在行: 来自别名 HGSQLUsers.UserName 你可以使用它，但在配额中。

我还建议您为每个表使用别名，如下所示 - 对我来说更具可读性

SELECT  I.ID, I.ImportExportSettingID,
        SETT.Name,
        I.UserID,
       Coalesce( HGSQLUsers .UserName  ,'') as "HGSQLUsers.UserName"
FROM ImportExportBadFile I
LEFT OUTER JOIN Users HGSQLUsers ON I.UserID = HGSQLUsers.ID
INNER JOIN ImportExportSettings SETT ON I.ImportExportSettingID = SETT.ID  
ORDER BY  I.DateTimeStamp DESC