我想在大查询中加入两个具有公共列的表？答案

【问题标题】：I want to join two tables with a common column in Big query?我想在大查询中加入两个具有公共列的表？
【发布时间】：2018-01-09 19:42:03
【问题描述】：

要加入表格，我使用以下查询。

SELECT *
FROM(select user as uservalue1 FROM [projectname.FullData_Edited]) as FullData_Edited 
JOIN (select user as uservalue2 FROM [projectname.InstallDate]) as InstallDate 
ON FullData_Edited.uservalue1=InstallDate.uservalue2;

查询有效，但连接表只有两列 uservalue1 和 uservalue2。我想保留两个表中的所有列。知道如何实现吗？

【问题讨论】：

标签： sql join google-bigquery legacy-sql

【解决方案1】：

#legacySQL
SELECT <list of fields to output>
FROM [projectname:datasetname.FullData_Edited] AS FullData_Edited
JOIN [projectname:datasetname.InstallDate] AS InstallDate
ON FullData_Edited.user = InstallDate.user

或（更可取的）

#standardSQL
SELECT <list of fields to output>
FROM `projectname.datasetname.FullData_Edited` AS FullData_Edited
JOIN `projectname.datasetname.InstallDate` AS InstallDate
ON FullData_Edited.user = InstallDate.user

注意，在这种情况下使用SELECT * 会导致Ambiguous column name 错误，因此最好在输出中明确列出您需要的列/字段

解决方法是使用USING() 语法，如下例所示。
假设 user 是唯一的模棱两可的字段 - 它可以解决问题

#standardSQL
SELECT *
FROM `projectname.datasetname.FullData_Edited` AS FullData_Edited
JOIN `projectname.datasetname.InstallDate` AS InstallDate
USING (user)

例如：

#standardSQL
WITH `projectname.datasetname.FullData_Edited` AS (
  SELECT 1 user, 'a' field1
),
`projectname.datasetname.InstallDate` AS (
  SELECT 1 user, 'b' field2
)
SELECT *
FROM `projectname.datasetname.FullData_Edited` AS FullData_Edited
JOIN `projectname.datasetname.InstallDate` AS InstallDate
USING (user)

user    field1  field2   
1       a       b

而使用ON FullData_Edited.user = InstallDate.user 会出现以下错误

Error: Duplicate column names in the result are not supported. Found duplicate(s): user

【讨论】：

如果它不是唯一的模棱两可的文件呢？

【解决方案2】：

如果您想要所有列，请不要使用子查询：

SELECT *
FROM [projectname.FullData_Edited] as FullData_Edited JOIN
     [projectname.InstallDate] as InstallDate 
     ON FullData_Edited.uservalue1 = InstallDate.uservalue2;

您可能必须列出要避免列名重复的特定列。

当您使用它时，您还应该切换到标准 SQL。

【讨论】：

@VSR 。 . .请注意答案中的部分内容：“您可能必须列出要避免列名重复的特定列。”