问题
SQL Server does not offer the keyword USING in the context of a JOIN,
nor it provides a NATURAL JOIN.
Besides explicitly (manually) listing all the columns (link to otherwise duplicated question), is there an alternative to obtain a table in which the columns I am joining onto, which have the same name in the 2 joined tables, are not duplicated?
As an intermediate step, I have tried to save in a temporary table the result
SELECT INTO #MyTempTable * FROM tableA
INNER JOIN tableB
ON tableA.commonColumn = tableB.commonColumn;
But I already get an error:
Column names in each table must be unique. Column name 'commonColumn' in table '#MyTempTable' is specified more than once.
回答1:
No there is not. The only way to avoid this error in a SELECT INTO query is to write out your column list after SELECT, instead of SELECT *.
回答2:
The only alternative is to avoid SELECTing *:
TableA
foo | bar
---------
TableB
foo | baz
---------
When selecting * from both tables, you'll end up with 2 columns named foo, which is not allowed in a single table.
Name the selected columns using unique names, and this will work.
SELECT INTO #TableTemp t1.foo foo1, t1.bar, t2.foo foo2, t2.baz
FROM tableA t1
INNER JOIN tableB t2 ON t1.foo = t2.foo
But while you're at it, no need to insert the common column twice (as t1.foo = t2.foo). Select just one of them:
SELECT INTO #TableTemp t1.foo, t1.bar, t2.baz
FROM tableA t1
INNER JOIN tableB t2 ON t1.foo = t2.foo
EDIT: As stated by Philip Kelley, this problem only occurs when you try to save the resultset into a table. As long as you only select data, everything works fine with duplicate column names.
回答3:
I wouldn't recommend this, but in an effort to answer your question, you could create a sql clause builder function that automatically creates column aliases for your individual tables prefixed with the tablename (or customize it if you'd like):
declare @tableName nvarchar(100);
set @tableName = 'TestTable';
select @tableName + '.' + c.name + ' AS ' + @tableName + '_' + c.name
from sys.tables t
inner join sys.columns c on c.object_id = t.object_id
where
t.name = @tableName
Output for my little 2-column table
TestTable.TestColumn1 AS TestTable_TestColumn1
TestTable.TestColumn2 AS TestTable_TestColumn2
Hypothetical usage
declare @sql nvarchar(1000) = '';
set @sql = myClauseBuilder('TestTable');
set @sql = @sql + ',' + myClauseBuilder('TestTable2');
set @sql = @sql + ' FROM TestTable INNER JOIN TestTable2....(etc.)
exec @sql
回答4:
It is not easy. But it is possible. Developing from this, you would first create two separated lists with all the columns of the 2 tables, excluding the common column on which you want to join:
DECLARE @columnsA varchar(8000)
SELECT @columnsA = ISNULL(@columnsA + ', ','') + QUOTENAME(column_name)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'tableA' AND COLUMN_NAME <> 'commonColumn'
ORDER BY ORDINAL_POSITION
DECLARE @columnsB varchar(8000)
SELECT @columnsB = ISNULL(@columnsB + ', ','') + QUOTENAME(column_name)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'tableB' AND COLUMN_NAME <> 'commonColumn'
ORDER BY ORDINAL_POSITION
Then you would use them for your query, selecting the commonColumn only for one of the table:
EXEC ('SELECT tableA.commonColumn, ' + @columnA + ', ' + @columnsB + ' FROM tableA INNER JOIN tableB ON tableA.commonColumn = tableB.commonColumn;')
So there's at least one way to do it. :) It is apparently also moderately efficient. I am not an SQL expert, but I suppose there is a way to create a function out of this, maybe one function to "select all columns but [...]" and one function that would do the join as USING would do.
It becomes a little bit simpler if in listing we add also the containing table. In this way we need to extract only the column names from one table.
DECLARE @columnsB varchar(8000)
SELECT @columnsB = ISNULL(@columnsB + ', ','') + QUOTENAME(table_name) + '.' + QUOTENAME(column_name)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'tableB' AND COLUMN_NAME <> 'commonColumn'
ORDER BY ORDINAL_POSITION
And the query is modified into:
EXEC ('SELECT tableA.* ', ' + @columnsB + ' FROM tableA INNER JOIN tableB ON tableA.commonColumn = tableB.commonColumn;')
来源：https://stackoverflow.com/questions/32481055/alternatives-to-using-in-a-join-for-sql-server-for-avoiding-duplicated-columns-i