SQL query SELECT FROM [value from column of anothe

问题:

I have a table X where a trigger will insert a row when there's a changes to some tables. I've inserted the table name into table X.

Now, I would like to select the data from table X while inner join with the actual table itself. Is it possible by using a value from a column of the select table as the table for inner join?

The query should looks something like this

SELECT X.a, Y.b, Y.c FROM X
INNER JOIN [X.TableName] AS Y ON Y.ID = X.ID

回答1:

Executing

select 'SELECT X.a, Y.b, Y.c FROM X
INNER JOIN [' + X.TableName + '] AS Y ON X.ID = Y.ID 
where x.primarykey =' + x.primarykey from x

Will output a series of sql statements like

SELECT X.a, Y.b, Y.c FROM X
INNER JOIN [ customer ] AS Y ON X.ID = Y.ID
where x.primarykey = 1234

that you can then execute "sql to build sql" if you will.

回答2:

No, that is not possible. You can't use values as table names directly in a query, and you can't join each record against a different table.

You would have to make the join for a single record, and create the query dynamically to use a value as table name:

declare @name varchar(50)
set @name = select TableName from X where ID = 42
exec('select X.a, Y.b, Y.c from X innner join ' + @name + ' as Y on Y.DI = X.ID where X.ID = 42')

回答3:

With dynamic query:

DECLARE @table AS NVARCHAR(128);
DECLARE @sql NVARCHAR(4000);

-- of course you'll have to add your WHERE clause here 
SELECT @table = TableName FROM X;
SET @sql = 'SELECT X.a, Y.b, Y.c FROM X INNER JOIN '+@table+' AS Y ON Y.ID = X.ID';

EXEC(@sql);