Need to get multiple value from a table in the left join - sql

Question

I have a table having data as below.
Say I have two versions of the project and I need to migrate data from older version to a new version.
Let's say tblFolders in version1
+----+------------+--------------+--------------+
| id | FolderName | CreatedBy | ModifiedBy |
+----+------------+--------------+--------------+
| 1 | SIMPLE | 5 | 6 |
| 2 | SIMPLE1 | 8 | 1 |
+----+------------+--------------+--------------+
And another table having userid of both versions.
Let's say its tblUsersMapping
+----+----------------+-------------------+
| id | Version1UserID | Version2UserID |
+----+----------------+-------------------+
| 1 | 1 | 500 |
| 2 | 2 | 465 |
| 3 | 3 | 12 |
| 4 | 4 | 85 |
| 5 | 5 | 321 |
| 6 | 6 | 21 |
| 7 | 7 | 44 |
| 8 | 8 | 884 |
+----+----------------+-------------------+
Now I need to transfer data from version 1 to version 2. When I transferring data, CreatedBy and Modifiedby ids should by of the new version.
So though I have data as below
| 1 | SIMPLE | 5 | 6 |
It should be transferred as below
| 1 | SIMPLE | 321 | 21 |
For that, I have added a join so far between these two tables as below.
SELECT id,
foldername,
B.version2userid AS CreatedBy
FROM tblfolders A WITH(nolock)
LEFT JOIN tblusersmapping B WITH(nolock)
ON A.createdby = B.version1userid
This would give me a proper result for column CreatedBy.
But how can I get userid from tblUsersMapping for ModifiedBy column?
Doing below will not work and will give NULL for both the columns.
SELECT id,
foldername,
b.version2userid AS createdby,
b.version2userid AS modifiedby
FROM tblfolders A WITH(nolock)
LEFT JOIN tblusersmapping B WITH(nolock)
ON a.createdby = b.version1userid,
a.modifiedby = b.version1userid
One way is I can add another join with tblusersmapping table. But its not a good idea because tables can have a huge data and another join will affect the performance of the query.
My question is how can I get Version1UserID and Version2UserID from mapping table based on createdby and modifiedby columns?

Answer 1

You can use multiple select which may help you.
SELECT id,
foldername,
(SELECT version2userid from tblUsersMapping where Version1UserID=tblfolders.CreatedBy) AS CreatedBy,
(SELECT version2userid from tblUsersMapping where Version1UserID=tblfolders.ModifiedBy) AS ModifiedBy
FROM tblfolders

Answer 2

If you want to populate both the column where each column joins to to a different row, in that case you have to join the same table twice like following. You can't get it with a single table join the way you are expecting.
SELECT id,
foldername,
B.version2userid AS CreatedBy
C.Version2UserID AS ModifiedBy
FROM tblfolders A WITH(nolock)
LEFT JOIN tblusersmapping B WITH(nolock)
ON A.createdby = B.version1userid
LEFT JOIN tblusersmapping C WITH(nolock)
ON A.ModifiedBy = C.version1userid

Answer 3

Try this, it will work across all sample data,
select tf.id,tf.FolderName
,oa.Version2UserID as CreatedBy
,oa1.Version2UserID as ModifiedBy
from #tblFolders tf
outer apply(select top 1 Version2UserID
from #tblUsersMapping tu
where tu.Version1UserID= tf.CreatedBy order by id desc)oa
outer apply(select top 1 Version2UserID
from #tblUsersMapping tu
where tu.Version1UserID= tf.ModifiedBy order by id desc)oa1

Answer 4

You can use UDF to return modifiedby and INNER JOIN instead of LEFT JOIN (if requirement meets) as below. I think it will help in the preformance
CREATE TABLE tblFolders (id INT, folderName VARCHAR(20), createdBy INT, modifiedBy INT)
INSERT INTO tblFolders VALUES
(1,'SIMPLE', 5,6),
(2,'SIMPLE1', 8,1)
CREATE TABLE tblUsersMapping(id INT, Version1UserID INT, Version2UserID INT)
INSERT INTO tblUsersMapping VALUES
(1,1,500),
(2,2,465),
(3,3,12),
(4,4,85),
(5,5,321),
(6,6,21),
(7,7,44),
(8,8,884)
SELECT a.id,
a.foldername,
b.version2userid AS createdby,
dbo.FNAReturnModifiedBy(a.modifiedBy) AS modifiedby
FROM tblfolders A WITH(nolock)
INNER JOIN tblusersmapping B WITH(nolock) ON a.createdby = b.version1userid
--Function
IF OBJECT_ID(N'dbo.FNAReturnModifiedBy', N'FN') IS NOT NULL
DROP FUNCTION dbo.FNAReturnModifiedBy
GO
CREATE FUNCTION dbo.FNAReturnModifiedBy(#updated_by INT)
RETURNS INT AS
BEGIN
DECLARE #updateUserID INT
SELECT #updateUserID = Version2UserID
FROM tblusersmapping WHERE Version1UserID = #updated_by
RETURN #updateUserID
END
OUTPUT:
id foldername createdby modifiedby
1 SIMPLE 321 21
2 SIMPLE1 884 500

Answer 5

Note :
I did not know about how to find the query performance. I wrote only for your expected output.
I am using SQL Server 2012.
I did not use more than one Join.
Query did JOIN, GROUP BY, ROW_NUMBER (), CASE instead of two LEFT JOIN
Input :
create table ##ver (id int, FolderName varchar (10), CreatedBy int, ModifiedBy int)
insert into ##ver values
(1,'SIMPLE',5,6)
,(2,'SIMPLE1',8,1)
,(3,'File',7, 5)
select * from ##ver
create table ##veruser (id int, Version1UserID int, Version2UserID int)
insert into ##veruser values
(1 , 1 , 500)
,(2 , 2 , 465)
,(3 , 3 , 12 )
,(4 , 4 , 85 )
,(5 , 5 , 321)
,(6 , 6 , 21 )
,(7 , 7 , 44 )
,(8 , 8 , 884)
select * from ##veruser
Query :
select
id, FolderName
, max (case when rn = 1 then Version2UserID end) Version1UserID
, max (case when rn = 2 then Version2UserID end) Version2UserID
from (
select
v.id, v.FolderName, u.Version1UserID, u.Version2UserID
, ROW_NUMBER () over
(partition by v.id order by v.id, v.CreatedBy,
case
when v.CreatedBy > v.ModifiedBy then u.Version1UserID
end desc
) rn
, v.CreatedBy, v.ModifiedBy
from ##ver v
join ##veruser u
on u.Version1UserID in (v.CreatedBy, v.ModifiedBy)
) a
group by id, FolderName
order by id
Update 1:
Query does :
Join the tables.
Row numbering, over (),
Partition by Id.
Order by File id (v.id), Creator id ascending, If creator id greater
than modified id, then creator id descending. (Due to second step this reordering is must)
Depends on 'rn' values, rows are transfer to columns
(You can find many examples at here)
Output :
id FolderName Version1UserID Version2UserID
1 SIMPLE 321 21
2 SIMPLE1 884 500
3 File 44 321

Answer 6

Try this one.
Select a.id,folderName,b.Version2UserId as createdby,c.Version2UserId as modifiedby
from tblFolders as a WITH(nolock)
inner join tblUsersMapping as b WITH(nolock) on a.createdby =b .Version1UserID
inner join tblUsersMapping as c WITH(nolock) on a.modifiedBy =c .Version1UserID