Updating Uncommitted data to a cell with in an UPDATE statement - sql

Question

I want to convert a table storing in Name-Value pair data to relational form in SQL Server 2008.
Source table
Strings
ID Type String
100 1 John
100 2 Milton
101 1 Johny
101 2 Gaddar
Target required
Customers
ID FirstName LastName
100 John Milton
101 Johny Gaddar
I am following the strategy given below,
Populate the Customer table with ID values in Strings Table
INSERT INTO CUSTOMERS SELECT DISTINCT ID FROM Strings
You get the following
Customers
ID FirstName LastName
100 NULL NULL
101 NULL NULL
Update Customers with the rest of the attributes by joining it to Strings using ID column. This way each record in Customers will have corresponding 2 matching records.
UPDATE Customers
SET FirstName = (CASE WHEN S.Type=1 THEN S.String ELSE FirstName)
LastName = (CASE WHEN S.Type=2 THEN S.String ELSE LastName)
FROM Customers
INNER JOIN Strings ON Customers.ID=Strings.ID
An intermediate state will be llike,
ID FirstName LastName ID Type String
100 John NULL 100 1 John
100 NULL Milton 100 2 Milton
101 Johny NULL 101 1 Johny
101 NULL Gaddar 101 2 Gaddar
But this is not working as expected. Because when assigning the values in the SET clause it is setting only the committed values instead of the uncommitted. Is there anyway to set uncommitted values (with in the processing time of query) in UPDATE statement?
PS: I am not looking for alternate solutions but make my approach work by telling SQL Server to use uncommitted data for UPDATE.

Answer 1

The easiest way to do it would be to split the update into two:
UPDATE Customers
SET FirstName = Strings.String
FROM Customers
INNER JOIN Strings ON Customers.ID=Strings.ID AND Strings.Type = 1
And then:
UPDATE Customers
SET LastName = Strings.String
FROM Customers
INNER JOIN Strings ON Customers.ID=Strings.ID AND Strings.Type = 2
There are probably ways to do it in one query such as a derived table, but unless that's a specific requirement I'd just use this approach.

Answer 2

Have a look at this, it should avoid all the steps you had
DECLARE #Table TABLE(
ID INT,
Type INT,
String VARCHAR(50)
)
INSERT INTO #Table (ID,[Type],String) SELECT 100 ,1 ,'John'
INSERT INTO #Table (ID,[Type],String) SELECT 100 ,2 ,'Milton'
INSERT INTO #Table (ID,[Type],String) SELECT 101 ,1 ,'Johny'
INSERT INTO #Table (ID,[Type],String) SELECT 101 ,2 ,'Gaddar'
SELECT IDs.ID,
tName.String NAME,
tSur.String Surname
FROM (
SELECT DISTINCT ID
FROM #Table
) IDs LEFT JOIN
#Table tName ON IDs.ID = tName.ID AND tName.[Type] = 1 LEFT JOIN
#Table tSur ON IDs.ID = tSur.ID AND tSur.[Type] = 2
OK, i do not think that you will find a solution to what you are looking for. From UPDATE (Transact-SQL) it states
Using UPDATE with the FROM Clause
The results of an UPDATE statement are
undefined if the statement includes a
FROM clause that is not specified in
such a way that only one value is
available for each column occurrence
that is updated, that is if the UPDATE
statement is not deterministic. For
example, in the UPDATE statement in
the following script, both rows in
Table1 meet the qualifications of the
FROM clause in the UPDATE statement;
but it is undefined which row from
Table1 is used to update the row in
Table2.
USE AdventureWorks;
GO
IF OBJECT_ID ('dbo.Table1', 'U') IS NOT NULL
DROP TABLE dbo.Table1;
GO
IF OBJECT_ID ('dbo.Table2', 'U') IS NOT NULL
DROP TABLE dbo.Table2;
GO
CREATE TABLE dbo.Table1
(ColA int NOT NULL, ColB decimal(10,3) NOT NULL);
GO
CREATE TABLE dbo.Table2
(ColA int PRIMARY KEY NOT NULL, ColB decimal(10,3) NOT NULL);
GO
INSERT INTO dbo.Table1 VALUES(1, 10.0), (1, 20.0), (1, 0.0);
GO
UPDATE dbo.Table2
SET dbo.Table2.ColB = dbo.Table2.ColB + dbo.Table1.ColB
FROM dbo.Table2
INNER JOIN dbo.Table1
ON (dbo.Table2.ColA = dbo.Table1.ColA);
GO
SELECT ColA, ColB
FROM dbo.Table2;

Answer 3

Astander is correct (I am accepting his answer). The update is not happening because of a read UNCOMMITTED issue but because of the multiple rows returned by the JOIN. I have verified this. UPDATE picks only the first row generated from the multiple records to update the original table. This is the behavior for MSSQL, Sybase and such RDMBMSs but Oracle does not allow this kind of an update an d it throws an error. I have verified this thing for MSSQL.
And again MSSQL does not support updating a cell with UNCOMMITTED data. Don't know the status with other RDBMSs. And I have no idea if anyRDBMS provides with in the query ISOLATION level management.
An alternate solution will be to do it in two steps, Aggregate to unpivot and then insert. This has lesser scans compared to methods given in above answers.
INSERT INTO Customers
SELECT
ID
,MAX(CASE WHEN Type = 1 THEN String ELSE NULL END) AS FirstName
,MAX(CASE WHEN Type = 2 THEN String ELSE NULL END) AS LastName
FROM Strings
GROUP BY ID
Thanks to my friend Roji Thomas for helping me with this.