Question

I have the following sql

DECLARE @tmpSelectedData   TABLE  -- table variable
        (SlNo  INT IDENTITY(1,1) PRIMARY KEY
         ,dataID INT NULL
         ,ValID INT NULL
         ,DdrID INT NULL
         ,InrID INT NULL
         ,IprID INT NULL)

-- inserting into table variable
INSERT INTO @tmpSelectedData(dataID)  
    SELECT 
        SQ.dataID    
    FROM 
        @SelectedQuestions SQ 

    -- Update the table variable with some values

    tblData
       dataID,ValID,DdrID,InrID,IprID 
        1-    2-  3  - 4-   5
        2-    7-  4  - 5-       8
        3-    8-  2  - 4-       3
        4-    0-  1  - 2-       5

    @SelectedData  
        dataID
         2
         3
         4

    @tmpSelectedData    
        dataID,ValID,DdrID,InrID,IprID 
            2-    
            3-    
        4-    

UPDATE @tmpSelectedData   
SET IprID = D.dataID,
    DdrID = D.DdrID,
    InrID = D.InrID 
FROM tblData D 
INNER JOIN @SelectedData SD ON SD.dataID = D.dataID  

Using this query, all the rows of @tmpSelectedData will be updated with the value which is corresponding to the first row of tblData

Expected result in @tmpSelectedData:

   dataID,ValID,DdrID,InrID,IprID 
        2-    7-  4  - 5-       8
        3-    8-  2  - 4-       3
        4-    0-  1  - 2-       5

Actual result in @tmpSelectedData:

   dataID,ValID,DdrID,InrID,IprID 
        2-    7-  4  - 5-       8
        3-    7-  4  - 5-       8
        4-    7-  4  - 5-       8

it will update only with the first value

UPDATE @tmpSelectedData   
SET dataID = D.dataID,
    DdrID = D.DdrID,
    InrID = D.InrID 
FROM tblData D 
INNER JOIN @tmpSelectedData SD ON SD.IprID = D.IprID  

When I changed it to @tmpSelectedData from @SelectedData (second query), it will works and update as the expected result.

What is the difference between two queries ?

Answer 1

This is one of the issues with the UPDATE ... FROM syntax – if it, in effect, attempts to update the same row multiple times, the end result will be one of those updates, but which one is indeterminate – and it doesn’t give an error or warning message when this happens.

What you have in your first UPDATE query is a FROM clause that doesn’t reference the table to be updated at all – it’s effectively an uncorrelated query, such that the entire resultset generated by the FROM clause applies to all rows in the target table. One row (in this case, the "first", although that is ill defined) has been the row to "win" the update (it should be noted, equally, that there’s no guarantee that the same row will "win" against each row in the table being updated).

In your second query, because the table to be updated is referenced in the FROM clause:

Specifies that a table, view, or derived table source is used to provide the criteria for the update operation. For more information, see FROM (Transact-SQL).

If the object being updated is the same as the object in the FROM clause and there is only one reference to the object in the FROM clause, an object alias may or may not be specified.