Question

Sorry for the title, perhaps it’s not very clear.

I have some SQL queries in a script that depend on each other.
The script uses a temporary table in which the data is inserted (the #temp_data table).

This is the expected output:

___________________________________
| speed1 |  speed2   |  distance   |  
|   1    |   NULL    |     10      |  
|   3    |   NULL    |     40      |
|   5    |   NULL    |     90      |
|  NULL  |    1      |     10      |
|  NULL  |    3      |     40      |
|  NULL  |    5      |     90      |

Here is the query structure (I didn’t include the actual query since it’s too big):

-- First group
queryForSpeed1
queryToUpdateDistanceBasedOnSpeed1

-- Second group 
queryForSpeed2
queryToUpdateDistanceBasedOnSpeed2

If I run the first group of queries (queryForSpeed1 and queryToUpdateDistanceBasedOnSpeed1) separately from the second group then I get the expected output: only the speed1 and distance columns contain data:

___________________________________
| speed1 |  speed2   |  distance   |  
|   1    |   NULL    |     10      |  
|   3    |   NULL    |     40      |
|   5    |   NULL    |     90      |
|  NULL  |   NULL    |    NULL     |
|  NULL  |   NULL    |    NULL     |
|  NULL  |   NULL    |    NULL     |

The same happens when I run the second group:

___________________________________
| speed1 |  speed2   |  distance   |  
|  NULL  |   NULL    |    NULL     |  
|  NULL  |   NULL    |    NULL     |
|  NULL  |   NULL    |    NULL     |
|  NULL  |    1      |     10      |
|  NULL  |    2      |     40      |
|  NULL  |    3      |     90      |

BUT, when I run both groups: all the distances are NULL:

___________________________________
| speed1 |  speed2   |  distance   |  
|   1    |   NULL    |    NULL     |  
|   3    |   NULL    |    NULL     |
|   5    |   NULL    |    NULL     |
|  NULL  |    1      |    NULL     |
|  NULL  |    2      |    NULL     |
|  NULL  |    3      |    NULL     |

I believe this is somehow related to transaction management and temporary tables, although I wasn’t able to find anything relevant to solve the problem on Google.

From what I’ve read, SQL Server keeps a transaction log where it stores every update, insert and whatever… when it arrives at the end of the script it actually does all those insertions and updates.

So the update I did for the distance column finds all the speeds as being NULL because the data wasn’t yet inserted in the temporary table from the previous updates, but at the end of the query the speeds are inserted in the table so that’s why they are visible.

I played a bit with the GO statement to execute my script in batches, but no luck so far…

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What am I doing wrong? Can someone point me in the right direction, please?

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EDIT

Here is the actual query.

Answer 1

The problem is not related to transactions, but rather to the way you conduct updates to #temp_speed_profile. The second pass through #temp_speed_profile retrieves all six records. Speed_new is null in first record of Voyage_Id, consequently @distance becomes null. As you retain the value of @distance in next turn, it remains null.

Problem goes away when using different temporary tables because second pass works on second set of data only.

A note on cursors – when defining one make sure to add local and fast_forward. Local because it is limiting cursors’ scope, and fast_forward to optimize fetches.