Comments on: How do to update rows without a cursor?

Question: How do to update rows without a cursor?

apark — Wed, 24 Jan 2007 13:02:04 -0800

How do I update rows in a table in a SQL*Server 2005 database with data from another row in the same table?

I have an event log type table that contains data as follows:
EVENT_TYPE START_TIME END_TIME
type1 10
type3 4
type2 5
type1 11
type1 15
type2 7
type2 9
type3 15
type1 23

For whatever reason, it turns out that I need to create a column called END_TIME and populate that with the BEGIN_TIME of the next event with the same EVENT_TYPE.

So, I would need to end up with

EVENT_TYPE START_TIME END_TIME
type1 10 11
type3 4 15
type2 5 7
type1 11 15
type1 15 23
type2 7 9
type2 9 x
type3 15 x
type1 23 x

So, if you see the first "type1" event, the END_TIME is the START_TIME of the next "type1" event which is 11.

Now, I suppose I can use a cursor to do this, but this DB has 10s of millions of rows and it's likely to take days, if not weeks to do this using a cursor. I've been Googling around and I see that some kind of Set base approach performs much better than a Cursor based approach.

My SQL-fu fails me here. Can you help?

FYI, I'm using SQL*Server 2005. I'm pertty weak at T-SQL so,... TIA!

By: Doofus Magoo

Doofus Magoo — Wed, 24 Jan 2007 13:21:44 -0800

A subquery should do the trick, but I'm not sure what the performance would be like. I tested this using your sample dataset, and it seemed to work fine:

UPDATE [tablename]

SET END_TIME=

(

SELECT MIN(START_TIME) FROM [tablename] b

WHERE event_type=b.event_type

AND b.START_TIME>tablename.START_TIME

)

By: Doofus Magoo

Doofus Magoo — Wed, 24 Jan 2007 13:26:23 -0800

Whoops, scratch that. It's updating them absolutely, not based on EVENT_TYPE. It threw me when I was looking at the results because all of the type1 come before all of the type2, which come before all of the type3. Sorry about the confusion.

By: Doofus Magoo

Doofus Magoo — Wed, 24 Jan 2007 13:27:45 -0800

This should do the trick. It's the same as my first post, but I explicitly reference the table in the outer part of the nested query:

UPDATE [tablename]

SET END_TIME=

(

SELECT MIN(START_TIME) FROM [tablename] b

WHERE tablename.event_type=b.event_type

AND b.START_TIME>tablename.START_TIME

)

By: apark

apark — Wed, 24 Jan 2007 13:53:52 -0800

Thanks Doofus Magoo. Looks plausible to me. I just kicked it off on the server. Let's see what happens!

By: apark

apark — Wed, 24 Jan 2007 15:01:39 -0800

Doofus Magoo, you're the man. It worked like a charm. Thank you!