Question

I am looking at this query and I am kinda confused by a few things.

-- T-SQL large update table
USE tempdb;
SELECT * INTO SOD
FROM AdventureWorks2008.Sales.SalesOrderDetail
GO
--(121317 row(s) affected)

-- SQL update in batches of 10,000
WHILE (2 > 1)
  BEGIN
    BEGIN TRANSACTION
    UPDATE TOP ( 10000 ) SOD
    SET    UnitPriceDiscount = 0.08,
           ModifiedDate = CONVERT(DATETIME,CONVERT(CHAR(10),getdate(),112))
    WHERE  ModifiedDate < CONVERT(DATETIME,CONVERT(CHAR(10),getdate(),112))

    IF @@ROWCOUNT = 0
      BEGIN
        COMMIT TRANSACTION
         BREAK
      END
    COMMIT TRANSACTION
    -- 1 second delay
    WAITFOR DELAY '00:00:01'
  END -- WHILE
GO

/* Messages

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(10000 row(s) affected)

(1317 row(s) affected)

(0 row(s) affected)
*/
-- Cleanup
DROP TABLE SOD
GO

------------

1. Is it making a temp table? SELECT * INTO SOD if so is this really needed? Can I just use the while part and below?
2. How does IF @@ROWCOUNT = 0 ever become zero? Does it do some self incrementing or something?

Edit

This is what I have now but I still think there is an endless loop or something

BEGIN TRAN

declare
  @rows_updated int ,
  @rowCount int,
  @batch_size   int
  set @rows_updated = -1
  set @batch_size   = 10000
  set @rowCount = 0;

Declare @xx VARCHAR(20) DECLARE @length INT
SET @length = 17 SET @xx = 'XXXXXXXXXXXXXXXX'

while ( @rows_updated != 0 )
  begin
    update top(@batch_size)  myTbl
    SET myNumber = SUBSTRING(@xx, 0, @length - len(RIGHT(myNumber, 4))) + RIGHT(myNumber, 4) 
    WHERE myDate <'2011-Jan-02' 
    set @rows_updated = @@rowcount
   set @rowCount += 10000
    print @rowCount
  end

ROLLBACK

I did a count

select count(*) from myTbl
where  myDate < '2011-Jan-02' 

this brings back a count of 1,448,982

the last print out I got was 31,110,000

Edit 2

I added this and now it stops but it still is not 100% where it should be at

while (Select Count(*) From myTbl Where myDate  <'2011-Jan-02' ) >=  @rowCount

Edit 3

I think edit 2 is just doing the same 10,000 rows over and over again.

Answer 1

The provided script does the following:

• Creates/Loads the permanent table SOD

• while (true)

  • begins a transaction
  • updates at most 10,000 rows of the SOD table, where the modified date is less than the current date, setting the unit price discount to 0.08 and setting the modified date to the current date.
  • commits the transaction
  • if no rows were modified, exits the loop
  • repeat

First, the begin transaction and commit transaction are unnecessary with a normal installation of SQL Server, unless you’ve explicitly changed the IMPLICIT_TRANSACTIONS setting by executing SET IMPLICIT_TRANSACTIONS ON, in which case the BEGIN TRANSACTION statement is unecessary. The reason is the by default, SQL Servers runs in auto-commit mode, so every statement fires, sotto voce, a commit on success or a rollback on failure.

Also, the logic is a little clunky. I’d write the loop like this:

declare
  @rows_updated int ,
  @batch_size   int

set @rows_updated = -1
set @batch_size   = 10000

while ( @rows_updated != 0 )
  begin

    update top @batch_size SOD
    set UnitPriceDiscount = 0.08 ,
        ModifiedDate      = convert(datetime,convert(char(10),getdate(),112))
    where ModifiedDate    < convert(datetime,convert(char(10),getdate(),112))

    set @rows_updated = @@rowcount

  end

which might make what’s going on a little more transparent.