Skip to main content

About TSEqual function (SQL 2K)

One of the common problem the database developers face in their day to day life is record concurrency issues. Lets try to address that with the help of timestamp data type.

Lets assume that Sarah and Ram are reading a same record. First Ram updates that record with some new data. Later if Sarah also updates the record (mind you she is viewing the old content only still) then it would overwrite the changes made by Ram.

There are two ways of solving this issue they are:

1. Pessimistic Locking
2. Optimistic Locking

Pessimistic Locking: First person who reads the record would put a lock on it so that nobody else can change it until he is done with it. Only when he releases the record the other user can make use of it. This method is not recommended because it might also takes hours or days together for the first person to release his lock due to various reasons.

Optimistic Locking: The record would be locked only when a user wants to modify its content.

SQL Server has a TSEqual (I presume it means TimeStamp Equal) function which compares TimeStamp values in the table and the T-SQL statement. If the timestamp values doesn't match it would throw an error and abort the operation. Let see this in action:

Declare @tStampOriginal TimeStamp

--Here tStamp is a TimeStamp column and TimeStampExample is the name of the table
Select @tStampOriginal = tStamp from TimeStampExample

-- Compares current timestamp value with the original value before updating
Update TimeStampExample Set LastName = 'Kapil' Where LastName = 'Tendulkar' and TSEqual (tStamp, @tStampOriginal)

The above batch of code should execute fine without any issues.

Declare @tStampOriginal TimeStamp

--here tStamp is a TimeStamp column and TimeStampExample is the name of the table
Select @tStampOriginal = tStamp from TimeStampExample

--dummy update statement to change the timestamp value
Update TimeStampExample Set LastName = 'Gandhi' Where LastName = 'Gandhi'

-- Compares current timestamp value with the original value before updating
Update TimeStampExample Set LastName = 'Kapil' Where LastName = 'Tendulkar' and TSEqual (tStamp, @tStampOriginal)

This batch would fail because the @tStampOriginal contains the initial timestamp values which has changed during the dummy update statement.

Comments

Popular posts from this blog

Registry manipulation from SQL

Registry Manupulation from SQL Server is pretty easy. There are 4 extended stored procedure in SQL Server 2000 for the purpose of manupulating the server registry. They are: 1) xp_regwrite 2) xp_regread 3) xp_regdeletekey 4) xp_regdeletevalue Let us see each one of them in detail! About xp_regwrite This extended stored procedure helps us to create data item in the (server’s) registry and we could also create a new key. Usage: We must specify the root key with the @rootkey parameter and an individual key with the @key parameter. Please note that if the key doesn’t exist (without any warnnig) it would be created in the registry. The @value_name parameter designates the data item and the @type the type of the data item. Valid data item types include REG_SZ and REG_DWORD . The last parameter is the @value parameter, which assigns a value to the data item. Let us now see an example which would add a new key called " TestKey ", and a new data item under it called TestKeyValue :

Screen scraping using XmlHttp and Vbscript ...

I wrote a small program for screen scraping any sites using XmlHttp object and VBScript. I know I haven't done any rocket science :) still I thought of sharing the code with you all. XmlHttp -- E x tensible M arkup L anguage H ypertext T ransfer P rotocol An advantage is that - the XmlHttp object queries the server and retrieve the latest information without reloading the page. Source code: < html > < head > < script language ="vbscript"> Dim objXmlHttp Set objXmlHttp = CreateObject("Msxml2.XMLHttp") Function ScreenScrapping() URL == "UR site URL comes here" objXmlHttp.Open "POST", url, False objXmlHttp.onreadystatechange = getref("HandleStateChange") objXmlHttp.Send End Function Function HandleStateChange() If (ObjXmlHttp.readyState = 4) Then msgbox "Screenscrapping completed .." divShowContent.innerHtml = objXmlHttp.responseText End If End Function </ script > < head > < body > &l

Script table as - ALTER TO is greyed out - SQL SERVER

One of my office colleague recently asked me why we are not able to generate ALTER Table script from SSMS. If we right click on the table and choose "Script Table As"  ALTER To option would be disabled or Greyed out. Is it a bug? No it isn't a bug. ALTER To is there to be used for generating modified script of Stored Procedure, Functions, Views, Triggers etc., and NOT for Tables. For generating ALTER Table script there is an work around. Right click on the table, choose "Modify" and enter into the design mode. Make what ever changes you want to make and WITHOUT saving it right click anywhere on the top half of the window (above Column properties) and choose "Generate Change Script". Please be advised that SQL Server would drop actually create a new table with modifications, move the data from the old table into it and then drop the old table. Sounds simple but assume you have a very large table for which you want to do this! Then it woul