Category: SQL


While I was going through a review of a sql server function, it is observed there are many checks implemented if any of input parameter is null then return null. Then, it got strike on mind about “WITH RETURNS NULL ON NULL INPUT” in SQL Server. This is available in SQL server from 2015 version, however, I have not seen people used it efficiently. So thought of sharing about this intelligent way of handling such situations.

Let us start with a sample function as below:

Mighty Sample function

create or alter function fn_validateinparam(@col1 varchar(50), @col2 varchar(50))
returns  varchar(100)

	Declare @outputval varchar(100)
	If (@Col1 is null or @Col2 is null)
		set @outputval= NULL
		set @outputval= 'Valid params'

	return @outputval

Select dbo.fn_validateinparam('You','me')
Select dbo.fn_validateinparam('','')
Select dbo.fn_validateinparam('You',NULL)
Select dbo.fn_validateinparam(NULL,NULL)
Select dbo.fn_validateinparam(NULL,'me')


The above is a sample function that we can see in many places to check the null param check. There is an option introduced in SQL Server 2005 “WITH RETURNS NULL ON NULL INPUT” to handle this with a grace as below. This way, if any of your input parameters is passed as NULL value, then the function will immediately return NULL value without actually invoking the body of the function. That clears the theory and its time for us to check with sample code as below.

create or alter function fn_Efficientvalidateinparam(@col1 varchar(50), @col2 varchar(50))
returns  varchar(100)

	Declare @outputval varchar(100)
	If (@Col1 is null or @Col2 is null)
		set @outputval= 'Wrong params'
		set @outputval= 'Valid params'

	return @outputval

Select dbo.fn_Efficientvalidateinparam('You','me')
Select dbo.fn_Efficientvalidateinparam('','')
Select dbo.fn_Efficientvalidateinparam('You',NULL)
Select dbo.fn_Efficientvalidateinparam(NULL,NULL)
Select dbo.fn_Efficientvalidateinparam(NULL,'me')

Sample output

The below screenshot explains that the option returns without getting into execution of the function whenever one of the parameter is having NULL value. (Please note I used “Wrong value” to assign to the output variable to understand it actually returns in the beginning or only after the execution.)

If we have not used this technique so far, start using this one, it helps to avoid lots of junk codes in the definition that makes it clean. In addition, it has also observed a small factor of performance improvement for the function.

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Palindrome in SQL Server

What is Palindrome?

From Books OnLine, “A palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward, such as madam, racecar. There are also numeric palindromes, including date/time stamps using short digits 11/11/11 11:11 and long digits 02/02/2020. Sentence-length palindromes ignore capitalization, punctuation, and word boundaries.”

Code implementation

The below function eliminates characters other than alphanumeric characters. The function is modified to accommodate alphabets to the old function written to extract only numbers.

create or alter function dbo.fn_extractalphanumeric (@ValueString nvarchar(max))
returns nvarchar(max)

Declare @retvaluestring nvarchar(max)

Num2 (n) AS (SELECT 1 FROM Num1 AS X, Num1 AS Y),
Num3 (n) AS (SELECT 1 FROM Num2 AS X, Num2 AS Y),
Num4 (n) AS (SELECT 1 FROM Num3 AS X, Num3 AS Y),
Select @retvaluestring =(
                       THEN SUBSTRING(string, n, 1)
                      ELSE ''
                  END + ''
           FROM Nums
           WHERE n <= LEN(string)
           FOR XML PATH( '' )) AS stringout
FROM (Select * from (values(@ValueString)) A(string))A)
return @retValuestring

Test Script

DECLARE @Sample TABLE(string VARCHAR(2000));
Values( 'hhjjj12345hhhkk'),('-7655'),('asd5-5dffgdfg105'),('hhjjj12345hhhkk'),('aA1b2c3d4ee55fff666gggg7777h'),('Al1')
,('2Bob$'),('!Carl%'),('%D#?:"{}|\][+_)(*&^	%$#@!~`_+?>a*vi()(d&*(&*#?:"{}|\][J+o_n)es(*&^%$#@!~`_+?><,./;:"' ),
('    Eddie     '),('Noon'),('My Gym'),('malayalam'),('11:11'),('11/11/1111'),('racecar')

Select string, dbo.fn_extractalphanumeric(string),
	case when reverse(dbo.fn_extractalphanumeric(string)) = dbo.fn_extractalphanumeric(string) 
			then 'Palindrome' else 'Not Palindrome' End stringtype
From @Sample

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Fibonacci series in SQL Server

Today, I noticed that my elder kid was learning about fibonacci series. While observing her, I was just trying to write SQL script to produce the series just for fun and sharing it with you all.

What is fibonacci series?

A series of numbers in which each number ( Fibonacci number ) is the sum of the two preceding numbers. The simplest is the series 1, 1, 2, 3, 5, 8, etc. Fibonacci was not the first to know about the sequence, it was known in India hundreds of years before! Leonardo Pisano Bogollo is known as fibonacci man, and he lived between 1170 and 1250 in Italy. “Fibonacci” was his nickname, which roughly means “Son of Bonacci”.

Fibonacci Day is November 23rd, as it has the digits “1, 1, 2, 3” (11 represents month number-23 represents day part) which is part of the sequence. So next Nov 23 let everyone know about this fact.(Honestly, I never noticed this until now.)

Formula & Code Implementation

The simple formula is Fn = Fn-1 + Fn-2

The code implementation as below :

I used Common Table Expression (CTE) to generate it easily over while loop. If you guys are comfortable with while loop, you can even generate with loop.

Declare @UntilNo integer = 100
;With Fibonacci (N, NextN) AS
 SELECT 0, 1
    SELECT NextN, NextN+ N
    FROM Fibonacci
    WHERE N < @UntilNo
SELECT Substring(
    (SELECT cast(', ' as varchar(max)) + cast(N as varchar(max)
FROM Fibonacci
FOR XML PATH('')),3,10000000) AS list

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The transaction log for database ‘database name’ is full due to ‘REPLICATION’

Today, we are going to see an exception from SQL Server “The transaction log for database ‘dbname’ is full due to ‘REPLICATION'”.

Recently, we received a backup of a database for a troubleshooting purpose from the production. The database had configured with CDC in Production environment. While we get this database and restored in our local environments, it is observed that the size of the database is very huge and if you look at the size in details, its log that is majorly contributing the size.

Since its huge in size, we tried to shrink the file, please note this is a non-production environment, shrinkfile is not advised to run without a careful consideration and validation. Few references on shrink file written earlier. Since, in our case it was a testing environment, we were free to use this command to reclaim the space.

Msg 9002, Level 17, State 6, Line 10 The transaction log for database ‘Restoredbname’ is full due to ‘REPLICATION’.

It is also observed that log_reuse_wait_desc was showing “REPLICATION” for the database.

Select log_reuse_wait_desc,* From sys.databases
As we know this db was enabled with CDC in Production environment, the first attempt was to disable CDC on restored database.

use Restoredbname

Msg 22831, Level 16, State 1, Procedure sp_cdc_disable_db_internal, Line 262 [Batch Start Line 2] Could not update the metadata that indicates database Restoredbname is not enabled for Change Data Capture. The failure occurred when executing the command ‘(null)’. The error returned was 9002: ‘The transaction log for database ‘Restoredbname’ is full due to ‘REPLICATION’.’. Use the action and error to determine the cause of the failure and resubmit the request.

We also noticed checkpoint was also not successful due to serious disk issue.

Could not write a checkpoint record in database Restoredbname because the log is out of space. Contact the database administrator to truncate the log or allocate more space to the database log files. Msg 5901, Level 16, State 1, Line 10 One or more recovery units belonging to database ‘Restoredbname’ failed to generate a checkpoint. This is typically caused by lack of system resources such as disk or memory, or in some cases due to database corruption. Examine previous entries in the error log for more detailed information on this failure. Msg 9002, Level 17, State 6, Line 10 The transaction log for database ‘Restoredbname’ is full due to ‘REPLICATION’.

Finally, we decided to apply sp_repldone on the database as below. When xactid is NULL, xact_seqno is NULL, and reset is 1, all replicated transactions in the log are marked as distributed. This is useful when there are replicated transactions in the transaction log that are no longer valid and you want to truncate the log. sp_removedbreplication stored procedure removes all replication objects on the publication database on the Publisher instance of SQL Server or on the subscription database on the Subscriber instance of SQL Server. Once we executed the commands, we were able to shrink the file and the size has been reclaimed to os.

EXEC sp_repldone @xactid = NULL, @xact_segno = NULL, @numtrans = 0, @time= 0, @reset = 1

The solution discussed is NOT a general solution nor for Production environments. This should ONLY be used for non-production or lower environments where the restored database is used for testing purpose and truncating log is not a concern!

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How to capture deadlock occurrences in SQL Server using sys.dm_os_performance_counters

Today, let us quickly see how to monitor deadlocks in your SQL Server. There are multiple ways to get the deadlock information.

But, this blog post details an approach to capture the number of deadlocks using performance monitor dynamic management view in a SQL Server.

This approach does not really cover the deadlock graph capturing or analyzing the deadlock graph etc. This enables you to just understand if there are deadlocks in your system and it captures the data as you configured. This data can be used to generate nice reports as per your requirement.

Object creation script

First step is to create the objects to store the deadlock information. Please note, we are not doing to capture the deadlock information, but just the occurrence of deadlocks. So we need a table with DateAdded to denote the captured date and time and deadlocks column to denote the number of deadlocks occurred.
Drop Table  if exists DeadlockTracker

CREATE TABLE DeadlockTracker(
    DateAdded datetime NOT NULL
    , Deadlocks int NOT NULL

CREATE UNIQUE CLUSTERED INDEX IX_DeadlockTracker_DateAdded_U_C ON DeadlockTracker
) WITH (FillFactor = 100)

Capture Query

The next step is to define the capture process. The idea is to frequently query a dynamic management view – sys.dm_os_performance_counter and log the data in the table as defined int he first step.

We need to capture these information in a defined interval, may be every 15 minutes. We can configure a SQL job to run every 15 minutes.
DECLARE @CounterPrefix NVARCHAR(30)
                            THEN 'SQLServer:'
                        ELSE 'MSSQL$' + @@SERVICENAME + ':'

INSERT INTO DeadlockTracker(DateAdded,  Deadlocks)
SELECT DateAdded            = GetDate()
     , Deadlocks             = (SELECT cntr_value FROM sys.dm_os_performance_counters 
									WHERE object_name like @CounterPrefix + '%'
										AND instance_name IN ('', '_Total')
										AND counter_name ='Number of Deadlocks/sec')

Sample Report Dashboard Queries

This is the last step which is nothing but visualization of data captured. This is absolutely depending on data analyst discretion, but a very basic report sample has provided for your reference. You can change the query to get the data group by month/days/quarter etc.

Select *, Deadlocks - Lag(Deadlocks) Over(Order by DateAdded asc) From  DeadlockTracker

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