Tag Archives: #TSQL

Articles on T-SQL. This can be a script or a syntax element

#0425 – SQL Server – Backup exists but doesn’t display on the restore window in SSMS. Why? How to fix?


Recently, I ran into a forum post where the ask was to figure out why a perfectly valid backup was not visible when attempting to restore it via the wizard in SSMS. Today, I will reproduce the issue, explain the root cause and provide the solution for the same.

Building the scenario

In one of the my test SQL Servers, I have a copy of the [AdventureWorks2019] sample database, which I have backed up using the following simple script.

USE [master];
GO
BACKUP DATABASE [AdventureWorks2019]
 TO DISK = 'C:\Program Files\Microsoft SQL Server\MSSQL14.SQL2K17\MSSQL\Backup\AdventureWorks2019.bak'
    WITH COMPRESSION;
GO

Now, to simulate the movement of the backup to a different machine, I created a new folder under my default “Documents” folder and placed the backup there.

Screenshot showing the placement of the target folder where the backup is placed
Screenshot showing the placement of the target folder where the backup is placed

Reproducing the symptom

Restoring via SSMS

  1. Connect to the target SQL Server using SSMS
  2. Right-click on the “Databases” folder in the Object Explorer
  3. Choose to Restore a database
  4. Under “Source”, select the radio-option for restoring from a “Device”
  5. Use the ellipsis to open the “Select Backup Devices” window and open the File explorer by choosing “Add”
  6. Navigate to the folder where the backup has been placed
    1. Expected Result: We should be able to see the folder and the backup file
    2. Actual Result: The backup file is not seen (the folder may or may not be seen)
Screenshot showing that the backup exists, but it is not seen in the "Locate Backup File" window
Screenshot showing that the backup exists, but it is not seen in the “Locate Backup File” window

Restoring via T-SQL

While the UI keeps things a bit mysterious, attempting to restore via T-SQL does point us to the right direction.

USE [master];
GO
RESTORE DATABASE [AdventureWorks2019_Copy]
    FROM DISK = 'C:\Users\sqltwins\Documents\AdventureWorksBackup\AdventureWorks2019.bak';
GO

Here’s the error that we run into:

Msg 3201, Level 16, State 2, Line 3
Cannot open backup device 'C:\Users\sqltwins\Documents\AdventureWorksBackup\AdventureWorks2019.bak'. Operating system error 5(Access is denied.).
Msg 3013, Level 16, State 1, Line 3
RESTORE DATABASE is terminating abnormally.

Notice that the error clearly says – “Access is denied.

Root Cause

As highlighted by the results of the T-SQL script, SQL Server is actually running into a security problem. The operation is done under the context of the SQL Server instance service user (i.e. the user under which the SQL Server service runs).

Because the user doesn’t have access to the folder we just created, the service cannot see the files underneath.

Solution

The solution is to use the SQL Server Configuration Manager to figure out the user under which the SQL Server service runs.

Once the user is identified, provide access to the target folder to the user and the files should now be visible – both to SSMS and to T-SQL.

Screenshot showing the backup file is now visible once the SQL Server instance service has been granted to the folder
Screenshot showing the backup file is now visible once the SQL Server instance service has been granted to the folder

Further Reading/References:

Until we meet next time,

Be courteous. Drive responsibly.

#0424 – SQL Server – Null value is eliminated by an aggregate or other SET operation. – Why? How to fix?


I recently ran into a forum post where the poster wanted to know why they were getting the following warning during query execution:

"Null value is eliminated by an aggregate or other SET operation."

Having been asked this question a few times by a few of my office colleagues as well, I thought to write up a quick post on the reason behind this warning.

A quick test

The test below is simple – I am creating a sample test table which allows NULL values to be inserted. I am then trying to perform a simple aggregate function (SUM) over the NULL-able column. Upon checking the “Messages” tab, we see that no warning is returned.

USE [tempdb];
GO
SET NOCOUNT ON;

--Safety Check
IF OBJECT_ID('dbo.NULLAggregation','U') IS NOT NULL
BEGIN
    DROP TABLE [dbo].[NULLAggregation];
END
GO

--Create the test table
CREATE TABLE [dbo].[NULLAggregation] 
    ([Id]          INT           NOT NULL IDENTITY (1,1),
     [Value]       INT               NULL,
     [ValueString] VARCHAR(50)   NOT NULL,
     CONSTRAINT [pk_NULLAggregation] PRIMARY KEY ([Id])
    );
GO

--Insert the test data
INSERT INTO [dbo].[NULLAggregation] ([Value], [ValueString])
VALUES ( 1, 'One'),
       (10, 'Ten'),
       (22, 'Twenty-Two');
GO

--Perform the aggregation
--NOTE: No NULL values are in the table at this point
SELECT SUM([na].[Value]) AS [SumOfValues]
FROM [dbo].[NULLAggregation] AS [na];
GO

/****************************************
RESULT
-----------
SumOfValues
-----------
33
****************************************/

Now, I add a single record with a NULL value in the [Value] column and repeat the aggregation. While the result is the same, we have a warning in the “Messages” tab.

INSERT INTO [dbo].[NULLAggregation] ([Value], [ValueString])
VALUES ( NULL, 'One');
GO

SELECT SUM([na].[Value]) AS [SumOfValues]
FROM [dbo].[NULLAggregation] AS [na];
GO

/****************************************
RESULT
-----------
SumOfValues
-----------
33
Warning: Null value is eliminated by an aggregate or other SET operation.
****************************************/
A screenshot showing the warning encountered in SSMS when an aggregation operation is performed on a NULL value.
Warning encountered when aggregating on NULL values

The reason for the warning

To begin – this is just a warning and not an error. If your script/job is failing it is probably failing due to some other data condition OR as an indirect result of operating on NULL values.

The warning simply suggests that an aggregation operation is being done on a NULL value. If no NULL values are present in the dataset being evaluated, then the warning is not encountered.

Solutions

The important thing is to consider what is important for the business/domain.

  • If processing on NULL values are okay for the business/domain, then one of the following two (2) workaround can be applied:
    • The warning can either be ignored OR
    • Use the “SET ANSI_WARNINGS OFF” option for your query/procedure
  • If processing on NULL values is not acceptable for your business/domain, then
    • Either use a simple WHERE clause to remove the records with NULL values from the aggregation
    • Use input validations in the application code and NOT NULL checks in the database to stop the NULL values from being entered by the users

NOTE: If you do decide to use the “SET ANSI_WARNINGS OFF” option, please do so with caution. It can have unintended consequences with string operations as well (https://docs.microsoft.com/en-us/sql/t-sql/statements/set-ansi-warnings-transact-sql?view=sql-server-ver15)

I trust you found this post useful.

Until we meet next time,

Be courteous. Drive responsibly.

#0417 – SQL Server – Select row count of local temp tables


I was recently contacted by a fellow team member who was interested in finding out the number of records in a temporary table which was being used as part of a long-running script.

As I had mentioned in one of my previous posts, local temporary tables are only accessible to the connection that created them. Hence, if a script is already running, the only connection that can execute queries against the local temporary table is the connection itself making this requirement a tricky one to work with.

The Solution

The solution for this is to realize that all tables – permanent or local consume storage either in a system/user database or in the tempdb. Hence, we can access this meta-data to fulfill our requirement. Let’s check it out with a demo.

Demo

In any window, we can access the dynamic management view (DMV): [sys].[dm_db_partition_stats]. As we know, this DMV returns page and row-count information for every partition in the current database.

So, let’s open a new query window in the SQL Server Management Studio and run the following query:

--Create a new test database
IF (DB_ID('SQLTwins') IS NULL)
BEGIN
    CREATE DATABASE [SQLTwins];
END
GO

USE [SQLTwins];
GO

--Window 01
BEGIN
    IF OBJECT_ID('tempdb..#SQLTwinsDemo','U') IS NOT NULL
    BEGIN
        DROP TABLE [dbo].[#SQLTwinsDemo];
    END

    CREATE TABLE [dbo].[#SQLTwinsDemo] ([Number] INT         NOT NULL,
                                        [Value]  VARCHAR(50) NOT NULL
                                       );

    INSERT INTO [dbo].[#SQLTwinsDemo] ([Number],
                                       [Value]
                                      )
    VALUES (9, 'Nine' ),
           (8, 'Eight'),
           (7, 'Seven'),
           (6, 'Six'  ),
           (5, 'Five' ),
           (4, 'Four' ),
           (3, 'Three'),
           (2, 'Two'  ),
           (1, 'One'  );
END

Now, in another window, try to run a simple row count query. As expected, it would return an error.

USE [SQLTwins];
GO
SELECT COUNT(*) FROM [dbo].[#SQLTwinsDemo];
GO
Msg 208, Level 16, State 0, Line 3
Invalid object name '#SQLTwinsDemo'.

Now, let’s use the DMV: [sys].[dm_db_partition_stats] in another window to get the row count information.

USE [SQLTwins];
GO
--Now, do this in Window #2
BEGIN
    SELECT [st].[name] AS [TableName],
           [partitionStatistics].[row_count] AS [RowCount]
    FROM [tempdb].[sys].[dm_db_partition_stats] AS [partitionStatistics]
    INNER JOIN [tempdb].[sys].[tables] AS [st] ON [st].[object_id] = [partitionStatistics].[object_id]
    WHERE [st].[name] LIKE '%SQLTwinsDemo%'
      AND ([partitionStatistics].[index_id] = 0  --Table is a heap
           OR
           [partitionStatistics].[index_id] = 1  --Table has a clustered index
          )
END
Fetching RowCount for local temporary tables using SQL Server DMVs

Hope it helps!

Until we meet next time,

Be courteous. Drive responsibly,

#0416 – SQL Server – Msg 8101 – Use column lists when working with IDENTITY columns


I have often written about IDENTITY columns on my blog. Identity columns, most commonly used to implement auto-increment keys, have been around for more than a decade now. Yet, I often see teams run into interesting use cases especially in cases where data is being migrated from one system to another.

Today’s post is based on one such incident that came to my attention.

The team was trying to migrate data from one table to another as part of an exercise to change the database structure for more efficiency. When moving the data from one table to another, they were using the option (SET IDENTITY_INSERT ON) in order to explicitly insert values into the Identity column. However, they were running into an error.

Msg 8101, Level 16, State 1, Line 24
An explicit value for the identity column in table 'dbo.tIdentity' can only be specified when a column list is used and IDENTITY_INSERT is ON.

Here is a simulation of what they were doing:

USE tempdb;
GO
SET NOCOUNT ON;
--Prepare the environment
--   Create a table, and add some test data into it
--Safety Check
IF OBJECT_ID('tIdentity','U') IS NOT NULL
    DROP TABLE dbo.tIdentity;
GO
--Create a table
CREATE TABLE dbo.tIdentity (IdentityId INT IDENTITY(1,1),
                            IdentityValue VARCHAR(10)
                           );
GO
--Turn Explicit IDENTITY_INSERT ON and insert duplicate IDENTITY values
SET IDENTITY_INSERT dbo.tIdentity ON;
GO
--NOTICE: No column list has been supplied in the INSERT
INSERT INTO dbo.tIdentity
VALUES (1, 'One'),
       (2, 'Two');
GO

--RESULTS
--Msg 8101, Level 16, State 1, Line 24
--An explicit value for the identity column in table 'dbo.tIdentity' can only be pecified when a column list is used and IDENTITY_INSERT is ON.

The Solution

Let’s re-read the error. It clearly gives an indication of what the issue is – if we need to insert an explicit value into Identity columns, we need to explicitly use column lists in our insert statements, as shown below.

USE tempdb;
GO
SET NOCOUNT ON;
--Prepare the environment
--Create a table, and add some test data into it
--Safety Check
IF OBJECT_ID('tIdentity','U') IS NOT NULL
    DROP TABLE dbo.tIdentity;
GO

--Create a table
CREATE TABLE dbo.tIdentity (IdentityId INT IDENTITY(1,1),
                            IdentityValue VARCHAR(10)
                           );
GO

--Turn Explicit IDENTITY_INSERT ON and insert duplicate IDENTITY values
SET IDENTITY_INSERT dbo.tIdentity ON;
GO

--NOTE: Column list has been supplied in the INSERT,
--      so, no errors will be encountered    
INSERT INTO dbo.tIdentity ([IdentityId], [IdentityValue])
VALUES (1, 'One'),
       (2, 'Two');
GO

--Confirm that data has been inserted
SELECT IdentityId,
       IdentityValue
FROM dbo.tIdentity;
GO

--Now that data has been inserted, turn OFF IDENTITY_INSERT
SET IDENTITY_INSERT dbo.tIdentity OFF;
GO

-----------------------------------------------------------------
--RESULTS
----------
--IdentityId  IdentityValue
--1           One
--2           Two
 -----------------------------------------------------------------

Hope you will find this helpful.

Untill we meet next time,

Be courteous. Drive responsibly.

#0415 – SQL Server – Performance Tuning – Use STRING_AGG to generate comma separated strings


With more and more data being exchanged over APIs, generating comma-separated strings are becoming a much more common requirement.

A few years ago, I wrote about two different ways to generate comma-separated strings. The most common one I find to be in use when generating comma-separated values from a table is the intermediate conversion of XML. This however, is a very costly mechanism and can potentially take minutes for the query to run depending upon the amount of data involved.

SQL Server 2017 brings a new aggregate function that can be used to generate comma-separated values extremely fast. The function is STRING_AGG().

Here’s a sample of it’s usage:


 --WARNING: THIS SCRIPT IS PROVIDED AS-IS AND WITHOUT
-- WARRANTY.
-- FOR DEMONSTRATION PURPOSES ONLY
--Step 01: Generate Temp table to store source data
DECLARE @NamesTable TABLE ([Id] INT,
[Name] NVARCHAR(50)
);
--Step 02: Generate test data
INSERT INTO @NamesTable
VALUES (1, 'A'),
(2, 'D'),
(2, 'C'),
(3, 'E'),
(3, 'H'),
(3, 'G');
--Step 03: Using STRING_AGG to generate comma-separated strings
SELECT STRING_AGG(tbl.Name, ',') AS [CommaSeparatedString]
FROM @NamesTable AS tbl;
GO
/RESULTS**
CommaSeparatedString
A,D,C,E,H,G
*/

Advantages of STRING_AGG:

  • Can be used just like any other aggregate function in a query
  • Can work with any user supplied separator – doesn’t necessarily have to be a comma
  • No manual step required – Separators are not added at the end of the concatenated string
  • STRING_AGG() is significantly faster than using XML based methods
  • Can be used with any compatibility level as long as the version is SQL Server 2017 (or higher) and Azure SQL database

Here’s an example of how STRING_AGG can be used with any separator:

 --WARNING: THIS SCRIPT IS PROVIDED AS-IS AND WITHOUT
-- WARRANTY.
-- FOR DEMONSTRATION PURPOSES ONLY
--Step 01: Generate Temp table to store source data
DECLARE @NamesTable TABLE ([Id] INT,
[Name] NVARCHAR(50)
);
--Step 02: Generate test data
INSERT INTO @NamesTable
VALUES (1, 'A'),
(2, 'D'),
(2, 'C'),
(3, 'E'),
(3, 'H'),
(3, 'G');
--Step 03: Using STRING_AGG to generate comma-separated strings
SELECT STRING_AGG(tbl.Name, '-*-') AS [CustomSeparatorString]
FROM @NamesTable AS tbl;
GO
/RESULTS**
CustomSeparatorString
A--D--C--E--H--G /

A minor challenge

As with every new feature, there may be a small usability challenge with STRING_AGG. One cannot use keywords like DISTINCT to ensure that only distinct values are used for generating the comma-separated string. There is however a Azure feedback item open where you can exercise your vote if you feel this feature is useful.

Further Reading

  • Different ways to generate a comma-separated string from a table [Blog Link]
  • STRING_AGG() Aggregate Function [MSDN BOL]

Until we meet next time,

Be courteous. Drive responsibly.