Tag Archives: Tips

General Microsoft SQL Server tips

#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.

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#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.