#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.

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Import Event Viewer Logs into Excel

#0414 – Analyzing Event Viewer Logs in Excel


When troubleshooting issues, the Event Viewer is one of the most handy of all tools. Assuming that appropriate coding practices were used during application development, the Event Viewer contains a log of most problems – in the system, in the configuration or in the application code.

The only problem is analyzing the Event Viewer logs when you have a thousand events. It becomes extremely difficult to try and answer questions like the following while going through events serially:

  1. Events logged by type for each source
  2. Events by severity
  3. Events by category
  4. And many more such analytical questions…

These analytical requirements are best achieved with tools like Microsoft Excel. And so, I went about analyzing Event Viewer logs in Microsoft Excel in just 2 steps.

Step #1: Export the Event Viewer Logs to XML

  1. Once the Event Viewer is launched, navigate to the Event Log to be evaluated
  2. Right-click on the Event Log and choose “Save All Events As” option
  3. In the Save As dialog, choose to save the Events as an XML file
    • If asked to save display information, you can choose not to store any or choose a language of your choice

And that’s it – it completes the 1st step!

Screenshot showing how to Save the Event Viewer Logs
Save the Event Viewer Logs
Screenshot showing how to save the Event Viewer Logs as an XML file
Choose to save the Event Viewer Logs as an XML file

Step #2: Import the XML file into Excel

  1. Launch Microsoft Excel
  2. In the File -> Open dialog, choose to search files of “XML” type
  3. Select the exported Event Viewer Log file
  4. In the Import Options, you can choose to import as an “XML Table”
    • Excel will prompt to create/determine the XML schema automatically. It’s okay to allow Excel to do so

And that’s it – the Event Viewer Logs are now in Excel and you can use all native Excel capabilities (sort, filter, pivot and so on).

Choose to import the Event Viewer Logs into Excel as an XML table
Import the Event Viewer Logs as an XML table
Image showing the successfully imported Event Viewer data into Microsoft Excel
Event Viewer Logs successfully imported into Excel

I do hope you found this tip helpful. If you have more such thoughts and ideas, drop in a line in the Comments section below.

Until we meet next time,

Be courteous. Drive responsibly.