Tag Archives: #TSQL

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

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

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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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#0410 – SQL Server – Dividing a TimeSpan by an Integer to find average time per execution

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I recently encountered an interesting question on the forums the other day. The question was how to determine the average time taken by a single execution of the report provided we know how many times the report ran and the total time taken for all those executions.

The challenge is that the total time taken for all the report executions is a timespan value (datatype TIME in SQL Server). A TIME value cannot be divided by an INTEGER. If we try to do that, we run into an error – an operand clash.

USE [tempdb];
GO
DECLARE @timeSpan TIME = '03:18:20';
DECLARE @numberOfExecutions INT = 99;

SELECT @timeSpan/@numberOfExecutions;
GO

Msg 206, Level 16, State 2, Line 6
Operand type clash: time is incompatible with int

The solution is to realize that a timespan/TIME value is ultimately the number of seconds passed from a given instant. Once the timespan is converted to the appropriate unit (number of seconds), dividing by the number of executions should be quite simple.

Here’s the working example:

USE [tempdb];
GO
DECLARE @timeSpan TIME = '03:18:20';
DECLARE @numberOfExecutions INT = 99;

SELECT @timeSpan AS TotalActiveTime,
       DATEDIFF(SECOND,'1900-01-01 00:00:00.000',CAST(@timeSpan AS DATETIME)) AS TotalExecutionTimeInSeconds,
       DATEDIFF(SECOND,'1900-01-01 00:00:00.000',CAST(@timeSpan AS DATETIME))/(@numberOfExecutions * 1.0) AS TimePerExecution;
GO

/* RESULTS
TotalActiveTime  TotalExecutionTimeInSeconds TimePerExecution   
---------------- --------------------------- -------------------
03:18:20.0000000 11900                       120.20202020202020
*/

I trust this simple thought will help in resolving a business problem someday.

Until we meet next time,

Be courteous. Drive responsibly.