I'm finding a way to aggregate strings from different rows into a single row. I'm looking to do this in many different places, so having a function to facilitate this would be nice. I've tried solutions using COALESCE and FOR XML, but they just don't cut it for me.
String aggregation would do something like this:
id | Name Result: id | Names
-- - ---- -- - -----
1 | Matt 1 | Matt, Rocks
1 | Rocks 2 | Stylus
2 | StylusI've taken a look at CLR-defined aggregate functions as a replacement for COALESCE and FOR XML, but apparently SQL Azure does not support CLR-defined stuff, which is a pain for me because I know being able to use it would solve a whole lot of problems for me.
Is there any possible workaround, or similarly optimal method (which might not be as optimal as CLR, but hey I'll take what I can get) that I can use to aggregate my stuff?
78 Answers
SOLUTION
The definition of optimal can vary, but here's how to concatenate strings from different rows using regular Transact SQL, which should work fine in Azure.
;WITH Partitioned AS
( SELECT ID, Name, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Name) AS NameNumber, COUNT(*) OVER (PARTITION BY ID) AS NameCount FROM dbo.SourceTable
),
Concatenated AS
( SELECT ID, CAST(Name AS nvarchar) AS FullName, Name, NameNumber, NameCount FROM Partitioned WHERE NameNumber = 1 UNION ALL SELECT P.ID, CAST(C.FullName + ', ' + P.Name AS nvarchar), P.Name, P.NameNumber, P.NameCount FROM Partitioned AS P INNER JOIN Concatenated AS C ON P.ID = C.ID AND P.NameNumber = C.NameNumber + 1
)
SELECT ID, FullName
FROM Concatenated
WHERE NameNumber = NameCountEXPLANATION
The approach boils down to three steps:
Number the rows using
OVERandPARTITIONgrouping and ordering them as needed for the concatenation. The result isPartitionedCTE. We keep counts of rows in each partition to filter the results later.Using recursive CTE (
Concatenated) iterate through the row numbers (NameNumbercolumn) addingNamevalues toFullNamecolumn.Filter out all results but the ones with the highest
NameNumber.
Please keep in mind that in order to make this query predictable one has to define both grouping (for example, in your scenario rows with the same ID are concatenated) and sorting (I assumed that you simply sort the string alphabetically before concatenation).
I've quickly tested the solution on SQL Server 2012 with the following data:
INSERT dbo.SourceTable (ID, Name)
VALUES
(1, 'Matt'),
(1, 'Rocks'),
(2, 'Stylus'),
(3, 'Foo'),
(3, 'Bar'),
(3, 'Baz')The query result:
ID FullName
----------- ------------------------------
2 Stylus
3 Bar, Baz, Foo
1 Matt, RocksAre methods using FOR XML PATH like below really that slow? Itzik Ben-Gan writes that this method has good performance in his T-SQL Querying book (Mr. Ben-Gan is a trustworthy source, in my view).
create table #t (id int, name varchar(20))
insert into #t
values (1, 'Matt'), (1, 'Rocks'), (2, 'Stylus')
select id ,Names = stuff((select ', ' + name as [text()] from #t xt where xt.id = t.id for xml path('')), 1, 2, '')
from #t t
group by idSTRING_AGG() in SQL Server 2017, Azure SQL, and PostgreSQL:
GROUP_CONCAT() in MySQL
(Thanks to @Brianjorden and @milanio for Azure update)
Example Code:
select Id
, STRING_AGG(Name, ', ') Names
from Demo
group by IdSQL Fiddle:
3Although @serge answer is correct but i compared time consumption of his way against xmlpath and i found the xmlpath is so faster. I'll write the compare code and you can check it by yourself. This is @serge way:
DECLARE @startTime datetime2;
DECLARE @endTime datetime2;
DECLARE @counter INT;
SET @counter = 1;
set nocount on;
declare @YourTable table (ID int, Name nvarchar(50))
WHILE @counter < 1000
BEGIN insert into @YourTable VALUES (ROUND(@counter/10,0), CONVERT(NVARCHAR(50), @counter) + 'CC') SET @counter = @counter + 1;
END
SET @startTime = GETDATE()
;WITH Partitioned AS
( SELECT ID, Name, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Name) AS NameNumber, COUNT(*) OVER (PARTITION BY ID) AS NameCount FROM @YourTable
),
Concatenated AS
( SELECT ID, CAST(Name AS nvarchar) AS FullName, Name, NameNumber, NameCount FROM Partitioned WHERE NameNumber = 1 UNION ALL SELECT P.ID, CAST(C.FullName + ', ' + P.Name AS nvarchar), P.Name, P.NameNumber, P.NameCount FROM Partitioned AS P INNER JOIN Concatenated AS C ON P.ID = C.ID AND P.NameNumber = C.NameNumber + 1
)
SELECT ID, FullName
FROM Concatenated
WHERE NameNumber = NameCount
SET @endTime = GETDATE();
SELECT DATEDIFF(millisecond,@startTime, @endTime)
--Take about 54 millisecondsAnd this is xmlpath way:
DECLARE @startTime datetime2;
DECLARE @endTime datetime2;
DECLARE @counter INT;
SET @counter = 1;
set nocount on;
declare @YourTable table (RowID int, HeaderValue int, ChildValue varchar(5))
WHILE @counter < 1000
BEGIN insert into @YourTable VALUES (@counter, ROUND(@counter/10,0), CONVERT(NVARCHAR(50), @counter) + 'CC') SET @counter = @counter + 1;
END
SET @startTime = GETDATE();
set nocount off
SELECT t1.HeaderValue ,STUFF( (SELECT ', ' + t2.ChildValue FROM @YourTable t2 WHERE t1.HeaderValue=t2.HeaderValue ORDER BY t2.ChildValue FOR XML PATH(''), TYPE ).value('.','varchar(max)') ,1,2, '' ) AS ChildValues FROM @YourTable t1 GROUP BY t1.HeaderValue
SET @endTime = GETDATE();
SELECT DATEDIFF(millisecond,@startTime, @endTime)
--Take about 4 millisecondsUpdate: Ms SQL Server 2017+, Azure SQL Database
You can use: STRING_AGG.
Usage is pretty simple for OP's request:
SELECT id, STRING_AGG(name, ', ') AS names
FROM some_table
GROUP BY idWell my old non-answer got rightfully deleted (left in-tact below), but if anyone happens to land here in the future, there is good news. They have implimented STRING_AGG() in Azure SQL Database as well. That should provide the exact functionality originally requested in this post with native and built in support. @hrobky mentioned this previously as a SQL Server 2016 feature at the time.
--- Old Post: Not enough reputation here to reply to @hrobky directly, but STRING_AGG looks great, however it is only available in SQL Server 2016 vNext currently. Hopefully it will follow to Azure SQL Datababse soon as well..
3You can use += to concatenate strings, for example:
declare @test nvarchar(max)
set @test = ''
select @test += name from namesif you select @test, it will give you all names concatenated
4I found Serge's answer to be very promising, but I also encountered performance issues with it as-written. However, when I restructured it to use temporary tables and not include double CTE tables, the performance went from 1 minute 40 seconds to sub-second for 1000 combined records. Here it is for anyone who needs to do this without FOR XML on older versions of SQL Server:
DECLARE @STRUCTURED_VALUES TABLE ( ID INT ,VALUE VARCHAR(MAX) NULL ,VALUENUMBER BIGINT ,VALUECOUNT INT
);
INSERT INTO @STRUCTURED_VALUES
SELECT ID ,VALUE ,ROW_NUMBER() OVER (PARTITION BY ID ORDER BY VALUE) AS VALUENUMBER ,COUNT(*) OVER (PARTITION BY ID) AS VALUECOUNT
FROM RAW_VALUES_TABLE;
WITH CTE AS ( SELECT SV.ID ,SV.VALUE ,SV.VALUENUMBER ,SV.VALUECOUNT FROM @STRUCTURED_VALUES SV WHERE VALUENUMBER = 1 UNION ALL SELECT SV.ID ,CTE.VALUE + ' ' + SV.VALUE AS VALUE ,SV.VALUENUMBER ,SV.VALUECOUNT FROM @STRUCTURED_VALUES SV JOIN CTE ON SV.ID = CTE.ID AND SV.VALUENUMBER = CTE.VALUENUMBER + 1
)
SELECT ID ,VALUE
FROM CTE
WHERE VALUENUMBER = VALUECOUNT
ORDER BY ID
;Try this, i use it in my projects
DECLARE @MetricsList NVARCHAR(MAX);
SELECT @MetricsList = COALESCE(@MetricsList + '|', '') + QMetricName
FROM #Questions;
