Simulating group_concat MySQL function in Microsoft SQL Server 2005? [Answered]

Query explanation:

I’m trying to migrate a MySQL-based app over to Microsoft SQL Server 2005 (not by choice, but that’s life).

In the original app, we used almost entirely ANSI-SQL compliant statements, with one significant exception — we used MySQL’s group_concat function fairly frequently.

group_concat, by the way, does this: given a table of, say, employee names and projects…

SELECT empName, projID FROM project_members;

returns:

ANDY   |  A100
ANDY   |  B391
ANDY   |  X010
TOM    |  A100
TOM    |  A510

… and here’s what you get with group_concat:

SELECT 
    empName, group_concat(projID SEPARATOR ' / ') 
FROM 
    project_members 
GROUP BY 
    empName;

returns:

ANDY   |  A100 / B391 / X010
TOM    |  A100 / A510

So what I’d like to know is: Is it possible to write, say, a user-defined function in SQL Server that emulates the functionality of group_concat?

I have almost no experience using UDFs, stored procedures, or anything like that, just straight-up SQL, so please err on the side of too much explanation 🙂

Answer #1:

No REAL easy way to do this. Lots of ideas out there, though.

Best one I’ve found:

SELECT table_name, LEFT(column_names , LEN(column_names )-1) AS column_names
FROM information_schema.columns AS extern
CROSS APPLY
(
    SELECT column_name + ','
    FROM information_schema.columns AS intern
    WHERE extern.table_name = intern.table_name
    FOR XML PATH('')
) pre_trimmed (column_names)
GROUP BY table_name, column_names;

Or a version that works correctly if the data might contain characters such as <

WITH extern
     AS (SELECT DISTINCT table_name
         FROM   INFORMATION_SCHEMA.COLUMNS)
SELECT table_name,
       LEFT(y.column_names, LEN(y.column_names) - 1) AS column_names
FROM   extern
       CROSS APPLY (SELECT column_name + ','
                    FROM   INFORMATION_SCHEMA.COLUMNS AS intern
                    WHERE  extern.table_name = intern.table_name
                    FOR XML PATH(''), TYPE) x (column_names)
       CROSS APPLY (SELECT x.column_names.value('.', 'NVARCHAR(MAX)')) y(column_names) 

Answer #2:

I may be a bit late to the party but this method works for me and is easier than the COALESCE method.

SELECT STUFF(
             (SELECT ',' + Column_Name 
              FROM Table_Name
              FOR XML PATH (''))
             , 1, 1, '')

Answer #3:

Possibly too late to be of benefit now, but is this not the easiest way to do things?

SELECT     empName, projIDs = replace
                          ((SELECT Surname AS [data()]
                              FROM project_members
                              WHERE  empName = a.empName
                              ORDER BY empName FOR xml path('')), ' ', REQUIRED SEPERATOR)
FROM         project_members a
WHERE     empName IS NOT NULL
GROUP BY empName

Answer #4:

With the below code you have to set PermissionLevel=External on your project properties before you deploy, and change the database to trust external code (be sure to read elsewhere about security risks and alternatives [like certificates]) by running “ALTER DATABASE database_name SET TRUSTWORTHY ON”.

using System;
using System.Collections.Generic;
using System.Data.SqlTypes;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedAggregate(Format.UserDefined,
MaxByteSize=8000,
IsInvariantToDuplicates=true,
IsInvariantToNulls=true,
IsInvariantToOrder=true,
IsNullIfEmpty=true)]
    public struct CommaDelimit : IBinarySerialize
{


[Serializable]
 private class StringList : List<string>
 { }

 private StringList List;

 public void Init()
 {
  this.List = new StringList();
 }

 public void Accumulate(SqlString value)
 {
  if (!value.IsNull)
   this.Add(value.Value);
 }

 private void Add(string value)
 {
  if (!this.List.Contains(value))
   this.List.Add(value);
 }

 public void Merge(CommaDelimit group)
 {
  foreach (string s in group.List)
  {
   this.Add(s);
  }
 }

 void IBinarySerialize.Read(BinaryReader reader)
 {
    IFormatter formatter = new BinaryFormatter();
    this.List = (StringList)formatter.Deserialize(reader.BaseStream);
 }

 public SqlString Terminate()
 {
  if (this.List.Count == 0)
   return SqlString.Null;

  const string Separator = ", ";

  this.List.Sort();

  return new SqlString(String.Join(Separator, this.List.ToArray()));
 }

 void IBinarySerialize.Write(BinaryWriter writer)
 {
  IFormatter formatter = new BinaryFormatter();
  formatter.Serialize(writer.BaseStream, this.List);
 }
    }

I’ve tested this using a query that looks like:

SELECT 
 dbo.CommaDelimit(X.value) [delimited] 
FROM 
 (
  SELECT 'D' [value] 
  UNION ALL SELECT 'B' [value] 
  UNION ALL SELECT 'B' [value] -- intentional duplicate
  UNION ALL SELECT 'A' [value] 
  UNION ALL SELECT 'C' [value] 
 ) X 

And yields: A, B, C, D.

Answer #5:

SQL Server 2017 does introduce a new aggregate function

STRING_AGG ( expression, separator).

Concatenates the values of string expressions and places separator values between them. The separator is not added at the end of string.

The concatenated elements can be ordered by appending WITHIN GROUP (ORDER BY some_expression)

For versions 2005-2016 I typically use the XML method in the accepted answer.

This can fail in some circumstances however. e.g. if the data to be concatenated contains CHAR(29) you see

FOR XML could not serialize the data … because it contains a character (0x001D) which is not allowed in XML.

A more robust method that can deal with all characters would be to use a CLR aggregate. However applying an ordering to the concatenated elements is more difficult with this approach.

The method of assigning to a variable is not guaranteed and should be avoided in production code.

Hope you learned something from this post.

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