问题
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 which 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 :)
回答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)
回答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, '')
回答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
回答4:
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.
回答5:
Have a look at the GROUP_CONCAT project on Github, I think I does exactly what you are searching for:
This project contains a set of SQLCLR User-defined Aggregate functions (SQLCLR UDAs) that collectively offer similar functionality to the MySQL GROUP_CONCAT function. There are multiple functions to ensure the best performance based on the functionality required...
回答6:
To concatenate all the project manager names from projects that have multiple project managers write:
SELECT a.project_id,a.project_name,Stuff((SELECT N'/ ' + first_name + ', '+last_name FROM projects_v
where a.project_id=project_id
FOR
XML PATH(''),TYPE).value('text()[1]','nvarchar(max)'),1,2,N''
) mgr_names
from projects_v a
group by a.project_id,a.project_name
回答7:
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
回答8:
Tried these but for my purposes in MS SQL Server 2005 the following was most useful, which I found at xaprb
declare @result varchar(8000);
set @result = '';
select @result = @result + name + ' '
from master.dbo.systypes;
select rtrim(@result);
@Mark as you mentioned it was the space character that caused issues for me.
回答9:
About J Hardiman's answer, how about:
SELECT empName, projIDs=
REPLACE(
REPLACE(
(SELECT REPLACE(projID, ' ', '-somebody-puts-microsoft-out-of-his-misery-please-') AS [data()] FROM project_members WHERE empName=a.empName FOR XML PATH('')),
' ',
' / '),
'-somebody-puts-microsoft-out-of-his-misery-please-',
' ')
FROM project_members a WHERE empName IS NOT NULL GROUP BY empName
By the way, is the use of "Surname" a typo or am i not understanding a concept here?
Anyway, thanks a lot guys cuz it saved me quite some time :)
回答10:
For my fellow Googlers out there, here's a very simple plug-and-play solution that worked for me after struggling with the more complex solutions for a while:
SELECT
distinct empName,
NewColumnName=STUFF((SELECT ','+ CONVERT(VARCHAR(10), projID )
FROM returns
WHERE empName=t.empName FOR XML PATH('')) , 1 , 1 , '' )
FROM
returns t
Notice that I had to convert the ID into a VARCHAR in order to concatenate it as a string. If you don't have to do that, here's an even simpler version:
SELECT
distinct empName,
NewColumnName=STUFF((SELECT ','+ projID
FROM returns
WHERE empName=t.empName FOR XML PATH('')) , 1 , 1 , '' )
FROM
returns t
All credit for this goes to here: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/9508abc2-46e7-4186-b57f-7f368374e084/replicating-groupconcat-function-of-mysql-in-sql-server?forum=transactsql
来源：https://stackoverflow.com/questions/451415/simulating-group-concat-mysql-function-in-microsoft-sql-server-2005