虽然为时已晚，而且已经有了许多解决方案。下面是MySQL的简单解决方案：

SELECT t1.id,
        GROUP_CONCAT(t1.id) ids
 FROM table t1 JOIN table t2 ON (t1.id = t2.id)
 GROUP BY t1.id

以下是实现这一目标的完整解决方案：

-- Table Creation
CREATE TABLE Tbl
( CustomerCode    VARCHAR(50)
, CustomerName    VARCHAR(50)
, Type VARCHAR(50)
,Items    VARCHAR(50)
)

insert into Tbl
SELECT 'C0001','Thomas','BREAKFAST','Milk'
union SELECT 'C0001','Thomas','BREAKFAST','Bread'
union SELECT 'C0001','Thomas','BREAKFAST','Egg'
union SELECT 'C0001','Thomas','LUNCH','Rice'
union SELECT 'C0001','Thomas','LUNCH','Fish Curry'
union SELECT 'C0001','Thomas','LUNCH','Lessy'
union SELECT 'C0002','JOSEPH','BREAKFAST','Bread'
union SELECT 'C0002','JOSEPH','BREAKFAST','Jam'
union SELECT 'C0002','JOSEPH','BREAKFAST','Tea'
union SELECT 'C0002','JOSEPH','Supper','Tea'
union SELECT 'C0002','JOSEPH','Brunch','Roti'

-- function creation
GO
CREATE  FUNCTION [dbo].[fn_GetItemsByType]
(   
    @CustomerCode VARCHAR(50)
    ,@Type VARCHAR(50)
)
RETURNS @ItemType TABLE  ( Items VARCHAR(5000) )
AS
BEGIN

        INSERT INTO @ItemType(Items)
    SELECT  STUFF((SELECT distinct ',' + [Items]
         FROM Tbl 
         WHERE CustomerCode = @CustomerCode
            AND Type=@Type
            FOR XML PATH(''))
        ,1,1,'') as  Items



    RETURN 
END

GO

-- fianl Query
DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT distinct ',' + QUOTENAME(Type) 
                    from Tbl
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT CustomerCode,CustomerName,' + @cols + '
             from 
             (
                select  
                    distinct CustomerCode
                    ,CustomerName
                    ,Type
                    ,F.Items
                    FROM Tbl T
                    CROSS APPLY [fn_GetItemsByType] (T.CustomerCode,T.Type) F
            ) x
            pivot 
            (
                max(Items)
                for Type in (' + @cols + ')
            ) p '

execute(@query) 

这对我有用（SQL Server 2016）：

SELECT CarNamesString = STUFF((
         SELECT ',' + [Name]
            FROM tbl_cars
            FOR XML PATH('')
         ), 1, 1, '')

来源如下：https://www.mytecbits.com/

还有一个针对MySQL的解决方案（因为该页面显示在Google for MySQL中）：

SELECT [Name],
       GROUP_CONCAT(DISTINCT [Name]  SEPARATOR ',')
       FROM tbl_cars

来自MySQL文档。

在Oracle中，它是wm_concat。我相信这个功能在10g及更高版本中可用。

SELECT PageContent = Stuff(
    (   SELECT PageContent
        FROM dbo.InfoGuide
        WHERE CategoryId = @CategoryId
          AND SubCategoryId = @SubCategoryId
        for xml path(''), type
    ).value('.[1]','nvarchar(max)'),
    1, 1, '')
FROM dbo.InfoGuide info

如果要处理null，可以通过添加where子句或在第一个子句周围添加另一个COALENCE来完成。

DECLARE @Names VARCHAR(8000) 
SELECT @Names = COALESCE(COALESCE(@Names + ', ', '') + Name, @Names) FROM People