SQL Help: Select statement Concatenate a One to Many relationship

问题

For example I have two tables. The first table is student while the second table are the courses that the a student is taking. How can I use a select statement so that I can see two columns student and courses so that the courses are separated by commas.

Thanks.

回答1:

Assuming you're using SQL Server 2005:

This should do what you're after - obviously replace fields as you need:

For demo purposes, consider the following two table structures:

Students(
  STU_PKEY Int Identity(1,1) Constraint PK_Students_StuPKey Primary Key,
  STU_NAME nvarchar(64)
)

Courses(
  CRS_PKEY Int Identity(1, 1) Constraint PK_Courses_CrsPKey Primary Key,
  STU_KEY Int Constraint FK_Students_StuPKey Foreign Key References Students(STU_PKEY),
  CRS_NAME nvarchar(64)
)

Now this query should do the job you're after:

Select  s.STU_PKEY, s.STU_NAME As Student,
        Stuff((
            Select  ',' + c.CRS_NAME
            From    Courses c
            Where   s.STU_PKEY = c.STU_KEY
            For     XML Path('')
        ), 1, 1, '') As Courses 
From    Students s
Group By s.STU_PKEY, s.STU_NAME

Way simpler than the currently accepted answer...

回答2:

create table Project (ProjectId int, Description varchar(50));
insert into Project values (1, 'Chase tail, change directions');
insert into Project values (2, 'ping-pong ball in clothes dryer');

create table ProjectResource (ProjectId int, ResourceId int, Name varchar(15));
insert into ProjectResource values (1, 1, 'Adam');
insert into ProjectResource values (1, 2, 'Kerry');
insert into ProjectResource values (1, 3, 'Tom');
insert into ProjectResource values (2, 4, 'David');
insert into ProjectResource values (2, 5, 'Jeff');

SELECT *, 
  (SELECT Name + ' ' AS [text()] 
   FROM ProjectResource pr 
   WHERE pr.ProjectId = p.ProjectId 
   FOR XML PATH ('')) 
AS ResourceList 
FROM Project p

--    ProjectId    Description                        ResourceList
--    1            Chase tail, change directions      Adam Kerry Tom 
--    2            ping-pong ball in clothes dryer    David Jeff

回答3:

It depends on which server you're using. SQL Server? MySQL? Other?

回答4:

Generally, what you're talking about is a join:

SELECT 
    S.*,
    SC.*
  FROM
    Students S
    INNER JOIN Student_Courses SC
      ON S.student_id = SC.student_id

However, that will give you one row per course. SQL doesn't make it easy to get the set of courses as a comma delimited list in a single row (that's not a set-based operation). Depending on the vendor, there are different ways to do it, involving looping.

回答5:

I think this MySQL page will help you with that. http://dev.mysql.com/doc/refman/4.1/en/group-by-modifiers.html

回答6:

You can use a UDF that cursors through the related records and concats a return string together but this will be expensive - if you give it a go make sure your cursor is READ_ONLY FAST_FORWARD

回答7:

Are you using 2005 or 2008? If so, then lookup the PIVOT comand. It should be more efficient than a function with a cursor.

回答8:

So you want to see:

'Jade', 'Math, English, History'
'Kieveli', 'History, Biology, Physics'

Yes, comma separated is always desirable, but SQL isn't good at it. Here's the approach I was always planning on using:

Create a Function on the SQL server that uses a cursor to loop over the subquery (sorry - I didn't fully test this):

CREATE FUNCTION commacourselist(@studentname varchar(100))
RETURNS @List varchar(4096)
AS
BEGIN

DECLARE @coursename varchar(100)

DECLARE FOR
SELECT course.name FROM course WHERE course.studentname = @studentname
OPEN coursecursor

FETCH NEXT FROM coursecursor INTO @coursename 
WHILE @@FETCH_STATUS = 0
BEGIN
 IF @List = ''
 BEGIN
  SET @List = @coursename
 END
 ELSE
 BEGIN
  SET @List = @List + ',' + @coursename 
 END
 FETCH NEXT FROM coursecursor INTO @coursename 
END
CLOSE coursecursor 
DEALLOCATE coursecursor 

RETURN
END
GO

Then call the function in the query:

SELECT student.name, 
       commacourselist( student.name )  
FROM student

来源：https://stackoverflow.com/questions/368459/sql-help-select-statement-concatenate-a-one-to-many-relationship

标签

sql

sql-server

concatenation