问题
I have a table where each record represents a person and there are many columns used to indicate what events they attended:
CREATE TABLE EventAttendees
(
Person VARCHAR(100),
[Event A] VARCHAR(1),
[Event B] VARCHAR(1),
[Event C] VARCHAR(1)
)
INSERT INTO EventAttendees
SELECT 'John Smith','x',NULL,NULL
UNION
SELECT 'Jane Doe',NULL,'x','x'
UNION
SELECT 'Phil White','x',NULL,'x'
UNION
SELECT 'Sarah Jenkins','x','x','x'
Which looks like this for example:
SELECT * FROM Event Attendees
/---------------|---------|---------|---------\
| Person | Event A | Event B | Event C |
|---------------|---------|---------|---------|
| John Smith | x | NULL | NULL |
| Jane Doe | NULL | x | x |
| Phil White | x | NULL | x |
| Sarah Jenkins | x | x | x |
\---------------|---------|---------|---------/
I want to generate a list of who attended which events, so my desired output is:
/---------------|---------|
| Person | Event |
|---------------|---------|
| John Smith | Event A |
| Jane Doe | Event B |
| Jane Doe | Event C |
| Phil White | Event A |
| Phil White | Event C |
| Sarah Jenkins | Event A |
| Sarah Jenkins | Event B |
| Sarah Jenkins | Event C |
\---------------|---------/
In reality I have many more than 3 events, but the above is for ease of explanation (This is not a homework question btw). As the Events might change in the future and I have no control over the data I am being passed, I really need a dynamic solution which can handle any number of possible event columns.
I'm assuming I can do something with UNPIVOT, but I just can't figure it out, or find a good example on SO or elsewhere to work from - can someone help?
回答1:
I do this using outer apply:
select ea.person, v.EventName
from EventAttendees ea outer apply
(values ('Event A', [Event A]),
('Event B', [Event B]),
('Event C', [Event C])
) v(EventName, EventFlag)
where v.EventFlag = 'x'
回答2:
You can do it with unpivot as you said, you would just need to ensure you are telling it what event it is for, otherwise you just get an X:
CREATE TABLE #tmpEventAttendees
(
Person VARCHAR(100),
[Event A] VARCHAR(1),
[Event B] VARCHAR(1),
[Event C] VARCHAR(1)
)
INSERT INTO #tmpEventAttendees
SELECT 'John Smith','x',NULL,NULL
UNION
SELECT 'Jane Doe',NULL,'x','x'
UNION
SELECT 'Phil White','x',NULL,'x'
UNION
SELECT 'Sarah Jenkins','x','x','x'
SELECT Person, [Event]
FROM
(
SELECT Person ,
CASE WHEN [Event A] IS NOT NULL THEN 'Event A' END AS [Event A] ,
CASE WHEN [Event B] IS NOT NULL THEN 'Event B' END AS [Event B] ,
CASE WHEN [Event C] IS NOT NULL THEN 'Event C' END AS [Event C]
FROM #tmpEventAttendees
) AS cp
UNPIVOT
(
[Event] FOR [Events] IN ([Event A], [Event B], [Event C])
) AS up;
DROP TABLE #tmpEventAttendees
回答3:
Try something like
SELECT * FROM (
SELECT Person, CASE WHEN [Event A] = 'x' THEN 'Event A' END AS [Event] FROM EventAttendees
UNION
SELECT Person, CASE WHEN [Event B] = 'x' THEN 'Event B' END AS [Event] FROM EventAttendees
UNION
SELECT Person, CASE WHEN [Event C] = 'x' THEN 'Event C' END AS [Event] FROM EventAttendees
) AS EventAttendees
WHERE Event is not null
order by Person
For dynamic sql you can try something like this:
DECLARE @name varchar(30)
DECLARE @sql varchar(1000) = 'SELECT * FROM (';
DECLARE NameCursor CURSOR
FOR select name from sys.all_columns where object_id = (select object_id from sys.tables where name='EventAttendees') and name!='Person'
OPEN NameCursor
FETCH NEXT FROM NameCursor INTO @name
WHILE @@FETCH_STATUS = 0
BEGIN
SET @sql += 'SELECT Person, CASE WHEN [' + @name+'] = ''x'' THEN ''' + @name +''' END AS [Event] FROM EventAttendees'
FETCH NEXT FROM NameCursor INTO @name
IF(@@FETCH_STATUS = 0)
BEGIN
SET @sql += ' UNION ';
END
END;
CLOSE NameCursor;
DEALLOCATE NameCursor;
SET @sql += ') AS EventAttendees
WHERE Event is not null
order by Person';
execute (@sql);
回答4:
Figured out the solution I was thinking of, but yes, it does require dynamic SQL to get the relevant column names to feed into the UNPIVOT:
declare @sql varchar(max)
set @sql =
'select Person, EventName
from EventAttendees
unpivot
(
Attended for EventName in (' + (select
stuff((
select ',' + QUOTENAME(c.[name])
from sys.columns c
join sys.objects o on c.object_id = o.object_id
where o.[name] = 'EventAttendees'
and c.column_id > 1
order by c.[name]
for xml path('')
),1,1,'') as colList) + ')
) unpiv
where unpiv.Attended = ''x''
order by Person, EventName'
exec (@sql)
In this example, I am making the assumption that the Event columns are from the second column in the table onwards, but obviously I could use some different logic within the subquery to identify the relevant columns if necessary.
On my example data, this gives the desired result:
/---------------------------\
| Person | EventName |
|---------------|-----------|
| Jane Doe | Event B |
| Jane Doe | Event C |
| John Smith | Event A |
| Phil White | Event A |
| Phil White | Event C |
| Sarah Jenkins | Event A |
| Sarah Jenkins | Event B |
| Sarah Jenkins | Event C |
\---------------------------/
I think I prefer this to using a cursor, although I haven't actually confirmed what performance difference (if any) there is between the two dynamic approaches.
Thanks for everyone's help and suggestions on this question though, greatly appreciated as always!
来源：https://stackoverflow.com/questions/46217564/converting-single-row-into-multiple-rows-based-on-values-in-columns