Question

This problem has been sending me insane, although admittedly I’ve not written SQL for a long time.

I have 2 tables:

CREATE TABLE [Entities].[Events](
[EventID] [int] IDENTITY(1,1) NOT NULL,
[EventVenueID] [int] NULL,
[EntityID] [int] NOT NULL,
[OrganisationID] [int] NULL,
[Title] [nvarchar](300) NOT NULL,
[DateStart] [datetime] NOT NULL,
[DateFinish] [datetime] NOT NULL,
[Notes] [nvarchar](max) NULL,
[MinimumProviders] [int] NOT NULL,
[MinimumAttendees] [int] NOT NULL,
[ShowCalendar] [bit] NOT NULL,
[CreationDate] [datetime] NOT NULL,
[IsEnabled] [bit] NOT NULL)

CREATE TABLE [Entities].[EventParticipants](
[ParticipantID] [int] IDENTITY(1,1) NOT NULL,
[EventID] [int] NOT NULL,
[PersonID] [int] NOT NULL,
[ParticipantType] [int] NOT NULL,
[ParticipantStatus] [int] NOT NULL,
[AttendanceStatus] [int] NULL)

And a query:

SELECT
e.EventID,
ev.VenueName,
e.EntityID,
o.Name AS 'Organisation',
e.Title,
e.DateStart,
e.DateFinish,
e.Notes,
e.MinimumProviders,
e.MinimumAttendees,
e.CreationDate,
COUNT(epp.ParticipantID) AS 'ProvidersConfirmed',
COUNT(epa.ParticipantID) AS 'AttendeesConfirmed',
e.IsEnabled
  FROM
Entities.Events e
  LEFT OUTER JOIN
Entities.EventVenues ev
  ON
e.EventVenueID = ev.EventVenueID
  LEFT OUTER JOIN
Entities.Organisations o
  ON
e.OrganisationID = o.OrganisationID
  LEFT OUTER JOIN
Entities.EventParticipants epp
  ON
e.EventID = epp.EventID AND epp.ParticipantType = 1 AND epp.ParticipantStatus = 3
  LEFT OUTER JOIN
Entities.EventParticipants epa
  ON
e.EventID = epa.EventID AND epa.ParticipantType = 2 AND epa.ParticipantStatus = 3
  GROUP BY
e.EventID,
ev.VenueName,
e.EntityID,
o.Name,
e.Title,
e.DateStart,
e.DateFinish,
e.Notes,
e.MinimumProviders,
e.MinimumAttendees,
e.CreationDate,
e.IsEnabled

The count is producing strange results, for example in the EventParticipants table I have the following data:

Participant Type = 1 (3 records)
Participant Type = 2 (1 record)
Participant Status = 3 (4 records)

The queries count should return:

ProvidersConfirmed = 3
AttendeesConfirmed = 1

However, it’s returning

ProvidersConfirmed = 3
AttendeesConfirmed = 3

Can anyone help?

Answer 1

Basically all your joins are throwing off this query. When you join from Event to EventParticipants There is one event record and three EventParticipant records, this equates to three duplicate eventIDs in your dataset. Then you join to EventParticipants again. Because you’re joining 3 rows with one row, you get the result repeated 3 times.

There are ways to trim down the results, but it would probably be better to only JOIN once and do some selective math in your aggregate, like this:

SELECT
e.EventID,
ev.VenueName,
e.EntityID,
o.Name AS 'Organisation',
e.Title,
e.DateStart,
e.DateFinish,
e.Notes,
e.MinimumProviders,
e.MinimumAttendees,
e.CreationDate,
COUNT(CASE WHEN ep.ParticipantType = 1 THEN 1 ELSE NULL END) AS 'ProvidersConfirmed',
COUNT(CASE WHEN ep.ParticipantType = 2 THEN 1 ELSE NULL END) AS 'AttendeesConfirmed',
e.IsEnabled
  FROM
Entities.Events e
  LEFT OUTER JOIN
Entities.EventVenues ev
  ON
e.EventVenueID = ev.EventVenueID
  LEFT OUTER JOIN
Entities.Organisations o
  ON
e.OrganisationID = o.OrganisationID
  LEFT OUTER JOIN
Entities.EventParticipants epp
  ON
e.EventID = epp.EventID AND epp.ParticipantStatus = 3
  GROUP BY
e.EventID,
ev.VenueName,
e.EntityID,
o.Name,
e.Title,
e.DateStart,
e.DateFinish,
e.Notes,
e.MinimumProviders,
e.MinimumAttendees,
e.CreationDate,
e.IsEnabled

And SQL’s COUNT will throw away all the null values so it will only count unique values.