SQL Server aggregation with several joins - sql

Question

I'm trying to join data from 3 tables in SQL Servre and display in result:
Alias of an entity
if the entity is virtual
the last date (if known)
the value (if known)
I tried this :
select
sr.alias, c.virtual, max(d.date) date
from
App_references sr
join
Sensor c on (c.id_capteur = sr.id_capteur)
left join
Sensor_data d on (c.id_capteur = d.id_capteur)
group by
d.id_capteur, sr.alias, c.virtual
order by
sr.alias
Here is the database scheme:
CREATE TABLE [dbo].[App_reference]
(
[id_ref] [int] IDENTITY(1,1) NOT NULL,
[alias] [varchar](60) NOT NULL,
[id_capteur] [int] NOT NULL,
CONSTRAINT [PK_App_reference] PRIMARY KEY CLUSTERED
(
[id_ref] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Sensor]
(
[id_capteur] [int] IDENTITY(1,1) NOT NULL,
[name] [varchar](50) NOT NULL,
[virtual] [tinyint] NULL,
[unite] [varchar](5) NULL,
[id_type] [int] NOT NULL,
CONSTRAINT [PK_Sensor] PRIMARY KEY CLUSTERED
(
[id_capteur] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Sensor_data]
(
[id_entry] [int] IDENTITY(1,1) NOT NULL,
[id_capteur] [int] NOT NULL,
[value] [xml] NOT NULL,
[date] [datetime] NOT NULL,
CONSTRAINT [PK_Sensor_data] PRIMARY KEY CLUSTERED
(
[id_entry] ASC,
[id_capteur] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
Supposing each columns like "id_%" are linked by foreign key.
The request on top pass well, I got value :
alias virtual date
Place 1 (Physique) 0 2017-04-27 14:58:42.423
Place 2 1 NULL
Place 3 1 NULL
But I tried to select the value too by doing this :
select
sr.alias, c.virtual, max(d.date) date, d.value
from
Citopia_test.dbo.Smartparking_reference sr
join
Citopia_test.dbo.Sensor c on (c.id_capteur = sr.id_capteur)
left join
Citopia_test.dbo.Sensor_data d on (c.id_capteur = d.id_capteur)
group by
d.id_capteur, sr.alias, c.virtual
order by
sr.alias
And I got this error :
Column 'Sensor_data.value' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
So I tried several things like adding column in the group by but nothing changes.

Answer 1

You probably want the value from the record with max date. Use ROW_NUMBER to get those records.
select alias, virtual, date, value
from
(
select
sr.alias, c.virtual, d.date, d.value,
row_number() over (partition by sr.alias order by d.date desc) as rn
from Citopia_test.dbo.Smartparking_reference sr
join Citopia_test.dbo.Sensor c on (c.id_capteur = sr.id_capteur)
left join Citopia_test.dbo.Sensor_data d on (c.id_capteur = d.id_capteur)
) numbered
where rn = 1
order by sr.alias;
This gets you one row per sr.alias. If you want one row per sr.alias + c.virtual then change the partition by clause accordingly.