Simple Car Rental database
ok.  It looks like you have 4 tables here with nothing to join them together.  Perhaps the easiest solution at this point would be to create a mapping table called something like CarRental which would have the columns Carpoolid, cilentsid, dateinid, and dteoutid.

I think the way I would go would be to drop the datein and dateout table and put them on a table called car rental. 

    CREATE TABLE [dbo].[CarRental](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [CarpoolID] [int] NOT NULL,
    [ClientID] [int] NOT NULL,
    [DateIN] [datetime] NULL,
    [DateOut] [datetime] NOT NULL,
 CONSTRAINT [PK_CarRental] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[CarRental]  WITH CHECK ADD  CONSTRAINT [FK_CarRental_CarPool] FOREIGN KEY([CarpoolID])
REFERENCES [dbo].[CarPool] ([Id])
GO

ALTER TABLE [dbo].[CarRental] CHECK CONSTRAINT [FK_CarRental_CarPool]
GO

ALTER TABLE [dbo].[CarRental]  WITH CHECK ADD  CONSTRAINT [FK_CarRental_Clients] FOREIGN KEY([ClientID])
REFERENCES [dbo].[Clients] ([Id])
GO

ALTER TABLE [dbo].[CarRental] CHECK CONSTRAINT [FK_CarRental_Clients]
GO


Car rental now stores the relationship between the client and the car that they rented and for each record stores the datein and dateout.

If you dont want to script all of this out you can use the table designer to create the table by right clicking on tables and then choosing new table. You can use the database diagram to drag and drop columns in order to create your foreign key relationships by right clicking database diagrams and chosing New Database diagram.

If you use this structure then You would leave the DateIn field as NULL untill the car was returned.  Your query to find out which clients and cars have not been returned would be

SELECT carpool.brand, clients.name, clients.lastname
  from carrental 
       join carpool on carpool.id = carrental.carpoolid
       join clients on clients.id = carretnal.cientid
where carretnal.datein IS NULL


In this case you dont need to update the records for people who havent returned a car.  You only update the record to contain the date when they have returned the car.

UPDATE CarRental
SET Datein = GETDATE()
WHERE Clientid = @CLientid
  AND Carpoolid = @Carpoolid