SQL - 1 Column Twice in the SELECT Statement for different values - sql

Question

I have created a database of "trips" with the following
TripParent
Id
DrivingCompany
Client
TripDetails
Id
Destination
PlannedArrivalDate
StatusLog
Id
CatStatusId (Comes from another table just with the names)
DateTimeModified
Let me explain the tables, first of all I hid another fields to keep it simple, the "Parent" table has MANY TripDetails, so it is just a summary for the many "Details" it has. The TripDetails table its 1 row for 1 destination, let's say the Trip is going from A to C, then we have a Row for each "stop" (A, B, C).
And then we got the StatusLog table that has MANY Rows for each "TripDetails".
The problem is, I need a stored procedure that returns the DrivingCompany, Client, PlannedArrivalDate, RealArrivalDate and RealDepartureDate.
The "Real Dates" come from the StatusLog table. Status 1 means that the truck has arrived Destination (A/B/C) and the status 2 means that it has left said location.
So far I got the following
SELECT
TP.DrivingCompany, TP.Client, TD.PlannedArrivalDate,
'Real Arrival Date' = CASE SL.CatStatusId
WHEN 1 THEN SL.DateTimeModified
ELSE NULL
END,
'Real Departure Date' = CASE SL.CatStatusId
WHEN 2 THEN SL.DateTimeModified
ELSE NULL
END
FROM
TripParent TP
JOIN
TripDetails TD ON TD.TripParentId = TE.Id
JOIN
StatusLog SL ON SL.TripDetailsId = TD.Id
GROUP BY
TP.Id
ORDER BY
TD.Id
Is using the CASE the correct way to show the same column twice in the SELECT statement? I think I'm on the right track but I can't group by TP.Id and I also need to show ALL the rows, going by this query, It doesn't show the "TripDetails" that don't have a "StatusLog" row because they haven't arrived.
Any help is appreciated

Answer 1

Try this:
SELECT TP.Id,
TP.DrivingCompany,
TP.Client,
TD.PlannedArrivalDate,
'Real Arrival Date' = CASE SL.CatStatusId
WHEN 1 THEN SL.DateTimeModified
ELSE NULL
END,
'Real Departure Date' = CASE SL.CatStatusId
WHEN 2 THEN SL.DateTimeModified
ELSE NULL
END
FROM TripParent TP
JOIN TripDetails TD ON TD.TripParentId = TE.Id
LEFT JOIN StatusLog SL ON SL.TripDetailsId = TD.Id
GROUP BY TP.Id,
TP.DrivingCompany,
TP.Client,
TD.PlannedArrivalDate
ORDER BY TD.Id
They way you use CASE if perfectly fine, yes. Columns you want to group by must be contained in the SELECT. Using a LEFT JOIN for the log entries ensures that you get also rows without corresponding log.
When grouping, each column you display must be either contained in the GROUP BY clause or in an aggregate function in the SELECT. So you have to think about why you are grouping, do you want to create a sum, count, ... ? >> Change the two CASE columns accordingly.