Group by in complex query with cases [closed] - mysql

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I have a table:
Visit (FromId, ToId, VisitTime)
where FromId and ToId are FKs to table
UserProfile (uid, name, age ...)
As a user with my UID I want to select all profiles I have visited or who visited me in one result set ordered by VisitTime and with the indication of the "direction of the visit".
I get data using this select:
SELECT CASE WHEN a.FromID = 'yourIDHere'
THEN c.Name
ELSE b.Name
END Name,
CASE WHEN a.FromID = 'yourIDHere'
THEN c.Age
ELSE b.Age
END Age,
a.VisitTime,
CASE WHEN a.FromID = 'yourIDHere'
THEN 'You'
ELSE 'Friend'
END DirectionOfVisit
FROM Visit a
INNER JOIN UserProfile b
ON a.FromID = b.Uid
INNER JOIN UserProfile c
ON a.ToID = c.Uid
WHERE 'yourIDHere' IN (a.FromID, a.ToID)
ORDER BY a.VisitTime
Now it prints (pseudo output)
Jack (id1) | IN |12.12.2012
Jack (id1) | IN |11.12.2012
Jack (id1) | IN |11.12.2012
Jack (id1) | OUT | 13.12.2012
Jack (id1) | OUT | 12.12.2012
Michael (id5) | IN | 5.12.2012
Michael (id5) | OUT | 6.12.2012
Michael (id5) | OUT | 5.12.2012
I would like the list to be like this:
Jack | IN | 12.12.2012 (the most recent)
Jack | OUT | 13.12.2012 (the most recent)
Michael (id5) | IN | 5.12.2012 (the most recent)
Michael (id5) | OUT | 6.12.2012 (the most recent)
I know the GROUP command would solve it but it's too complex for me (beginner).

Answer 1

You could use GROUP BY along with an aggregate function to get the result. Since you want the most recent date for each name and type (IN/OUT), then you can use the max() aggregate function on the date column. You will then use a GROUP BY on the other columns you want to return:
The basic syntax will be:
select
name,
type,
max(date) date
from yourtable
group by name, type;
See SQL Fiddle with Demo
If you want to return the max date with your existing query, you can just expand the query to use:
select name, age, max(VisitTime), DirectionOfVisit
from
(
SELECT CASE WHEN a.FromID = 'yourIDHere'
THEN c.Name
ELSE b.Name
END Name,
CASE WHEN a.FromID = 'yourIDHere'
THEN c.Age
ELSE b.Age
END Age,
a.VisitTime,
CASE WHEN a.FromID = 'yourIDHere'
THEN 'You'
ELSE 'Friend'
END DirectionOfVisit
FROM Visit a
INNER JOIN UserProfile b
ON a.FromID = b.Uid
INNER JOIN UserProfile c
ON a.ToID = c.Uid
WHERE 'yourIDHere' IN (a.FromID, a.ToID)
) d
group by name, age, DirectionOfVisit;
See SQL Fiddle with Demo