MYSQL - Sum Interval Dates - mysql

Question

I came across the following problem:
I would like to sum the hours of each name, giving a total interval between START and END activities,
would be simple if I could subtract from each record the end of the beginning, more e.g., Mary, started 13th and was up to 15 and started another activity while 14 and 16, I would like the result of it was 3 (she used 3 hours of their time to perform both activities)
e.g.:
Name | START | END |
-----------------------------------------------------------
KATE | 2014-01-01 13:00:00 | 2014-01-01 14:00:00 |
MARY | 2014-01-01 13:00:00 | 2014-01-01 15:00:00 |
TOM | 2014-01-01 13:00:00 | 2014-01-01 16:00:00 |
KATE | 2014-01-01 12:00:00 | 2014-01-02 04:00:00 |
MARY | 2014-01-01 14:00:00 | 2014-01-01 16:00:00 |
TOM | 2014-01-01 12:00:00 | 2014-01-01 18:00:00 |
TOM | 2014-01-01 22:00:00 | 2014-01-02 02:00:00 |
result:
KATE 15 hours
MARY 3 hours
TOM 9 hours

Answer 1

Have you tried a group by and then an aggregate function?
SELECT Name, SUM(UNIX_TIMESTAMP(End) - UNIX_TIMESTAMP(Start)) FROM myTable
GROUP BY Name
Which will return a cumulative total of seconds from the intervals you have. You can then change the seconds to hours for display.
Also I would highly recommend grouping by a primary key or something instead of a string name, but I understand that this may have been just to simplify the question.

Answer 2

I found this problem interesting, so spent a little more time to develop a solution. What I came up with involves sorting the rows by name and start time, then using MySQL variables to account for overlapping ranges. I begin by sorting the table and supplementing it with columns that carry the name and times from one row to the next
SELECT [expounded below]
FROM (SELECT * FROM tbl ORDER BY Name, START, END) AS u,
(SELECT #x := 0, #gap := 0, #same_name:='',
#beg := (SELECT MIN(START) FROM tbl),
#end := (SELECT MAX(END) FROM tbl)) AS t
This adds the name and the outer bounds of the time range to each row of the table, as well as sorting the table so that
names are together in order by starting time. For each row, we will now have #same_name, #beg, and #end carrying values forward from one line to the next, and #x and #gap will accumulate the hours.
Now we have to do some reasoning about the possible overlaps that can occur. For any two intervals, they are either disjoint or have an intersection:
Non-overlapping: beg--------end START-------END
Overlapping: beg-----------end beg---------end
START--------------END START-----------END
Subset: beg---------------------------------end
START-----END
Once the rows are adjacent, we can decide if two ranges overlap by comparing their start and end points. They overlap
if the start of one is before the end of the other and vice versa:
IF( #end >= START && #beg <= END,
If they do overlap, then the total interval is the difference between the outer edges of the two intervals:
TIMESTAMPDIFF(HOUR, LEAST(#beg, START), GREATEST(#end, END))
If they don't overlap, then we can just add the new interval to the previous one.
We will also need to know the gap between intervals, which is the difference from the end of the first to the beginning of the second. This will be necessary to calculate the hours for a case of more than two intervals, where only some overlap.
1-----------2 3----------4
3--------------------5
Putting this together gets us a calculation per row, where each row calculates the union of the hours with the one
above it. For each variable, we have to reset it if the name changes:
SELECT Name, START, END,
#x := IF(#same_name = Name,
IF( #end >= START && #beg <= END, -- does it overlap?
TIMESTAMPDIFF(HOUR, LEAST(#beg, START), GREATEST(#end, END)),
#x + TIMESTAMPDIFF(HOUR, START, END) ),
TIMESTAMPDIFF(HOUR,START,END) ) AS hr,
#gap := IF(#same_name = Name,
IF(#end >= START && #beg <= END, -- does it overlap?
#gap,
#gap + TIMESTAMPDIFF(HOUR, #end, START)),
0) AS gap,
#beg := IF(#same_name = Name,
CAST(LEAST(#beg, START) AS DATETIME), -- expand interval
START) AS beg, -- reset interval
#end := IF(#same_name = Name,
CAST(GREATEST(#end, END) AS DATETIME),
END) AS finish,
#same_name := Name AS sameName
FROM
(SELECT * FROM xt ORDER BY Name, START, END) AS u,
(SELECT #x := 0, #gap := 0, #same_name:='', #beg := (SELECT MIN(START) FROM xt), #end := (SELECT MAX(END) FROM xt)) AS t
That still gives us as many rows as there were in the original table. The hours and gaps will accumulate for each name, so we have to select the highest values and group by Name:
SELECT Name, MAX(hr) - MAX(gap) AS HOURS
FROM ( [insert above query here] ) AS intermediateCalculcation
GROUP BY Name;
Edit
And of course a moment after hitting enter, it occurs to me that (a) there is a bug for names that have no overlapping intervals at all; and (b) all #x is really doing is building up the interval from MIN(START) to MAX(END) for eacdh name, which could be done with a simpler query and join. Um, exercise for the reader ? :-)