Finding a previous, non-contiguous date using SQL - mysql

Question

Suppose a table, tableX, like this:
| date | hours |
| 2014-07-02 | 10 |
| 2014-07-03 | 10 |
| 2014-07-07 | 20 |
| 2014-07-08 | 40 |
The dates are 'workdays' -- that is, no weekends or holidays.
I want to find the increase in hours between consecutive workdays, like this:
| date | hours |
| 2014-07-03 | 0 |
| 2014-07-07 | 10 |
| 2014-07-08 | 20 |
The challenge is dealing with the gaps. If there were no gaps, something like
SELECT t1.date1 AS 'first day', t2.date1 AS 'second day', (t2.hours - t1.hours)
FROM tableX t1
LEFT JOIN tableX t2 ON t2.date1 = DATE_add(t1.date1, INTERVAL 1 DAY)
ORDER BY t2.date1;
would get it done, but that doesn't work in this case as there is a gap between 2014-07-03 and 2014-07-07.

Answer 1

Just use a correlated subquery instead. You have two fields, so you can do this with two correlated subqueries, or a correlated subquery with a join back to the table. Here is the first version:
SELECT t1.date1 as `first day`,
(select t2.date1
from tableX t2
where t2.date1 > t.date1
order by t2.date asc
limit 1
) as `next day`,
(select t2.hours
from tableX t2
where t2.date1 > t.date1
order by t2.date asc
limit 1
) - t.hours
FROM tableX t
ORDER BY t.date1;

Answer 2

Another alternative is to rank the data by date and then subtract the hours of the previous workday's date from the hours of the current workday's date.
SELECT
ranked_t1.date1 date,
ranked_t1.hours - ranked_t2.hours hours
FROM
(
SELECT t.*,
#rownum := #rownum + 1 AS rank
FROM (SELECT * FROM tableX ORDER BY date1) t,
(SELECT #rownum := 0) r
) ranked_t1
INNER JOIN
(
SELECT t.*,
#rownum2 := #rownum2 + 1 AS rank
FROM (SELECT * FROM tableX ORDER BY date1) t,
(SELECT #rownum2 := 0) r
) ranked_t2
ON ranked_t2.rank = ranked_t1.rank - 1;
SQL Fiddle demo
Note:
Obviously an index on tableX.date1 would speed up the query.
Instead of a correlated subquery, a join is used in the above query.
Reference:
Mysql rank function on SO

Answer 3

Unfortunately, MySQL doesn't (yet) have analytic functions which would allow you to access the "previous row" or the "next row" of the data stream. However, you can duplicate it with this:
select h2.LogDate, h2.Hours - h1.Hours as Added_Hours
from Hours h1
left join Hours h2
on h2.LogDate =(
select Min( LogDate )
from Hours
where LogDate > h1.LogDate )
where h2.LogDate is not null;
Check it out here. Note the index on the date field. If that field is not indexed, this query will take forever.