Select data: paginating data from multiple tables with inconsistent dates - mysql

Question

I have 8 separate tables. Each table has id, a datetime field and some text.
I am presenting data combined from all tables on a single page in a timeline view, most recent entries are at the top, and entries from each table are mixed in this timeline.
Now the hard part - I need to add pagination to this website, so on each page I want to show exactly 10 days (except for the last page, that may have less than 10).
Each table may have variable number of rows.
I've been struggling with this for quite some time but have not yet come up with an elegant solution.
Here's an example (I'll pick only two tables for this example to make it simpler).
tableA
Apr 1 | a1
Apr 5 | a2
Apr 7 | a3
tableB
Apr 2 | b1
Apr 2 | b2
Apr 5 | b3
Apr 6 | b4
Global timeline would look like this
Apr 7 a3
Apr 6 b4
Apr 5 a2
Apr 5 b3
Apr 2 b2
Apr 2 b1
Apr 1 a1
And if each page shows only 3 days, I need for it to look like this:
--- p1 ---
Apr 7 a3
Apr 6 b4
Apr 5 a2
--- p2 ---
Apr 5 b3
Apr 2 b2
Apr 2 b1
--- p3 ---
Apr 1 a1
The problem is - I can't figure out a way to query for this data in an elegant way. Here's some live query that I've been messing with:
select date(d.entryTime) entryDate, date(wc.changeTime) wcDate
from diary_entry d
join water_change wc
on d.aquariumId = wc.aquariumId
where d.aquariumId = 2
group by entryDate
order by entryDate
limit 10, 5
so, for one table I have this query:
select date(d.entryTime) entryDate
from diary_entry d
where d.aquariumId = 2
group by entryDate
it yields 13 results
entryDate
2012-01-13
2012-01-14
2012-01-25
2012-01-26
2012-01-31
2012-02-04
2012-02-17
2012-02-20
2012-02-25
2012-03-17
2012-03-31
2012-04-01
2012-04-06
and for another:
select date(wc.changeTime) changeDate
from water_change wc
where wc.aquariumId = 2
group by changeTime
it yields 8 results
2012-01-22
2012-01-26
2012-02-17
2012-02-25
2012-03-04
2012-03-10
2012-04-04
2012-04-24
There are three common days between the two
2012-01-26
2012-02-17
2012-02-25
So the query that I need to produce would have to have
13 + 8 - 3 rows = 18 rows
And solution is found!
(select date(d.entryTime) activityDate
from diary_entry d
where d.aquariumId = 1
group by activityDate)
union
(select date(wc.changeTime) activityDate
from water_change wc
where wc.aquariumId = 1
group by activityDate
order by activityDate)
limit 10, 10

Answer 1

Query it using a Union.
SELECT TheDate,
TheText
FROM Table_A
WHERE [your critera]
UNION
SELECT TheDate,
TheText
FROM Table_B
WHERE [your critera]
...
If you need to only select the distinct ones, you can then wrap this as follows:
Select Distinct
TheDate,
TheText
From
(
SELECT TheDate,
TheText
FROM Table_A
WHERE [your critera]
UNION
SELECT TheDate,
TheText
FROM Table_B
WHERE [your critera]
...
);

Answer 2

It sounds like you've got the select and joins in place and are just missing the pageination. If that's the case, your script will have to keep track of what page you're on, and each click on "next page" will have to tell MySQL where to return the next 10 records from by appending this to your SELECT query:
SELECT...JOIN...WHERE...ORDER BY...LIMIT 20, 10
Will return the next 10 rows beginning with the 21st row in the full record set