calculating max and joining tables Just need a little help as new to SQL.

I have a table which contains readingtime (records every 4 seconds during the day) and an energy reading. I wanted to know how much energy was used in a day. 
this is the query that i used

SELECT date(readingtime), max(energy)-min(energy)as total 
FROM lights_1 
WHERE readingtime between 20140407000000 and 20140409235959 
GROUP BY date(readingtime) LIMIT 0,30;


This gave me the desired results for 1 table. I have 4 tables of these lights_2, Lights_3, lights_4 and i would like to display them together and hence match up the reading time so each day has the energy requirements for all for tables with the total. When i try and join tables i get a lot of errors!

Can anyone help.

thanks

Union gives

date(readingtime) total
2014-04-07        1244
2014-04-08        1039
2014-04-09        1389
2014-04-07        419
2014-04-08        1102
2014-04-09        482


I would like

date(readingtime) total_1  total_2
2014-04-07        1244      419
2014-04-08        1039      1039
2014-04-09        1389      1389
SELECT  date(lights_1.readingtime), 
        max(lights_1.energy)-min(lights_1.energy)as total_1, 
        max(lights_2.energy)-min(lights_2.energy)as total_2
FROM lights_1
join lights_2 on lights_1.readingtime = lights_2.readingtime
WHERE lights_1.readingtime between 20140407000000 and 20140409235959 
GROUP BY date(lights_1.readingtime) LIMIT 0,30