USE khms
SET 
@startTime='2015-01-01',
@endTime='2015-01-02',
@minute_rnd=1

SELECT * FROM 

(SELECT
round_minutes(date_time,@minute_rnd) dt,
temp_out,out_hum, wind_dir
FROM davis
WHERE date_time BETWEEN @startTime and @endTime
) davis

LEFT JOIN
(SELECT
round_minutes(date_time,@minute_rnd) dt,
cons,sigma
FROM dasibi
WHERE date_time BETWEEN @startTime and @endTime
) dasibi
USING (dt)

LEFT JOIN
(SELECT  
round_minutes(date_time,@minute_rnd) dt,
(n0*ifnull(flow,1)*ifnull(work_int,1)) sum_a 
FROM las
WHERE date_time BETWEEN @startTime and @endTime
) las
USING (dt)

WHERE wind_dir IN('W','NW','SW','S','SSW','NNW') 
LIMIT 0,30