т1:
хлор
вода

т2:
цех1
цех2

т3:
1 цех1 хлор 10  '2019-5-10'
2 цех1 хлор 12	'2019-5-11'
3 цех2 вода 15	'2019-5-10'
4 цех2 вода 17	'2019-5-12'

т4:

1 цех1	хлор	10 '2019-5-11'
0 цех1	вода	0	-
0 цех2	хлор	0	-
4 цех2	вода	17 '2019-5-12'

SELECT
st.location,
st.component,
bt.*
 FROM
tbl_log bt
RIGHT JOIN
(SELECT
    t1.loc_id id_location,
    t2.com_id id_comp,
    t1.loc_name location,
    t2.com_name component,
    MAX(t3.log_id) id,
    COALESCE(MAX(t3.log_datetime), 0) dtime
FROM
(( tbl_locations t1,
       tbl_components t2 )
LEFT JOIN
tbl_log t3
    ON
     t1.loc_id = t3.log_loc_id
   AND
    t2.com_id = t3.log_com_id
    AND
--     t3.log_datetime<'2019-05-10 11:37:52')
    t3.log_datetime<now())
    LEFT JOIN
    (SELECT
    t4.log_id id,
    t4.log_value val,
    t4.log_datetime dt
    FROM
    tbl_log t4) t5
    ON
    t5.id = t3.log_id
GROUP BY
 t1.loc_id,
 t2.com_id) st
 ON
 st.id=bt.log_id
ORDER BY
location,
component