т1:
хлор
вода

т2:
цех1
цех2

т3:
цех1 хлор 10
цех2 вода 15

нужно получить результат:
т4:
цех1	хлор	10
цех1	вода	0
цех2	хлор	0
цех2	вода	15

SELECT
loc_id,
com_id,
t3.val
FROM
  tbl_locations t1
  JOIN
  tbl_components t2
LEFT JOIN
  (
      SELECT
    log_loc_id loc,
    max(log_value) val
    FROM
    tbl_log
    GROUP BY
    loc
  ) AS t3
  ON
  t1.loc_id = t3.loc

SELECT
loc_id,
com_id,
t3.id,
t3.val,
t3.dtime
FROM
  tbl_locations t1
  JOIN
  tbl_components t2
LEFT JOIN
  (
      SELECT
    log_loc_id loc,
    log_com_id comp,
    log_loc_id id,
    max(log_datetime) dtime,
    max(log_value) val
    FROM
    tbl_log
    GROUP BY
    loc, comp
  ) AS t3
  ON
  t1.loc_id = t3.loc
  AND
  t2.com_id = t3.comp

SELECT t1.loc_id,
       t2.com_id,
       t1.loc_name,
       t2.com_name,
       max(t3.log_value),
       COALESCE(MAX(t3.log_datetime), 0) val
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
GROUP BY t1.loc_id,
         t2.com_id

SELECT
    t1.loc_id id_location,
    t2.com_id id_comp,
    t1.loc_name location,
    t2.com_name component,
    MAX(t3.log_value) val,
    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<now()
GROUP BY
 t1.loc_id,
 t2.com_id