有三个不同的查询语句
1：
select year_mon,sum(gasmon)
from a
where id='HBsRf0t6UI'
and class=11
group by year_mon
2：
select year_mon,sum(wellgasmon)
from b
where id='HBsRf0t6UI'
and class=11
group by year_mon
3：
select year_mon,sum(gasprodmon)
from c
where id='HBsRf0t6UI'
and class=11
group by year_mon

本人希望能显示出下面的效果：

year_mon  sum(gasmon)  sum(wellgasmon)    sum(gasprodmon)
200702     122                  222                       123
200703     333                  234                        342
200704     0                      2334                      0
200705     324                  2342                      234

这样的效果也行。

在高人的指点下，问题总算解决了

SELECT   year_mon, SUM (gasmon), SUM (wellgasmon), SUM (gasprodmon)
    FROM (SELECT   year_mon, SUM (gasmon) gasmon, 0 wellgasmon, 0 gasprodmon
              FROM a
             WHERE ID = 'HBsRf0t6UI' AND CLASS = 11
          GROUP BY year_mon
          UNION ALL
          SELECT   year_mon, 0, SUM (wellgasmon), 0
              FROM b
             WHERE ID = 'HBsRf0t6UI' AND CLASS = 11
          GROUP BY year_mon
          UNION ALL
          SELECT   year_mon, 0, 0, SUM (gasprodmon)
              FROM c
             WHERE ID = 'HBsRf0t6UI' AND CLASS = 11
          GROUP BY year_mon)
GROUP BY year_mon