要求

現在有三個表，分別是年、月、數量三列，現在想整合成一個結果： 四列，分別是年-月、數量a、數量b、數量c 三個表如下（舉個例子，非真實資料）

具體的結果如下

ps：就是相同年月的為一行 ，別的表沒有的資料補上0

select concat(pyear,'-',pmonth)pmonth,SUM(psum1) AS applyWat,SUM(psum2) AS planWat,SUM(psum3) AS actualWat from (
            (SELECT pyear ,pmonth,psum as psum1,'0' as psum2,'0' as psum3
                from
 
 zssl_planwater  where del_flag='0')
            UNION
            (SELECT atime ,amonth,'0' as psum1,asum as psum2,'0' as psum3
                from zssl_Applywater   where del_flag='0')
            UNION
            (SELECT pyear ,pmonth,'0' as psum1,'0' as psum2,psum as psum3
                from zssl_actual  where
 
 del_flag='0')
        ) as f
        where pyear is not null and pmonth  is not null

        GROUP BY concat(pyear,'-',pmonth)
        order by  concat(pyear,'-',pmonth) desc