很有可能你也不小心遇到这种需求了，看下下面几种。

select d1,d2,sum(d1,d2) from (select sum(a1,a2) as d1,sum(a3,a4) as d2,id from 500wan where id='1') as t;错

select d1,d2,(d1+d2) from (select sum(a1,a2) as d1,sum(a3,a4) as d2,id from 500wan where id='1') as t;错

select d1,d2,sum(d1,d2) from (select (a1+a2) as d1,(a3+a4) as d2,id from 500wan where id='1') as t;错

select d1,d2,(d1+d2) from (select (a1+a2) as d1,(a3+a4) as d2,id from 500wan where id='1') ;错

-------------------以上都是错的-----------------------------------------------

select d1,d2,(d1+d2) from (select (a1+a2) as d1,(a3+a4) as d2,id from 500wan where id='1') as t;对