SQL查詢和優化（九）

用left join 優化標量子查詢

多次訪問同乙個表時，盡量不用標量子查詢

select s.sid,

s.sname,
s.shot,
s.stype,
(select a.aid from a where a.aid =s.aids) aid,
(select a.aname from a where a.aid =s.aids) aname,
(select a.atime from a where a.aid =s.aids) aatime
from s;

select s.sid,s.sname,s.type,s.shot,a.aid,a.anme,a.aatime

from s
left join a on(a.aid=s.aids);

select d.department_id,

d.department_name,
d.location_id,
nvl((select
sum(e.salary)from hr.employees e
where e.department_id = d.department_id),0) as sum_sal
from hr.departments d;

select d.department_id,

d.department_name,
d.location_id,
nvl (e.sum_sal,0) as sum_sal
from hr.departments d 
left
join (select e.department_id,sum(e.salary)as sum_sal 
from hr.employees e
group
by e.department_id) e
on (e.department_id = d.department_id);

選自《oracle 查詢優化改寫技巧與案例》 有教無類 落落 著

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