189 8069 5689
--标量子查询
select e.empno, e.ename, e.sal, e.deptno,
(select d.dname from dept d where e.deptno = d.deptno)as dname
from emp e
--插入一条数据
insert into emp(empno,deptno) values(9999,null)--返回结果15条记录
--改成left join(hash outer)
select e.empno, e.ename, e.sal, e.deptno,d.dname
from emp e
left join dept d
on (e.deptno = d.deptno)
--NL outer
select /*+ use_nl(e,d) */e.empno, e.ename, e.sal, e.deptno,d.dname
from emp e
left join dept d
on (e.deptno = d.deptno)
/*Note:修改后plan一般有outer字样，如果没有，注意是否改错。*/
--用left join 优化标量子查询之聚合改写
select dp.department_id, dp.department_name, dp.location_id,
nvl((select sum(em.salary)
from hr.employees em
where em.department_id = dp.department_id),
0) as sum_dept_salary
from hr.departments dp
--错误写法
select dp.department_id, dp.department_name, dp.location_id,
nvl(sum(em.salary), 0) as sum_sal
from hr.departments dp
left join hr.employees em
on dp.department_id = em.department_id
--原标量子查询改写为：
select em.department_id, sum(em.salary) as sum_sal
from hr.employees em
group by em.department_id
--左联改写后的内联视图
select dp.department_id, dp.department_name, dp.location_id,
nvl(sum(e.sum_sal), 0) as sum_sal
from hr.departments dp
left join (select e.department_id, sum(e.salary) as sum_sal
from hr.employees e
group by e.department_id) e
on (dp.department_id = e.department_id)
group by dp.department_id, dp.department_name, dp.location_id
--
create table dept2 as select * from scott.dept;
insert into dept2 select * from scott.dept where deptno=10
select t1.job, t1.deptno,
(select distinct dname from dept2 b where b.deptno = t1.deptno) as dname
from scott.emp t1
order by 1, 2, 3
--以下改写结果变了
select distinct t1.job, b.deptno, b.dname
from scott.emp t1
left join dept2 b
on t1.deptno = b.deptno
--正确改写
select t1.job, t1.deptno, f.dname
from scott.emp t1
left join (select b.deptno, b.dname
from dept2 b
group by b.deptno, b.dname) f
on (f.deptno = t1.deptno)
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