子查询

--使用子查询提升count distinct的效率
数据库：38_6543 db_hyrhk

--1.count distinct很有用，但太慢
----Count distinct是SQL分析时的祸根所以拿它来做个例子
----首先，让我们以我们一直使用的一个简单查询开始：查看案件涉及当事人大于200的案件
select
t_aj_all.c_ajbh,
count(distinct t_dsr_allbak.c_bh)
from t_aj_all
join t_dsr_allbak on t_dsr_allbak.c_ajbh = t_aj_all.c_ajbh
group by t_aj_all.c_ajbh having count(distinct t_dsr_allbak.c_bh)>200
order by count desc
耗时：8.573s
----它慢是因为数据库遍历了所有当事人以及所有的案件，然后join它们，再将它们排序，这些都在真正的group和分组和聚合工作之前。

--2.先聚合，然后Join
----group-聚合后的任何工作代价都要低，因为数据量会更小。：

select
t_aj_all.c_ajbh,
dsr.ct
from t_aj_all
join (
select
c_ajbh,
count(distinct c_bh) as ct
from t_dsr_allbak
group by c_ajbh having count(distinct c_bh)>200
) dsr
on dsr.c_ajbh = t_aj_all.c_ajbh
order by dsr.ct desc
耗时：7.822s
----正如设计的，group-聚合在join之前。

--3.先将数据集缩小
----我们可以做的更好。通过在整个当事人表上group-聚合，我们处理了数据库中很多不必要的数据。
----我们可以预先计算差异，而不是处理全部数据，这样只需要一个哈希集合。然后我们在此基础上做一个简单的聚集即可。

select
t_aj_all.c_ajbh,
counts.ct
from t_aj_all
inner join (
select distinct_dsr.c_ajbh,
count(1) as ct
from (
select distinct c_ajbh, c_bh
from t_dsr_allbak
) as distinct_dsr
group by distinct_dsr.c_ajbh having count(1) >200
) as counts
on counts.c_ajbh = t_aj_all.c_ajbh
order by counts.ct desc
耗时：1.948s
----我们采取内部的count-distinct-group，然后将数据拆成两部分分成两块。第一块计算distinct (c_ajbh, c_bh) 。第二块在它们基础上运行一个简单group-count。跟上面一样，最后再join。
----通常，数据大小和类型很重要。上面的例子受益于基数中没多少换算。distinct (c_ajbh, c_bh)相对于数据总量来说数量也很少。不同的对数越多，用来group和计数的唯一数据就越多——代价便会越来越大。

--总结：下一遇到长时间运行的count distinct时，尝试一些子查询吧。

什么是临时表（with as）

with as 语法：WITH AS短语，也叫做子查询部分（subquery factoring），可以让你做很多事情，定义一个SQL片断，该SQL片断会被整个SQL语句所用到。如果WITH AS短语所定义的表名被调用两次以上，则优化器会自动将 WITH AS短语所获取的数据放入一个TEMP表里，如果只是被调用一次，则不会。而提示materialize则是强制将WITH AS 短语里的数据放入一个全局临时表里。很多查询通过这种方法都可以提高速度

是公用表表达式，可以理解为创建临时表。比如
WITH A AS(SELECT * FROM TABLE1)
SELECT * FROM A

with as语法
–针对一个别名
with tmp as (select * from tb_name)

–针对多个别名
with
   tmp as (select * from tb_name),
   tmp2 as (select * from tb_name2),
   tmp3 as (select * from tb_name3)

--相当于建了个e临时表
with e as (select * from scott.emp e where e.empno=7499)
select * from e;
--相当于建了e、d临时表
with
e as (select * from scott.emp),
d as (select * from scott.dept)
select * from e, d where e.deptno = d.deptno;

其实就是把一大堆重复用到的sql语句放在with as里面，取一个别名，后面的查询就可以用它，这样对于大批量的sql语句起到一个优化的作用，而且清楚明了

--样例：
--1.
select
t_aj_all.c_ajbh,
log_counts.ct
from t_aj_all
inner join (
select distinct_logs.c_ajbh,
count(1) as ct
from (
select distinct c_ajbh, c_bh
from t_dsr_allbak
) as distinct_logs
group by distinct_logs.c_ajbh having count(1) >200
) as log_counts
on log_counts.c_ajbh = t_aj_all.c_ajbh
order by log_counts.ct desc

with distinct_logs as
( select distinct c_ajbh, c_bh
from t_dsr_allbak
),
log_counts as
(
select distinct_logs.c_ajbh,
count(1) as ct
from distinct_logs group by distinct_logs.c_ajbh having count(1) >200
)
select
t_aj_all.c_ajbh,
log_counts.ct
from t_aj_all
inner join log_counts
on log_counts.c_ajbh = t_aj_all.c_ajbh
order by log_counts.ct desc

--2.
select
t_aj_all.c_ajbh,
dsr.ct
from t_aj_all
join (
select
c_ajbh,
count(distinct c_bh) as ct
from t_dsr_allbak
group by c_ajbh having count(distinct c_bh)>200
) dsr
on dsr.c_ajbh = t_aj_all.c_ajbh
order by dsr.ct desc

with dsr as ( select
c_ajbh,
count(distinct c_bh) as ct
from t_dsr_allbak
group by c_ajbh having count(distinct c_bh)>200)

select t_aj_all.c_ajbh,
dsr.ct
from t_aj_all
join dsr on dsr.c_ajbh = t_aj_all.c_ajbh
order by dsr.ct desc

区别：

EXPLAIN ANALYZE
with solution as(
       SELECT
       c_id,
       c_task_mc,
       d_createtime,
       n_status,
       c_graph_version
       FROM
       db_solution.t_solution_task
       ),
       temp as(
       SELECT
       c_id,
       MAX (d_createtime) AS d_createtime
       FROM
       db_solution.t_solution_task_his
       GROUP BY
       c_id
       ),
       solutionhis as(
       SELECT
       A .c_id,
       A .c_task_mc,
       A .d_createtime,
       A .n_status,
       A .c_graph_version
       FROM
       db_solution.t_solution_task_his A
       left join temp on A.c_id = temp.c_id and A.d_createtime=temp.d_createtime
       left join solution on A.c_id= solution.c_id
       where temp.c_id is not null and solution.c_id is null)
       select
       COUNT (DISTINCT task.c_id)
       from
       db_solution.t_task task
       left join solution on task.c_id = solution.c_id
       left join solutionhis on task.c_id = solutionhis.c_id
       LEFT JOIN db_solution.t_task_process_graph gra ON task.c_id = gra.c_task_id AND
       task.c_graph_version = gra.c_version


       EXPLAIN ANALYZE
       select
       COUNT (DISTINCT task.c_id)
       from
       db_solution.t_task task
       left join db_solution.t_solution_task solution on task.c_id = solution.c_id
       left join (       SELECT
       A .c_id,
       A .c_task_mc,
       A .d_createtime,
       A .n_status,
       A .c_graph_version
       FROM
       db_solution.t_solution_task_his A
       left join (       SELECT c_id,MAX (d_createtime) AS d_createtime FROM db_solution.t_solution_task_his GROUP BY c_id)temp on A.c_id = temp.c_id and A.d_createtime=temp.d_createtime
       left join db_solution.t_solution_task solution on A.c_id= solution.c_id
       where temp.c_id is not null and solution.c_id is null) solutionhis on task.c_id = solutionhis.c_id
       LEFT JOIN db_solution.t_task_process_graph gra ON task.c_id = gra.c_task_id AND
       task.c_graph_version = gra.c_version

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SQL --子查询与临时表

子查询

什么是临时表（with as）

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