SQL子连接案例

子查询

何时使用子查询

1. 子查询作为数据源

2. 数据加工

需求：根据不同顾客的所有的账户余额划分区间，进行分组

sql语句实现如下：

select 'small fry' name , 0 low_limit , 4999.99 high_limit
union all
select 'average joes' name ,5000 low_limit, 9999.99 high_limit
union all
select 'high hitters' name,10000 low_limit,9999999.99 high_limit;

需求：根据顾客，统计每一个分组里的顾客数目，以了解顾客的经济情况，根据不同的顾客提供对应的服务

思路：

1. 一个顾客有多个账户 （account表里）

2. group by每个顾客，统计每个顾客的总余额 (这里还涉及到账户类型一定要是account类型才是customer的accounts，因为还涉及到保险公司的账户类型，以及借贷的账户类型),所以还要跟产品表，产品类型表连接

3. 根据这个总余额 和 上面的余额分组做 内连接，进行范围查询。（范围查询的内连接，把两张表连接起来，形成一个新的临时表）

4. 最后，根据第3步得到的临时表，按分组名进行分组，group by groups.name,然后根据分组统计每个组里的人数。

   4.1 cust_rollup 临时表 为 每个customer的总余额

select groups.name,count(*) num_customers
from (select a.cust_id ,sum(a.avail_balance) cust_balance
      from account a
      inner join product p
      on a.product_cd = p.product_cd
      where p.product_type_cd = 'account'
      group by a.cust_id
     ) cust_rollup
     inner join 
     (select 'small fry' name , 0 low_limit , 4999.99 high_limit
      union all
      select 'average joes' name ,5000 low_limit, 9999.99 high_limit
      union all
      select 'high hitters' name,10000 low_limit,9999999.99 high_limit) groups
      on cust_rollup.cust_balance between groups.low_limit and groups.high_limit
 group by groups.name;

不进行group by,可以显示每个分组的顾客id

select groups.name,cust_id
from (select a.cust_id ,sum(a.avail_balance) cust_balance
      from account a
      inner join product p
      on a.product_cd = p.product_cd
      where p.product_type_cd = 'account'
      group by a.cust_id
     ) cust_rollup
     inner join 
     (select 'small fry' name , 0 low_limit , 4999.99 high_limit
      union all
      select 'average joes' name ,5000 low_limit, 9999.99 high_limit
      union all
      select 'high hitters' name,10000 low_limit,9999999.99 high_limit) groups
      on cust_rollup.cust_balance between groups.low_limit and groups.high_limit
order by groups.name,cust_id asc;

效果:

3. 面向任务的子查询