Oracle笔记--WITH AS 用法

读《Oracle 查询优化改写技巧与案例》笔记

语法

with tempName as (select ....)

select ...

易读，多次调用效率也高

1.统计当年星期一，星期二...星期日各有多少天

with x0 as

(select trunc(sysdate,'yyyy') btime from dual),

x1 as

(select btime,add_months(btime,12) etime from x0),

x2 as

(select btime,etime,etime-btime day_num from x1),

x3 as

(select btime+(level-1) as dtime from x2 connect by level <= day_num),

x4 as

(select dtime,to_char(dtime,'dy') dy from x3)

select dy,count(*) from x4 group by dy

2.统计当年每个月的天数

with x0 as

     (select trunc(sysdate,'yyyy') btime,add_months(trunc(sysdate,'yyyy'),12) from dual)

     select add_months(btime,level-1) yyyymm,add_months(btime,level)-add_months(btime,level-1) days 

            from x0 connect by level<=12

相同SQL

select

      to_char(add_months(trunc(sysdate,'yyyy'),level-1),'yyyymm') year_month,

      last_day(add_months(trunc(sysdate,'yyyy'),level-1))-add_months(trunc(sysdate,'yyyy'),level-1)+1 days

from  dual connect by level<=12