选读SQL经典实例笔记06_日期处理（上）.png

1. 计算一年有多少天

1.1. 方案

1.1.1. 找到当前年份的第一天

1.1.2. 加上1年以得到下一年的第一天

1.1.3. 得到的结果减去第一步得到的结果

1.2. DB2

1.2.1. sql

select days((curr_year + 1 year)) - days(curr_year)
   from (
 select (current_date -
         dayofyear(current_date) day +
          1 day) curr_year
   from t1
        ) x

1.3. Oracle

1.3.1. sql

select add_months(trunc(sysdate,'y'),12) - trunc(sysdate,'y')
   from dual

1.4. PostgreSQL

1.4.1. sql

select cast((curr_year + interval '1 year') as date) - curr_year
   from (
 select cast(date_trunc('year',current_date) as date) as curr_year
   from t1
        ) x

1.5. MySQL

1.5.1. sql

select datediff((curr_year + interval 1 year),curr_year)
   from (
 select adddate(current_date,-dayofyear(current_date)+1) curr_year
   from t1
        ) x

1.6. SQL Server

1.6.1. sql

select datediff(d,curr_year,dateadd(yy,1,curr_year))
   from (
 select dateadd(d,-datepart(dy,getdate())+1,getdate()) curr_year
   from t1
        ) x

2. 日期值里提取年月日时分秒

2.1. DB2

2.1.1. sql

select   hour( current_timestamp ) hr,
        minute( current_timestamp ) min,
        second( current_timestamp ) sec,
           day( current_timestamp ) dy,
         month( current_timestamp ) mth,
          year( current_timestamp ) yr
   from t1

2.2. Oracle

2.2.1. sql

select to_number(to_char(sysdate,'hh24')) hour,
        to_number(to_char(sysdate,'mi')) min,
        to_number(to_char(sysdate,'ss')) sec,
        to_number(to_char(sysdate,'dd')) day,
        to_number(to_char(sysdate,'mm')) mth,
        to_number(to_char(sysdate,'yyyy')) year
    from dual

2.3. PostgreSQL

2.3.1. sql

select to_number(to_char(current_timestamp,'hh24'),'99') as hr,
        to_number(to_char(current_timestamp,'mi'),'99') as min,
        to_number(to_char(current_timestamp,'ss'),'99') as sec,
        to_number(to_char(current_timestamp,'dd'),'99') as day,
        to_number(to_char(current_timestamp,'mm'),'99') as mth,
        to_number(to_char(current_timestamp,'yyyy'),'9999') as yr
   from t1

2.4. MySQL

2.4.1. sql

select date_format(current_timestamp,'%k') hr,
        date_format(current_timestamp,'%i') min,
        date_format(current_timestamp,'%s') sec,
        date_format(current_timestamp,'%d') dy,
        date_format(current_timestamp,'%m') mon,
        date_format(current_timestamp,'%Y') yr
   from t1

2.5. SQL Server

2.5.1. sql

select datepart( hour, getdate()) hr,
        datepart( minute,getdate()) min,
        datepart( second,getdate()) sec,
        datepart( day,   getdate()) dy,
        datepart( month, getdate()) mon,
        datepart( year, getdate()) yr
   from t1

3. 一个月的第一天和最后一天

3.1. DB2

3.1.1. sql

select (current_date - day(current_date) day +1 day) firstday,
        (current_date +1 month -day(current_date) day) lastday
   from t1

3.2. Oracle

3.2.1. sql

select trunc(sysdate,'mm') firstday,
        last_day(sysdate) lastday
   from dual

3.3. PostgreSQL

3.3.1. sql

select firstday,
        cast(firstday + interval '1 month'
                      - interval '1 day' as date) as lastday
   from (
 select cast(date_trunc('month',current_date) as date) as firstday
   from t1
        ) x

3.4. MySQL

3.4.1. sql

select date_add(current_date,
                 interval -day(current_date)+1 day) firstday,
        last_day(current_date) lastday
   from t1

3.5. SQL Server

3.5.1. sql

select dateadd(day,-day(getdate())+1,getdate()) firstday,
       dateadd(day,
               -day(getdate( )),
               dateadd(month,1,getdate())) lastday
   from t1

4. 当前月份的第一个和最后一个星期一

4.1. DB2

4.1.1. sql

with x (dy,mth,is_monday)
       as (
  select dy,month(dy),
          case when dayname(dy)='Monday'
               then 1 else 0
          end
     from (
   select (current_date-day(current_date) day +1 day) dy
     from t1
          ) tmp1
   union all
  select (dy +1 day), mth,
         case when dayname(dy +1 day)='Monday'
               then 1 else 0
         end
    from x
   where month(dy +1 day) = mth
  )
  select min(dy) first_monday, max(dy) last_monday
    from x
   where is_monday = 1

4.2. Oracle

4.2.1. sql

select next_day(trunc(sysdate,'mm')-1,'MONDAY') first_monday,
       next_day(last_day(trunc(sysdate,'mm'))-7,'MONDAY') last_monday
  from dual

4.3. PostgreSQL

4.3.1. sql

select first_monday,
          case to_char(first_monday+28,'mm')
               when mth then first_monday+28
                        else first_monday+21
          end as last_monday
     from (
   select case sign(cast(to_char(dy,'d') as integer)-2)
               when  0
               then dy
              when -1
              then dy+abs(cast(to_char(dy,'d') as integer)-2)
              when 1
              then (7-(cast(to_char(dy,'d') as integer)-2))+dy
         end as first_monday,
         mth
    from (
  select cast(date_trunc('month',current_date) as date) as dy,
         to_char(current_date,'mm') as mth
    from t1
         ) x
         ) y

4.4. MySQL

4.4.1. sql

select first_monday,
          case month(adddate(first_monday,28))
               when mth then adddate(first_monday,28)
                        else adddate(first_monday,21)
          end last_monday
     from (
   select case sign(dayofweek(dy)-2)
               when 0 then dy
               when -1 then adddate(dy,abs(dayofweek(dy)-2))
              when 1 then adddate(dy,(7-(dayofweek(dy)-2)))
         end first_monday,
         mth
    from (
  select adddate(adddate(current_date,-day(current_date)),1) dy,
         month(current_date) mth
    from t1
         ) x
         ) y

4.5. SQL Server

4.5.1. sql

with x (dy,mth,is_monday)
       as (
   select dy,mth,
          case when datepart(dw,dy) = 2
               then 1 else 0
          end
     from (
   select dateadd(day,1,dateadd(day,-day(getdate()),getdate())) dy,
          month(getdate()) mth
    from t1
         ) tmp1
   union all
  select dateadd(day,1,dy),
         mth,
         case when datepart(dw,dateadd(day,1,dy)) = 2
              then 1 else 0
         end
    from x
   where month(dateadd(day,1,dy)) = mth
  )
  select min(dy) first_monday,
         max(dy) last_monday
    from x
   where is_monday = 1

5. 一年中所有的星期五

5.1. DB2

5.1.1. sql

with x (dy,yr)
       as (
   select dy, year(dy) yr
     from (
   select (current_date -
            dayofyear(current_date) days +1 days) as dy
     from t1
           ) tmp1
    union all
  select dy+1 days, yr
    from x
   where year(dy +1 day) = yr
  )
  select dy
    from x
   where dayname(dy) = 'Friday'

5.2. Oracle

5.2.1. sql

with x
       as (
   select trunc(sysdate,'y')+level-1 dy
     from t1
     connect by level ＜=
        add_months(trunc(sysdate,'y'),12)-trunc(sysdate,'y')
   )
   select *
     from x
   where to_char( dy, 'dy') = 'fri'

5.3. PostgreSQL

5.3.1. sql

select cast(date_trunc('year',current_date) as date)
          + x.id as dy
    from generate_series (
          0,
          ( select cast(
                   cast(
             date_trunc('year',current_date) as date)
                        + interval '1 years' as date)
                        - cast(
                   date_trunc('year',current_date) as date) )-1
         ) x(id)
  where to_char(
           cast(
     date_trunc('year',current_date)
                as date)+x.id,'dy') = 'fri'

5.4. MySQL

5.4.1. sql

select dy
     from (
   select adddate(x.dy,interval t500.id-1 day) dy
     from (
   select dy, year(dy) yr
     from (
   select adddate(
          adddate(current_date,
                  interval -dayofyear(current_date) day),
                 interval 1 day ) dy
    from t1
         ) tmp1
         ) x,
         t500
   where year(adddate(x.dy,interval t500.id-1 day)) = x.yr
         ) tmp2
   where dayname(dy) = 'Friday'

5.5. SQL Server

5.5.1. sql

with x (dy,yr)
       as (
   select dy, year(dy) yr
     from (
   select getdate()-datepart(dy,getdate())+1 dy
      from t1
           ) tmp1
    union all
   select dateadd(dd,1,dy), yr
   from x
   where year(dateadd(dd,1,dy)) = yr
  )
  select x.dy
    from x
   where datename(dw,x.dy) = 'Friday'
  option (maxrecursion 400)

6. 判断闰年

6.1. DB2

6.1.1. sql

 with x (dy,mth)
     as (
   select dy, month(dy)
     from (
   select (current_date -
            dayofyear(current_date) days +1 days)
             +1 months as dy
     from t1
           ) tmp1
   union all
  select dy+1 days, mth
    from x
   where month(dy+1 day) = mth
  )
  select max(day(dy))
    from x

6.2. Oracle

6.2.1. sql

select to_char(
          last_day(add_months(trunc(sysdate,'y'),1)),
         'DD')
   from t1

6.3. PostgreSQL

6.3.1. sql

select max(to_char(tmp2.dy+x.id,'DD')) as dy
     from (
   select dy, to_char(dy,'MM') as mth
     from (
   select cast(cast(
               date_trunc('year',current_date) as date)
                          + interval '1 month' as date) as dy
     from t1
           ) tmp1
          ) tmp2, generate_series (0,29) x(id)
    where to_char(tmp2.dy+x.id,'MM') = tmp2.mth

6.4. MySQL

6.4.1. sql

select day(
         last_day(
         date_add(
         date_add(
         date_add(current_date,
                  interval -dayofyear(current_date) day),
                  interval 1 day),
                  interval 1 month))) dy
   from t1

6.5. SQL Server

6.5.1. sql

with x (dy,mth)
       as (
   select dy, month(dy)
     from (
   select dateadd(mm,1,(getdate()-datepart(dy,getdate()))+1) dy
     from t1
          ) tmp1
    union all
   select dateadd(dd,1,dy), mth
    from x
   where month(dateadd(dd,1,dy)) = mth
  )
  select max(day(dy))
    from x