我在 Oracle SQL 中的代码的查询优化

Question

Here is my code:

    select 
    (case when c.yr = 2019 and c.mon = 10 then 'October 2019' 
    when c.yr = 2019 and c.mon = 11 then 'November 2019' 
    when c.yr =2019 and c.mon = 12 then 'December 2019' end) as dae 
    from ( 
          select substr(d,-4,4) as yr, substr(d,1,2) as mon 
          from 
          (select '10/11/2019' as d from dual)  )c;                                                                                                                                                                                                                                                                       
`

So I don't want to hard code the dates for the next 5 years, Is there a function that makes this easier.

Here is the Sample Input I want to try

10/11/2019
11/11/2019
12/11/2019
01/11/2020

Expected Output

October 2019
November 2019
December 2019
January 2020

Answer 1

You could use to_date() to turn your string to a date, and then convert it back to a string in the desired format with to_char() :

to_char(to_date(d, 'mm/dd/yyyy'), 'Month yyyy')

Demo on DB Fiddle :

with t as (
    select '10/11/2019' d from dual
    union all select '11/11/2019' from dual
    union all select '12/11/2019' from dual
    union all select '01/11/2020' from dual
)
select to_char(to_date(d, 'mm/dd/yyyy'), 'Month yyyy') new_dt from t

| NEW_DT         |
| :------------- |
| October   2019 |
| November  2019 |
| December  2019 |
| January   2020 |

Answer 2

Use connect by to generate as many dates as you want. Here the gen_dates CTE starts with your start_date and returns a total of 4 months per your example. To increase the number of months to generate, increase the number 4 to a higher number.

with gen_dates(date_in) as (
  select add_months('11-OCT-2019', level -1) date_in
  from dual
  connect by level <= 4
)
select date_in, to_char(date_in, 'Month yyyy') date_out 
from gen_dates; 


DATE_IN   DATE_OUT      
--------- --------------
11-OCT-19 October   2019
11-NOV-19 November  2019
11-DEC-19 December  2019
11-JAN-20 January   2020

4 rows selected.