compare the value with the previous line oracle sql ORACLE

Question

I have a table like this.

Date	Enddate
20012022	21012022
21012022	23012022
23012022	24012022
20012022	26012022
26012022	27012022
27012022	27012022

The next date entry is equal to the last one enddate. How do I find lines that don't follow this rule? In the example, line 4 (previus enddate 24012022 - next date 20012022).

I tried use

lag()

I can't understand how it works... Thanks for helping..

Answer 1

Here's one option.

Sample data:

SQL> with test (datum, enddatum) as
  2    (select date '2022-01-20', date '2022-01-21' from dual union all
  3     select date '2022-01-21', date '2022-01-23' from dual union all
  4     select date '2022-01-23', date '2022-01-24' from dual union all
  5     select date '2022-01-20', date '2022-12-26' from dual union all
  6     select date '2022-12-26', date '2022-12-27' from dual union all
  7     select date '2022-12-27', date '2022-12-27' from dual
  8    ),

Query begins here: find previous enddatum so that you could compare it to datum (line #17):

  9  temp as
 10    (select datum,
 11       enddatum,
 12       lag(enddatum) over (order by enddatum) previous_enddatum
 13     from test
 14    )
 15  select datum, enddatum
 16  from temp
 17  where datum <> previous_enddatum;

DATUM      ENDDATUM
---------- ----------
20.01.2022 26.12.2022

SQL>

Answer 2

The LAG() function's result depends on query partition clause and order by clause. Here are two codes giving different results if ordered by Start or End date:
Your sample data:

WITH 
    tbl (START_DATE, END_DATE) as
        (   Select DATE '2022-01-20', DATE '2022-01-21' From dual Union All
            Select DATE '2022-01-21', DATE '2022-01-23' From dual Union All
            Select DATE '2022-01-23', DATE '2022-01-24' From dual Union All
            Select DATE '2022-01-20', DATE '2022-12-26' From dual Union All
            Select DATE '2022-12-26', DATE '2022-12-27' From dual Union All
            Select DATE '2022-12-27', DATE '2022-12-27' From dual
        )

1. Using Order By END_DATE:

Select  START_DATE, END_DATE, 
        CASE 
            WHEN START_DATE != LAG(END_DATE) OVER(ORDER BY END_DATE) 
            THEN 'Should be ' || LAG(END_DATE) OVER(ORDER BY END_DATE) 
        END "END_DATE_CHECK"
From    tbl

START_DATE END_DATE  END_DATE_CHECK    
---------- --------- -------------------
20-JAN-22  21-JAN-22                     
21-JAN-22  23-JAN-22                     
23-JAN-22  24-JAN-22                     
20-JAN-22  26-DEC-22 Should be 24-JAN-22 
26-DEC-22  27-DEC-22                     
27-DEC-22  27-DEC-22    

2. Using Order By START_DATE

Select  START_DATE, END_DATE, 
        CASE 
            WHEN START_DATE != LAG(END_DATE) OVER(ORDER BY START_DATE) 
            THEN 'Should be ' || LAG(END_DATE) OVER(ORDER BY START_DATE) 
        END "END_DATE_CHECK"
From    tbl

START_DATE END_DATE  END_DATE_CHECK    
---------- --------- -------------------
20-JAN-22  21-JAN-22                     
20-JAN-22  26-DEC-22 Should be 21-JAN-22 
21-JAN-22  23-JAN-22 Should be 26-DEC-22 
23-JAN-22  24-JAN-22                     
26-DEC-22  27-DEC-22 Should be 24-JAN-22 
27-DEC-22  27-DEC-22                   

It looks like there is something missing in your sample data (some ID column maybe). Let's say that there is some column the dates belong to and that we could partition the dates by that column like below. There is no checking problems at all:
3. Using Partition By

WITH 
    tbl (ID, START_DATE, END_DATE) as
        (   Select 1, DATE '2022-01-20', DATE '2022-01-21' From dual Union All
            Select 1, DATE '2022-01-21', DATE '2022-01-23' From dual Union All
            Select 1, DATE '2022-01-23', DATE '2022-01-24' From dual Union All
            Select 2, DATE '2022-01-20', DATE '2022-12-26' From dual Union All
            Select 2, DATE '2022-12-26', DATE '2022-12-27' From dual Union All
            Select 2, DATE '2022-12-27', DATE '2022-12-27' From dual
        )

Select  ID, START_DATE, END_DATE, 
        CASE 
            WHEN START_DATE != LAG(END_DATE) OVER(Partition By ID ORDER BY START_DATE) 
            THEN 'Should be ' || LAG(END_DATE) OVER(Partition By ID ORDER BY START_DATE) 
        END "END_DATE_CHECK"
From    tbl

        ID START_DATE END_DATE  END_DATE_CHECK    
---------- ---------- --------- -------------------
         1 20-JAN-22  21-JAN-22                     
         1 21-JAN-22  23-JAN-22                     
         1 23-JAN-22  24-JAN-22                     
         2 20-JAN-22  26-DEC-22                     
         2 26-DEC-22  27-DEC-22                     
         2 27-DEC-22  27-DEC-22                  

In this case there is no difference using Start or End date ordering... More about LAG() OVER() here .