How can I find duplicate consecutive values in this table?

Question

Say I have a table which I query like so:

select date, value from mytable order by date

and this gives me results:

date             


        

Answer 1

As @Janek Bogucki has pointed out LEAD and LAG are probably the easiest way to accomplish this - but just for fun let's try to do it by using only basic join operations:

SELECT mydate, VALUE FROM
  (SELECT a.mydate, a.value,
          CASE WHEN a.value = b.value THEN '*' ELSE NULL END AS flag1,
          CASE WHEN a.value = c.value THEN '*' ELSE NULL END AS flag2
     FROM
       (SELECT ROWNUM AS outer_rownum, mydate, VALUE
         FROM mytable
         ORDER BY mydate) a
     LEFT OUTER JOIN
       (select ROWNUM-1 AS inner_rownum, mydate, VALUE
         from mytable
         order by myDATE) b
       ON b.inner_rownum = a.outer_rownum
     LEFT OUTER JOIN
       (select ROWNUM+1 AS inner_rownum, mydate, VALUE
         from mytable
         order by myDATE) c
       ON c.inner_rownum = a.outer_rownum
     ORDER BY a.mydate)
  WHERE flag1 = '*' OR
        flag2 = '*';

Share and enjoy.