mysql - SQL query for run-length, or consecutive identical value encoding -


my goal take set of data ordered id , return resultset indicates number of consecutive rows val column identical. e.g. given data:

| id | val | |  1 |  33 | |  2 |  33 | |  3 |  44 | |  4 |  28 | |  5 |  44 | |  6 |  44 |

i see result:

| id | val | run_length | | 1  | 33  | 2          | | 3  | 44  | 1          | | 4  | 28  | 1          | | 5  | 44  | 2          |

the id column in resultset optional. in fact, if makes harder, leave column out of result. sort of having because "pins" resultset particular location in table.

i interested in result in free database engines. order of preference solution is:

  1. sqlite
  2. postgres
  3. mysql
  4. oracle
  5. sql server
  6. sybase

i'll choose #2 on list, because incredibly painful in sqlite single query. following standard sql:

select min(id), val, count(*) runlength (select t.*,              (row_number() on (order id) -               row_number() on (partition val order id              ) grp       data t      ) t group grp, val;

this uses difference of 2 row number calculations identify seuqnces of identical values. should work in recent versions of databases 2, 4, 5, , 6.


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