Question

I am working on a postgresql query that i am not sure how to produce the output.

Lets say i have a sql query whose output i want is

         name    date     visit_number  visit
          x    2011-01-01     123         ??   (value i want=1)
          y    2011-01-01     123         ??   (value i want=empty)
          a    2011-02-02     345         ??   (value i want=1) 
          b    2011-02-02     345         ??    (empty)
          c    2011-02-02     345         ??     (empty)

currently my sql query contains all the values except the last column visit. I want the visit column to work this way…if visit_number contains same value for multiple rows, i want the column visit to show the value 1 for the first row and just null or empty for the remaining rows where the visit_number is the same. How do i do that???

i could write the sample query in any way.it could simply be :

    select name,date,visit_number from sometable order by date;

I am using postgres 8.1 version.

Thanks

Answer 1

The first thing you should do is upgrade to a modern day version of PostgreSQL. Version 8.1 has reached end of life in November 2010.

In a more recent version you can conveniently solve this with window functions:

SELECT name, date, visit_number
     , CASE WHEN row_number() OVER (PARTITION BY visit_number
                                    ORDER BY date, name) = 1
          THEN 1
          ELSE NULL
       END AS visit
FROM   tbl
ORDER  BY date, name;

I ordered by name additionally to break ties.

---

For versions before PostgreSQL 8.4, this query should work (untested):

SELECT name, date, visit_number
     , CASE WHEN EXISTS (
         SELECT *
         FROM   tbl t1
         WHERE  t1.visit_number =  tbl.visit_number -- more to make it unique?
         AND    t1.date <= tbl.date -- or more columns to make order unambiguous
         AND    t1.name <  tbl.name
         )
       THEN NULL ELSE 1 END AS visit
FROM   tbl
ORDER  BY date, name;