dacology dacology - 4 months ago 28
SQL Question

Postgres - Transpose Rows to Columns

I have the following table, which gives multiple email addresses for each user.

enter image description here

I need to flatten this out to columns on a user query. To give me the "newest" 3 email addresses based on the creation date.

user.name | user.id | email1 | email2 | email3**

Mary | 123 | mary@gmail.com | mary@yahoo.co.uk | mary@test.com

Joe | 345 | joe@gmail.com | [NULL] | [NULL]

Answer

Use crosstab() from the tablefunc module.

SELECT * FROM crosstab(
   $$SELECT user_id, user_name, rn, email_address
     FROM  (
        SELECT u.user_id, u.user_name, e.email_address
             , row_number() OVER (PARTITION BY u.user_id
                            ORDER BY e.creation_date DESC NULLS LAST) AS rn
        FROM   usr u
        LEFT   JOIN email_tbl e USING (user_id)
        ) sub
     WHERE  rn < 4
     ORDER  BY user_id
   $$
  , 'VALUES (1),(2),(3)'
   ) AS t (user_id int, user_name text, email1 text, email2 text, email3 text);

I used dollar-quoting for the first parameter, which has no special meaning. It's just convenient if you have to escape single quotes in the query string which is a common case:

Detailed explanation and instructions here:

And in particular, for "extra columns":

The special difficulties here are:

  • The lack of key names.
    -> We substitute with row_number() in a subquery.

  • The varying number of emails.
    -> We limit to a max. of three in the outer SELECT
    and use crosstab() with two parameters, providing a list of possible keys.

Pay attention to NULLS LAST in the ORDER BY.