Postgres - Transpose Rows to Columns

问题

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

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]

回答1:

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:

• Insert text with single quotes in PostgreSQL

Detailed explanation and instructions here:

• PostgreSQL Crosstab Query

And in particular, for "extra columns":

• Pivot on Multiple Columns using Tablefunc

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.

回答2:

If anyone else that finds this question and needs a dynamic solution for this where you have an undefined number of columns to transpose to and not exactly 3, you can find a nice solution here: https://github.com/jumpstarter-io/colpivot

来源：https://stackoverflow.com/questions/23060256/postgres-transpose-rows-to-columns