How to create an Sql table from SELECT with a multiple row DISTINCT constraint?

Question

I want to create a table from a SELECT statement based on a multiple row DISTINCT, but PostgreSQL does not allow me to do it.

I need to create a table where the elements were previously selected from another table and they are all unique.

Here's my try:

CREATE TABLE new_name AS (

   SELECT DISTINCT(table.field1, table.field2), table.field1, table.field2, 
   FROM ...
   WHERE ...

);

If I run this statement, PostgreSQL says:

ERROR: column "row" has pseudo-type record

How could I solve this problem?

Answer 1

distinct is NOT a function. It always operates on all columns of the result.

The difference between select distinct (foo), bar and select distinct foo, bar is the same as the difference between select (foo), bar and select foo, bar. The parenthesis is just "noise".

When you write select (foo,bar) you are actually creating an anonymous record type in Postgres which results in a single column that has two attributes - which is not what you actually want.

---

As you are using Postgres, you can use the (proprietary) operator DISTINCT ON which - in contrast to the standard DISTINCT - does operate on a sub-set of the columns.

You have to specify an ORDER BY in that case to define which of the rows to take if there is more than one with the same combination of (field1, field2).

CREATE TABLE new_name 
AS 
SELECT DISTINCT ON (table.field1, table.field2), 
       table.field1, 
       table.field2, 
       .....
FROM ...
WHERE ...
ORDER BY ..

If you want to stick to ANSI SQL you will need a window function for this:

create table new_name
as
select column1, column2, column3, column4, column5, column6
from (
   select column1, column2, column3, column4, column5, column6, 
          row_number() over (partition by column1, column2 order by ...) as rn 
   from the_table
   where ...
) t
where rn = 1;

For a large table, DISTINCT ON is probably faster than the solution with the window function.