Postgres: select all row with count of a field greater than 1

Window Functions are really nice for this.

SELECT p.*, count(*) OVER (PARTITION BY name) FROM product p;

For a full example:

CREATE TABLE product (no SERIAL, name text, price NUMERIC(8,2), date DATE);

INSERT INTO product(name, price, date) values
('paper', 1.99, '2017-03-23'),
('paper', 2.99, '2017-05-25'),
('paper', 1.99, '2017-05-29'),
('orange', 4.56, '2017-04-23'),
('apple', 3.43, '2017-03-11')
;

WITH report AS (
  SELECT p.*, count(*) OVER (PARTITION BY name) as count FROM product p
)
SELECT * FROM report WHERE count > 1;

Gives:

 no |  name  | price |    date    | count
----+--------+-------+------------+-------
  1 | paper  |  1.99 | 2017-03-23 |     3
  2 | paper  |  2.99 | 2017-05-25 |     3
  3 | paper  |  1.99 | 2017-05-29 |     3
(3 rows)

Try this:

SELECT no, name, price, "date"
FROM (
  SELECT no, name, price, "date",
         COUNT(*) OVER (PARTITION BY name) AS cnt 
  FROM product_price_info ) AS t
WHERE t.cnt > 1

You can use the window version of COUNT to get the population of each name partition. Then, in an outer query, filter out name partitions having a population that is less than 2.

SELECT * 
FROM product_price_info 
WHERE name IN (SELECT name 
               FROM product_price_info 
               GROUP BY name HAVING COUNT(*) > 1)