Question

I am trying to find the first lowest value in a table, and the date on which it occurs, for every DISTINCT user in a table.

This is the table schema and some sample data:

CREATE TABLE diet_watch (
  entry_date date NOT NULL,
  user_id    int default 1,
  weight     double precision NOT NULL
);

INSERT INTO diet_watch VALUES ('2001-01-01', 1, 128.2);
INSERT INTO diet_watch VALUES ('2001-01-02', 1, 121.0);
INSERT INTO diet_watch VALUES ('2001-01-03', 1, 122.3);
INSERT INTO diet_watch VALUES ('2001-01-04', 1, 303.7);
INSERT INTO diet_watch VALUES ('2001-01-05', 1, 121.0);
INSERT INTO diet_watch VALUES ('2001-01-01', 2, 121.0);
INSERT INTO diet_watch VALUES ('2001-01-06', 2, 128.0);

The SQL I came up with is here in this snippet

I have since been informed that it is incorrect, perhaps someone can explain what the problem with my SQL is ?

Note: I would prefer ANSI SQL if possible, but I am using PostgreSQL, so if I have to use a specific flavour of SQL, it has to work on PG.

Answer 1

Note: Not sure if Window functions are ANSI SQL

WINDOW Functions are a part of the SQL:2003 specification:
http://en.wikipedia.org/wiki/Window_function_%28SQL%29#Window_function (Thx
@a_horse_with_no_name)

Try this:

http://sqlfiddle.com/#!1/7aa4e/22

SELECT *
  FROM 
    (
     SELECT a.*, 
            ROW_NUMBER() OVER(PARTITION BY user_id ORDER BY weight) AS Position
       FROM diet_watch a

    ) a
    WHERE a.Position = 1