ORDER BY

PostgreSQL Summary Stats and Window Functions

Michel Semaan

Data Scientist

Row numbers

Query

SELECT
  Year, Event, Country,
  ROW_NUMBER() OVER () AS Row_N
FROM Summer_Medals
WHERE
  Medal = 'Gold';

Result*

| Year | Event                      | Country | Row_N |
|------|----------------------------|---------|-------|
| 1896 | 100M Freestyle             | HUN     | 1     |
| 1896 | 100M Freestyle For Sailors | GRE     | 2     |
| 1896 | 1200M Freestyle            | HUN     | 3     |
| ...  | ...                        | ...     | ...   |

Enter ORDER BY

ORDER BY in OVER orders the rows related to the current row
- Example: Ordering by year in descending order in ROW_NUMBER's OVER clause will assign 1 to the most recent year's rows

Ordering by Year in descending order

Query

SELECT
  Year, Event, Country,
  ROW_NUMBER() OVER (ORDER BY Year DESC) AS Row_N
FROM Summer_Medals
WHERE
  Medal = 'Gold';

Result

| Year | Event         | Country | Row_N |
|------|---------------|---------|-------|
| 2012 | Wg 96 KG      | IRI     | 1     |
| 2012 | 4X100M Medley | USA     | 2     |
| 2012 | Wg 84 KG      | RUS     | 3     |
| ...  | ...           | ...     | ...   |
| 2008 | 50M Freestyle | BRA     | 637   |
| 2008 | 96 - 120KG    | CUB     | 638   |
| ...  | ...           | ...     | ...   |

Ordering by multiple columns

Query

SELECT
  Year, Event, Country,
  ROW_NUMBER() OVER
    (ORDER BY Year DESC, Event ASC) AS Row_N
FROM Summer_Medals
WHERE
  Medal = 'Gold';

Result

| Year | Event   | Country | Row_N |
|------|---------|---------|-------|
| 2012 | + 100KG | FRA     | 1     |
| 2012 | + 67 KG | SRB     | 2     |
| 2012 | + 78KG  | CUB     | 3     |
| ...  | ...     | ...     | ...   |

Ordering in- and outside OVER

Query

SELECT
  Year, Event, Country,
  ROW_NUMBER() OVER
    (ORDER BY Year DESC, Event ASC) AS Row_N
FROM Summer_Medals
WHERE
  Medal = 'Gold'
ORDER BY Country ASC, Row_N ASC;

Result

| Year | Event   | Country | Row_N |
|------|---------|---------|-------|
| 2012 | 1500M   | ALG     | 36    |
| 2000 | 1500M   | ALG     | 1998  |
| 1996 | 1500M   | ALG     | 2662  |
| ...  | ...     | ...     | ...   |

ORDER BY inside OVER takes effect before ORDER BY outside OVER

Reigning champion

A reigning champion is a champion who's won both the previous and current years' competitions
The previous and current year's champions need to be in the same row (in two different columns)

Enter LAG

LAG(column, n) OVER (...) returns column's value at the row n rows before the current row
- LAG(column, 1) OVER (...) returns the previous row's value

Current champions

Query

SELECT
  Year, Country AS Champion
FROM Summer_Medals
WHERE
  Year IN (1996, 2000, 2004, 2008, 2012)
  AND Gender = 'Men' AND Medal = 'Gold'
  AND Event = 'Discus Throw';

Result

| Year | Champion |
|------|----------|
| 1996 | GER      |
| 2000 | LTU      |
| 2004 | LTU      |
| 2008 | EST      |
| 2012 | GER      |

Current and last champions

Query

WITH Discus_Gold AS (
  SELECT
    Year, Country AS Champion
  FROM Summer_Medals
  WHERE
    Year IN (1996, 2000, 2004, 2008, 2012)
    AND Gender = 'Men' AND Medal = 'Gold'
    AND Event = 'Discus Throw')

SELECT
  Year, Champion,
  LAG(Champion, 1) OVER
    (ORDER BY Year ASC) AS Last_Champion
FROM Discus_Gold
ORDER BY Year ASC;

Result

| Year | Champion | Last_Champion |
|------|----------|---------------|
| 1996 | GER      | null          |
| 2000 | LTU      | GER           |
| 2004 | LTU      | LTU           |
| 2008 | EST      | LTU           |
| 2012 | GER      | EST           |

Let's practice!

PostgreSQL Summary Stats and Window Functions