ROLLUP

Data-Driven Decision Making in SQL

Bart Baesens

Professor Data Science and Analytics

Table renting_extended

The first few rows of the table renting_extended:

| renting_id | country  | genre  | rating |
|------------|----------|--------|--------|
| 2          | Belgium  | Drama  | 10     |
| 32         | Belgium  | Drama  | 10     |
| 203        | Austria  | Drama  | 6      |
| 292        | Austria  | Comedy | 8      |
| 363        | Belgium  | Drama  | 7      |
| .......... | ........ | ...... | ...... |

Query with ROLLUP

SELECT country, 
       genre, 
       COUNT(*)
FROM renting_extended
GROUP BY ROLLUP (country, genre);

Levels of aggregation
- Aggregation of each combination of country and genre
- Aggregation of country alone
- Total aggregation

Query with ROLLUP

SELECT country, 
       genre, 
       COUNT(*)
FROM renting_extended
GROUP BY ROLLUP (country, genre);

| country | genre  | count |
|---------|--------|-------|
| null    | null   | 22    |
| Austria | Comedy | 2     |
| Belgium | Drama  | 15    |
| Austria | Drama  | 4     |
| Belgium | Comedy | 1     |
| Belgium | null   | 16    |
| Austria | null   | 6     |

Order in ROLLUP

SELECT country, 
       genre, 
       COUNT(*)
FROM renting_extended
GROUP BY ROLLUP (genre, country);

| country | genre  | count |
|---------|--------|-------|
| null    | null   | 22    |
| Austria | Comedy | 2     |
| Belgium | Drama  | 15    |
| Austria | Drama  | 4     |
| Belgium | Comedy | 1     |
| null    | Comedy | 3     |
| null    | Drama  | 19    |

Summary ROLLUP

Returns aggregates for a hierarchy of values, e.g. ROLLUP (country, genre)
- Movie rentals for each country and each genre
- Movie rentals for each country
- Total number of movie rentals
In each step, one level of detail is dropped
Order of column names is important for ROLLUP

Number of rentals and ratings

SELECT country, 
       genre, 
       COUNT(*) AS n_rentals,
       COUNT(rating) AS n_ratings
FROM renting_extended
GROUP BY ROLLUP (genre, country);

| country  | genre  | n_rentals | n_ratings |
|----------|--------|-----------|-----------|
| null     | null   | 22        | 9         |
| Belgium  | Drama  | 15        | 6         |
| Austria  | Comedy | 2         | 1         |
| Belgium  | Comedy | 1         | 0         |
| Austria  | Drama  | 4         | 2         |
| null     | Comedy | 3         | 1         |
| null     | Drama  | 19        | 8         |

Let's practice!

Data-Driven Decision Making in SQL