Filtering and ordering

Decision making basato sui dati in SQL

Tim Verdonck

Professor Statistics and Data Science

WHERE

Select all customers from Italy:

SELECT *
FROM customers
WHERE country = 'Italy';

|customer_id | name              | country | gender | date_of_birth | date_account_start|
|------------|-------------------|---------|--------|---------------|-------------------|
| 53         | Teresio Panicucci | Italy   | male   | 1999-07-21    | 2018-11-06        |
| 54         | Demetrio Palermo  | Italy   | male   | 1997-10-10    | 2018-10-17        |
| 55         | Facino Milano     | Italy   | male   | 1973-05-23    | 2018-01-02        |

Operators in the WHERE clause

Comparison operators:
- Equal =
- Not equal <>
- Less than <
- Less than or equal to <=
- Greater than >
- Greater than or equal to >=
BETWEEN operator
IN operator
IS NULL and IS NOT NULL operators

Example comparison operators

Select all columns from movies where the genre is not Drama.

SELECT *
FROM movies
WHERE genre <> 'Drama';

Select all columns from movies where the price for renting is larger equal 2.

SELECT *
FROM movies
WHERE renting_price >= 2;

Example: BETWEEN operator

Select all columns of customers where the date when the account was created is between 2018-01-01 and 2018-09-30.

SELECT *
FROM customers
WHERE date_account_start BETWEEN '2018-01-01' AND '2018-09-30';

Example: IN operator

Select all actors with nationality USA or Australia.

SELECT *
FROM actors
WHERE nationality IN ('USA', 'Australia')

Example: NULL operator

Select all columns from renting where rating is NULL.

SELECT *
FROM renting
WHERE rating IS NULL

Select all columns from renting where rating is not NULL.

SELECT *
FROM renting
WHERE rating IS NOT NULL

Boolean operators AND

Select customer name and the date when they created their account for customers who are from Italy AND who created an account between 2018-01-01 and 2018-09-30.

SELECT name, date_account_start
FROM customers
WHERE country = 'Italy'
AND date_account_start BETWEEN '2018-01-01' AND '2018-09-30';

| name               | date_account_start |
|--------------------|--------------------|
| Facino Milano      | 2018-01-02         |
| Mario Lettiere     | 2018-01-30         |
| Rocco Buccho       | 2018-02-27         |
| Cristoforo Mancini | 2018-01-12         |

Boolean operators OR

Select customer name and the date when they created their account for customers who are from Italy _OR_ who created an account between 2018-01-01 and 2018-09-30.

SELECT name, date_account_start
FROM customers
WHERE country = 'Italy'
OR date_account_start BETWEEN '2018-01-01' AND '2018-09-30';

| name                  | country | date_account_start |
|-----------------------|-------- |--------------------|
| Rowanne Couperus      | Belgium | 2018-08-26         |
| Annelous Sneep        | Belgium | 2018-05-12         |
| Jaëla van den Dolder  | Belgium | 2018-02-08         |
| ...                   | ...     | ...                |

ORDER BY

Order the results of a query by rating.

SELECT *
FROM renting
WHERE rating IS NOT NULL
ORDER BY rating;

| renting_id | customer_id | movie_id | rating | date_renting |
|------------|-------------|----------|--------|--------------|
| 552        | 28          | 56       | 1      | 2017-03-27   |
| 558        | 41          | 19       | 3      | 2019-01-13   |
| 444        | 120         | 59       | 3      | 2018-08-10   |
| 200        | 86          | 46       | 3      | 2018-08-26   |
| 234        | 104         | 28       | 4      | 2018-10-04   |

ORDER BY ... DESC

Order the results of a query by rating in descending order.

SELECT *
FROM renting
WHERE rating IS NOT NULL
ORDER BY rating DESC;

| renting_id | customer_id | movie_id | rating | date_renting |
|------------|-------------|----------|--------|--------------|
| 243        | 7           | 5        | 10     | 2019-01-11   |
| 18         | 36          | 39       | 10     | 2019-03-20   |
| 396        | 7           | 40       | 10     | 2018-09-11   |
| 487        | 61          | 48       | 10     | 2017-08-14   |
| 476        | 78          | 42       | 10     | 2018-07-04   |

Let's practice!

Decision making basato sui dati in SQL