NULL-waarden

Gevorderde SQL

Jasmin Ludolf

Data Science Content Developer, DataCamp

Ontbrekende waarden

COUNT(field_name) bevat alleen waarden die er zijn
COUNT(*) inclusief ontbrekende waarden

null

Ontbrekende waarden:
- Menselijke fout
- Informatie niet beschikbaar
- Onbekend

null

SELECT COUNT(*) AS count_records
FROM people;

|count_records|
|-------------|
|8397         |

SELECT *
FROM people;

|id|name              |birthdate |deathdate|
|--|------------------|----------|---------|
|1 |50 Cent           |1975-07-06|null     |
|2 |A. Michael Baldwin|1963-04-04|null     |
|3 |A. Raven Cruz     |null      |null     |
...

IS NULL

SELECT name
FROM people
WHERE birthdate IS NULL;

|name         |
|-------------|
|A. Raven Cruz|
|A.J. DeLucia |
|Aaron Hann   |
...

IS NOT NULL

SELECT COUNT(*) AS no_birthdates
FROM people
WHERE birthdate IS NULL;

|no_birthdates|
|-------------|
|2245         |

SELECT COUNT(name) AS count_birthdates
FROM people
WHERE birthdate IS NOT NULL;

|count_birthdates|
|----------------|
|6152            |

COUNT() vs IS NOT NULL

SELECT 
   COUNT(certification) 
   AS count_certification
FROM films;

|count_certification|
|-------------------|
|4666               |

SELECT 
   COUNT(certification) 
   AS count_certification
FROM films
WHERE certification IS NOT NULL;

|count_certification|
|-------------------|
|4666               |

NULL in het kort

NULL-waarden zijn ontbrekende waarden
Veelvoorkomend
Gebruik IS NULL of IS NOT NULL om:
- ontbrekende waarden te identificeren
- ontbrekende waarden te selecteren
- ontbrekende waarden uit te sluiten

Laten we oefenen!

Gevorderde SQL