NULL-Werte

SQL für Fortgeschrittene

Jasmin Ludolf

Data Science Content Developer, DataCamp

Fehlende Werte

COUNT(field_name) enthält nur nicht-fehlende Werte
COUNT(*) enthält fehlende Werte

null

Fehlende Werte
- Menschliches Versagen
- Information nicht verfügbar
- Unbekannt

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 einfach ausgedrückt

NULL-Werte sind fehlende Werte
Sehr häufig
Verwende IS NULL oder IS NOT NULL, um:
- Identifiziere fehlende Werte
- Wähle fehlende Werte aus
- Schließe fehlende Werte aus

Lass uns üben!

SQL für Fortgeschrittene