Konvertera datatyper

Rapportering i SQL

Tyler Pernes

Learning & Development Consultant

Kapitlets mål

"Stökiga" datatyper

Problem 1: Typspecifika funktioner

Problem 2: Kombinera tabeller (JOIN)

Problem 2: Kombinera tabeller (UNION)

Tolka felmeddelanden

Fel vid typspecifik funktion:

SELECT AVG(first_name)
FROM athletes;

ERROR: Function avg(character varying) does not exist

Fel vid JOIN:

SELECT country, continent
FROM countries AS c1
JOIN continents AS c2
ON c1.continent_id = c2.id;

ERROR: Operator does not exist: integer = character varying

Lösning: Använd CAST()

Syntax:

CAST(field AS type)

Exempel:

CAST(birthday AS date)
CAST(country_id AS int)

CAST med funktioner

SELECT DATE_PART('month',birthdate)
FROM birthdates;

ERROR: Can't run DATE_PART on string.

SELECT DATE_PART('month', 
       CAST(birthdate AS date)) 
FROM birthdates;

+-----+
| 04  |
| 05  |
+-----+

CAST med JOINs

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = b.id;

ERROR: Cannot join ON varchar = int.

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = CAST(b.id AS varchar);

Query Ran Successfully!

Hantera datatypsproblem

Åtgärda problemen när de uppstår
Läs felmeddelanden!

Hantera datatypsproblem

SELECT column_name, data_type
FROM information_schema.columns
WHERE table_name = 'countries';

+--------------+--------------------+
| column_name  | data_type          |
|--------------|--------------------|
| id           | integer            |
| country      | character varying  |
| region       | character varying  |
+--------------+--------------------+

Dokumentation för datatyper

https://www.postgresql.org/docs/9.5/datatype.html

Nu kör vi en övning!

Rapportering i SQL