The keys to the database

Analisi esplorativa dei dati in SQL

Christina Maimone

Data Scientist

Foreign keys

Foreign key example linking films to actors

Reference another row
- In a different table or the same table
- Via a unique ID

>> Primary key column containing unique, non-NULL values

Entity relationship diagram

Entity relationship diagram showing link between tag_company and company tables

Entity relationship diagram showing self-reference in company table

Entity relationship diagram

Entity relationship diagram with primary keys

coalesce(value_1, value_2 [, ...])

SELECT *
  FROM prices;

 column_1 | column_2 
----------+----------
          |       10
          |         
       22 |         
        3 |        4

SELECT coalesce(column_1, column_2)
  FROM prices;

 coalesce 
----------
       10

       22
        3

Analisi esplorativa dei dati in SQL