Joins in Snowflake

Einführung in Snowflake SQL

George Boorman

Senior Curriculum Manager, DataCamp

JOINS

  • INNER JOIN
  • OUTER JOINS
    • LEFTOUTERLEFT oder JOINLEFTJOINJOIN
    • RIGHTOUTERRIGHT oder JOINRIGHTJOINJOIN
    • FULLOUTERFULL oder JOINFULLJOINJOIN
  • CROSS JOINS
  • SELF JOINS
  • NATURAL JOIN
  • LATERAL JOIN
Einführung in Snowflake SQL

Pizza-Datensatz

Pizza database schema diagram

Einführung in Snowflake SQL

NATURAL JOIN

  • NATURAL JOIN gleicht Spalten automatisch ab und löscht doppelte Spalten

Syntax:

SELECT ...
FROM <table_one> [
                     {
                       | NATURAL [ { LEFT | RIGHT | FULL } [ OUTER ] ]
                     }
                   ]
                   JOIN <table_two>
[ ... ]
Einführung in Snowflake SQL

NATURAL JOIN

Ohne NATURAL JOIN

SELECT * 
FROM pizzas AS p 
JOIN  pizza_type AS t 
    ON t.pizza_type_id = p.pizza_type_id

Join result without natural join

Mit NATURAL JOIN

SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t

  Join result with natural join

Einführung in Snowflake SQL

NATURAL JOIN

 

❌ NICHT ERLAUBT ❌

select *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t
    ON  t.pizza_type_id = p.pizza_type_id

Syntax Error Image

Einführung in Snowflake SQL

NATURAL JOIN

$$

✅ ERLAUBT ✅

  • WHERE-Klausel
SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t
WHERE pizza_type_id = 'bbq_ckn'
Einführung in Snowflake SQL

LATERAL JOIN

  • LATERAL JOIN: ermöglicht es einer Unterabfrage in FROM auf Spalten in vorhergehenden Tabellen oder Sichten zuzugreifen

Syntax:

SELECT ...
FROM <left_hand_expression> , -- 
LATERAL 
(<right_hand_expression>)

  • left_hand_expression - Tabelle, Sicht oder Unterabfrage

  • right_hand_expression - Inline-Sicht oder Unterabfrage

Einführung in Snowflake SQL

LATERAL JOIN mit Unterabfrage

SELECT 
    p.pizza_id, 
    lat.name, 
    lat.category 
FROM pizzas AS p,

LATERAL -- Keyword LATERAL
        ( SELECT * 
          FROM pizza_type AS t

         -- Referencing outer query column: p.pizza_type_id
          WHERE p.pizza_type_id = t.pizza_type_id

        ) AS lat
Einführung in Snowflake SQL

Warum ein LATERAL JOIN?

SELECT 
    *
FROM orders AS o,
LATERAL (
   -- Subquery calculating total_spent
    SELECT 
        SUM(p.price * od.quantity) AS total_spent
    FROM order_details AS od
    JOIN pizzas AS p 
          ON od.pizza_id = p.pizza_id
    WHERE o.order_id = od.order_id
) AS t
ORDER BY o.order_id
Einführung in Snowflake SQL

Lass uns üben!

Einführung in Snowflake SQL

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