Join in Snowflake

Introduzione a Snowflake SQL

George Boorman

Senior Curriculum Manager, DataCamp

JOIN

  • INNER JOIN
  • OUTER JOIN
    • LEFT OUTER JOIN o LEFT JOIN
    • RIGHT OUTER JOIN o RIGHT JOIN
    • FULL OUTER JOIN o FULL JOIN
  • CROSS JOIN
  • SELF JOIN
  • NATURAL JOIN
  • LATERAL JOIN
Introduzione a Snowflake SQL

Dataset Pizza

Diagramma schema database Pizza

Introduzione a Snowflake SQL

NATURAL JOIN

  • NATURAL JOIN abbina automaticamente le colonne ed elimina i duplicati

Sintassi:

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

NATURAL JOIN

Senza NATURAL JOIN

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

Risultato join senza natural join

Con NATURAL JOIN

SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t

  Risultato join con natural join

Introduzione a Snowflake SQL

NATURAL JOIN

 

❌ NON CONSENTITO ❌

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

Errore di sintassi

Introduzione a Snowflake SQL

NATURAL JOIN

$$

✅ CONSENTITO ✅

  • Clausola WHERE
SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t
WHERE pizza_type_id = 'bbq_ckn'
Introduzione a Snowflake SQL

LATERAL JOIN

  • LATERAL JOIN: permette a una subquery in FROM di riferirsi a colonne di tabelle o viste precedenti.

Sintassi:

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

  • left_hand_expression - Tabella, vista o subquery

  • right_hand_expression - Inline view o subquery

Introduzione a Snowflake SQL

LATERAL JOIN con subquery

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

LATERAL -- Parola chiave LATERAL
        ( SELECT * 
          FROM pizza_type AS t

         -- Riferimento a colonna della query esterna: p.pizza_type_id
          WHERE p.pizza_type_id = t.pizza_type_id

        ) AS lat
Introduzione a Snowflake SQL

Perché LATERAL JOIN?

SELECT 
    *
FROM orders AS o,
LATERAL (
   -- Subquery che calcola 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
Introduzione a Snowflake SQL

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Introduzione a Snowflake SQL

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