Spojování v Snowflake

Introduction to Snowflake SQL

George Boorman

Senior Curriculum Manager, DataCamp

JOINS

  • INNER JOIN
  • OUTER JOINS
    • LEFT OUTER JOIN nebo LEFT JOIN
    • RIGHT OUTER JOIN nebo RIGHT JOIN
    • FULL OUTER JOIN nebo FULL JOIN
  • CROSS JOINS
  • SELF JOINS
  • NATURAL JOIN
  • LATERAL JOIN
Introduction to Snowflake SQL

Datová sada Pizza

Diagram schématu pizzové databáze

Introduction to Snowflake SQL

NATURAL JOIN

  • NATURAL JOIN automaticky páruje sloupce a odstraňuje duplicitní

Syntax:

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

NATURAL JOIN

Bez NATURAL JOIN

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

Výsledek spojení bez NATURAL JOIN

S NATURAL JOIN

SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t

  Výsledek spojení s NATURAL JOIN

Introduction to Snowflake SQL

NATURAL JOIN

 

❌ NENÍ POVOLENO ❌

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

Obrázek chyby syntaxe

Introduction to Snowflake SQL

NATURAL JOIN

$$

✅ POVOLENO ✅

  • Klauzule WHERE
SELECT *
FROM pizzas AS p 
NATURAL JOIN pizza_type AS t
WHERE pizza_type_id = 'bbq_ckn'
Introduction to Snowflake SQL

LATERAL JOIN

  • LATERAL JOIN: umožňuje poddotazu v FROM odkazovat na sloupce z předcházejících tabulek nebo pohledů.

Syntax:

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

  • left_hand_expression – tabulka, pohled nebo poddotaz

  • right_hand_expression – inline pohled nebo poddotaz

Introduction to Snowflake SQL

LATERAL JOIN s poddotazem

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

LATERAL -- Klíčové slovo LATERAL
        ( SELECT * 
          FROM pizza_type AS t

         -- Odkaz na sloupec vnějšího dotazu: p.pizza_type_id
          WHERE p.pizza_type_id = t.pizza_type_id

        ) AS lat
Introduction to Snowflake SQL

Proč LATERAL JOIN?

SELECT 
    *
FROM orders AS o,
LATERAL (
   -- Poddotaz počítající 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
Introduction to Snowflake SQL

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Introduction to Snowflake SQL

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