Subconsulta y expresiones comunes de tabla

Introducción a Snowflake SQL

George Boorman

Senior Curriculum Manager, DataCamp

Subconsulta

Consultas anidadas
Se utiliza en las cláusulas « FROM », « WHERE », « HAVING » o « SELECT ».

Ejemplo:

SELECT column1 
FROM table1 
WHERE column1 = (SELECT column2 FROM table2 WHERE condition)

Tipos: Subconsultas correlacionadas y no correlacionadas

Subconsulta no correlacionada

-- Main query returns pizzas priced at the maximum value found in the subquery
SELECT pizza_id
FROM pizzas
-- Uncorrelated subquery that identifies the highest pizza price
WHERE price = (
    SELECT MAX(price)
    FROM pizzas
)

La subconsulta no interactúa con la consulta principal.

Subconsulta correlacionada

La subconsulta hace referencia a columnas de la consulta principal.

SELECT pt.name, 
       pz.price, 
       pt.category
FROM pizzas AS pz
JOIN pizza_type AS pt 
    ON pz.pizza_type_id = pt.pizza_type_id
WHERE pz.price = (
  -- Identifies max price for each pizza category
    SELECT MAX(p2.price) -- Max price
    FROM pizzas AS p2
    WHERE -- Correlated: uses outer query column 
      p2.pizza_type_id = pz.pizza_type_id
)

Expresiones de tabla comunes

Sintaxis general:

-- WITH keyword
WITH cte1 AS ( -- CTE name
        SELECT col_1, col_2
            FROM table1
    )
    ...
SELECT ... 
FROM cte1 -- Query CTE
;

Expresiones de tabla comunes

WITH max_price AS ( -- CTE called max_price
    SELECT pizza_type_id, 
           MAX(price) AS max_price
    FROM pizzas
    GROUP BY pizza_type_id
)

-- Main query 
SELECT pt.name, 
       pz.price, 
       pt.category
FROM pizzas AS pz
JOIN pizza_type AS pt ON pz.pizza_type_id = pt.pizza_type_id
JOIN max_price AS mp  -- Joining with CTE max_price
     ON pt.pizza_type_id = mp.pizza_type_id 
WHERE pz.price < mp.max_price -- Compare the price with max_price CTE column

Múltiples CTE

-- Define multiple CTEs separated by commas
WITH cte1 AS (
    SELECT ...
    FROM ...
),

cte2 AS (
    SELECT ...
    FROM ...
)

-- Main query combining both CTEs
SELECT ...
FROM cte1
JOIN cte2 ON ...
WHERE ...

¿Por qué utilizar CTE?

Gestión de operaciones complejas
Modular
Legible
Reutilizable

¡Vamos a practicar!

Introducción a Snowflake SQL