Common table expressions

Introduzione a BigQuery

Matthew Forrest

Field CTO

Che cos'è una CTE?

-- Le parole chiave WITH avviano la CTE
WITH our_cte AS (
  SELECT column1, column2
  FROM table
)


/* La CTE è tra parentesi e può essere usata come
una tabella nella query principale sotto */
SELECT *
FROM our_cte

Subquery vs CTE

Subquery

SELECT *
FROM (
  SELECT order_id, status
  FROM ecommerce.ecomm_order_status
) AS subquery

CTE

WITH cte_name AS (
  SELECT order_id, status
  FROM ecommerce.ecomm_order_status
)
SELECT *
FROM cte_name

Scrivere CTE

-- Tutte le CTE iniziano con la parola chiave WITH
WITH cte_name AS


-- La CTE è racchiusa tra parentesi
(SELECT col_1, col_2
 FROM tablename WHERE col_1 > 10
)


-- La nuova CTE "cte_name" è accessibile solo nell'ambito di questa query

SELECT * FROM cte_name LIMIT 10

Usare più CTE

WITH orders AS (
  SELECT order_id, status
  FROM ecommerce.ecomm_order_status
),

-- Le CTE aggiuntive non riutilizzano la parola chiave WITH e sono separate da virgole
order_details AS (
SELECT orders.*, details.order_items
FROM ecommerce.ecomm_orders details 
```

Usare le CTE per filtrare i dati

WITH filtered_data AS (
  SELECT *
  FROM ecommerce.ecomm_orders
  -- Qui filtriamo i dati nella nostra CTE
  WHERE order_status = 'delivered'
)
SELECT order_id, order_status
FROM filtered_data

| order_id | order_status |
|----------|--------------|
| 1        | delivered    |
| 2        | delivered    |

Usare le CTE per ottimizzare le query

WITH precomputed_data AS (
  SELECT order_id, SUM(payment_value) AS total
  FROM ecommerce.ecomm_payments
  GROUP BY order_id
)
SELECT *
FROM precomputed_data
WHERE total > 1000

| order_id | total   |
|----------|---------|
| 1        | 50000   |
| 2        | 8000    |

Usare le CTE per unire dati

WITH cte1 AS (
  SELECT order_id, status
  FROM ecommerce.ecomm_order_status
),
cte2 AS (
  SELECT order_id, order_items
  FROM ecommerce.ecomm_orders
)
SELECT *
FROM cte1
JOIN cte2 USING (order_id)

Esercitiamoci!

Introduzione a BigQuery