Expressões de tabela comuns

Manipulação de dados em SQL

Mona Khalil

Data Scientist, Greenhouse Software

Ao adicionar subconsultas...

A complexidade das consultas aumenta rápido!
- Pode ser difícil acompanhar as informações

Solução: Expressões de tabela comuns!

Expressões de tabela comuns

Expressões de tabela comuns (CTEs)

Tabela declarada antes da consulta principal
Nomeada e referenciada depois em uma instrução FROM

Configuração de CTEs

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

Pegue uma subconsulta no FROM

SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10) AS s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Coloque no começo

(
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Coloque no começo

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Mostre-me a CTE

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)
SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Mostre-me todas as CTEs

WITH s1 AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10),
s2 AS (                              -- New subquery
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) <= 1
)
SELECT
  c.name AS country,
  COUNT(s1.id) AS high_scores,
  COUNT(s2.id) AS low_scores         -- New column
FROM country AS c
INNER JOIN s1
ON c.id = s1.country_id
INNER JOIN s2                        -- New join
ON c.id = s2.country_id
GROUP BY country;

Por que usar CTEs?

Executada uma vez
- A CTE é então guardada na memória
- Melhora o desempenho das consultas
Melhorar a organização das consultas
Referenciar outras CTEs
Referenciar a si mesma (SELF JOIN)

Vamos praticar!

Manipulação de dados em SQL