Common Table Expressions

Datahantering i SQL

Mona Khalil

Data Scientist, Greenhouse Software

När man lägger till underfrågor...

Frågekomplexiteten ökar snabbt!
- Informationen kan bli svår att hålla reda på

Lösning: Common Table Expressions!

Common Table Expressions

Common Table Expressions (CTEs)

Tabell deklareras före huvudfrågan
Namnges och refereras senare i FROM-satsen

Skapa CTEs

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

Ta en underfråga i 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       |

Placera den i början

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

Placera den i början

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

Visa mig CTE:n

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       |

Visa mig alla CTE:er

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;

Varför använda CTEs?

Körs en gång
- CTE:n lagras sedan i minnet
- Förbättrar frågeprestanda
Förbättrar frågans struktur
Kan referera andra CTE:er
Kan referera sig själv (SELF JOIN)

Nu kör vi en övning!

Datahantering i SQL