Subqueries and common table expressions (cte)

Improving Query Performance in PostgreSQL

Amy McCarty

Instructor

About subqueries

What?

Join alternative
Simple query

Why?

Can return one result
Readable
SQL instructions similar to joins

How?

In SELECT, FROM, or WHERE clauses

SELECT subquery

row	script_word	word_length
1	goat	4
2	goat	4
3	dog	3
15,782	...	...

row	english_word	word_length
1	goat	4
2	turkey	6
3	ant	3
171,476	...	...

SELECT subquery

SELECT AVG(word_length) AS avg_movie
  , (SELECT AVG(word_length) 
      FROM english_language) 
      AS avg_english
FROM MOVIE

avg_movie	avg_english
3	4.5

WHERE subquery

row	script_word	word_length
1	goat	4
2	goat	4
3	dog	3
15,782	...	...

row	english_word	word_length
1	goat	4
2	turkey	6
3	ant	3
171,476	...	...

WHERE subquery

SELECT AVG(word_length) AS avg_movie
FROM english_language
WHERE word IN 
  (SELECT DISTINCT word FROM movie)

avg_movie
3

FROM subquery

SELECT AVG(word_length) AS avg_movie 
FROM (SELECT * FROM movie)

Decreases readability
Limits query plan flexibility

About common table expressions (CTEs)

What?

Join alternative
Standalone query with temporary results set

Why?

Can return one result
Readable
Creates a temporary table

How?

WITH statements

CTE structure

WITH english_cte AS
(    
  SELECT word_length
      , COUNT(word) AS english_word_count
    FROM english_language 
    GROUP BY word_length
)

SELECT movie.word_length
  , COUNT(movie.word) AS movie_word_count
  , cte.english_word_count
FROM movie
INNER JOIN english_cte cte 
ON movie.word_length = cte.word_length
GROUP BY movie.word_length, cte.english_word_count

Let's practice!

Improving Query Performance in PostgreSQL