SQL count if columns

Question

What is the best way to create columns which count the number of occurrences of data in a table? The table needs to be grouped by one column.

I have seen

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Answer 1

In Postgres 9.4 there is new, cleaner aggregate FILTER option:

SELECT category
     , count(*) FILTER (WHERE question1 = 0) AS zero
     , count(*) FILTER (WHERE question1 = 1) AS one
     , count(*) FILTER (WHERE question1 = 2) AS two
FROM   reviews
GROUP  BY 1;

Details for the new FILTER clause:

• How can I simplify this game statistics query?

If you want it short:

SELECT category
     , count(question1 = 0 OR NULL) AS zero
     , count(question1 = 1 OR NULL) AS one
     , count(question1 = 2 OR NULL) AS two
FROM   reviews
GROUP  BY 1;

Overview over possible variants:

• For absolute performance, is SUM faster or COUNT?

Proper crosstab query

crosstab() yields the best performance and is shorter for longer lists of options:

SELECT * FROM crosstab(
     'SELECT category, question1, count(*)::int AS ct
      FROM   reviews
      GROUP  BY 1, 2
      ORDER  BY 1, 2'
   , 'VALUES (0), (1), (2)'
   ) AS ct (category text, zero int, one int, two int);

Detailed explanation:

• PostgreSQL Crosstab Query