Generating Groups

Single-Column Grouping

• example

  SELECT actor_id, count(*)
  FROM film_actor
  GROUP BY actor_id;
  

Multicolumn Grouping

• example

  SELECT fa.actor_id, f.rating, count(*)
  FROM film_actor fa
  INNER JOIN film f
    on fa.film_id = f.film_id
  GROUP BY fa.actor_id, f.rating
  ORDER BY 1,2;
  

Grouping via Expressions

• example

  SELECT EXTRACT(YEAR  FROM rental_date) Year, count(*) how_many
  FROM rental
  GROUP BY extract(YEAR FROM rental_date);
  

Generating Rollups

• GROUP BY $fld_name1, $fld_name2 WITH ROLLUP 会对fld_name1进行汇总