Data aggregation

    Find out the number of unique episodes within every season of every series.

    Note

    We assume that you already created tables in step Creating a table and populated them with data in step Adding data to a table.

    SELECT
        series_id,
        season_id,
        COUNT(*) AS cnt  -- Aggregation function COUNT returns the number of rows
                         -- output by the query.
                         -- Asterisk (*) specifies that COUNT
                         -- counts the total number of rows in the table.
                         -- COUNT(*) returns the number of rows in
                         -- the specified table, preserving the duplicate rows.
                         -- It counts each row separately.
                         -- The result includes rows that contain null values.
    FROM episodes
    
    GROUP BY
        series_id,       -- The query result will follow the listed order of columns.
        season_id        -- Multiple columns are separated by a comma.
                         -- Other columns can be listed after a SELECT only if
                         -- they are passed to an aggregate function.
    ORDER BY
        series_id,
        season_id
    ;
    
    COMMIT;