通过csv文件提供的数据库表内容：
links.csv的格式：
movies.csv格式，一个movie可以有多种风格(genres)，通过|分隔：
ratings.csv:
用户给movie打得分：
tags.csv：movie的标签
练习一：
列出四张表的总记录数：
select "links" as "table name", count(1) as "row count" from "MOVIELENS"."public.aa.movielens.hdb::data.LINKS"union allselect "movies" as "table name", count(1) as "row count" from "MOVIELENS"."public.aa.movielens.hdb::data.MOVIES"union allselect "ratings" as "table name", count(1) as "row count" from "MOVIELENS"."public.aa.movielens.hdb::data.RATINGS"union allselect "tags" as "table name", count(1) as "row count" from "MOVIELENS"."public.aa.movielens.hdb::data.TAGS";
执行结果：
练习2：计算总共9125部电影，一共包含多少艺术类别？
DOBEGIN DECLARE genreArray NVARCHAR(255) ARRAY; DECLARE tmp NVARCHAR(255); DECLARE idx INTEGER; DECLARE sep NVARCHAR(1) := "|"; DECLARE CURSOR cur FOR SELECT DISTINCT "GENRES" FROM "MOVIELENS"."public.aa.movielens.hdb::data.MOVIES"; DECLARE genres NVARCHAR (255) := ""; idx := 1; FOR cur_row AS cur() DO SELECT cur_row."GENRES" INTO genres FROM DUMMY; tmp := :genres; WHILE LOCATE(:tmp,:sep) > 0 DO genreArray[:idx] := SUBSTR_BEFORE(:tmp,:sep); tmp := SUBSTR_AFTER(:tmp,:sep); idx := :idx + 1; END WHILE; genreArray[:idx] := :tmp; END FOR; genreList = UNNEST(:genreArray) AS ("GENRE"); SELECT "GENRE" FROM :genreList GROUP BY "GENRE";END;
执行结果，总共包含18种：
练习3：计算每种艺术类别总共包含多少部电影：
DOBEGIN DECLARE genreArray NVARCHAR(255) ARRAY; DECLARE tmp NVARCHAR(255); DECLARE idx INTEGER; DECLARE sep NVARCHAR(1) := "|"; DECLARE CURSOR cur FOR SELECT DISTINCT "GENRES" FROM "MOVIELENS"."public.aa.movielens.hdb::data.MOVIES"; DECLARE genres NVARCHAR (255) := ""; idx := 1; FOR cur_row AS cur() DO SELECT cur_row."GENRES" INTO genres FROM DUMMY; tmp := :genres; WHILE LOCATE(:tmp,:sep) > 0 DO genreArray[:idx] := SUBSTR_BEFORE(:tmp,:sep); tmp := SUBSTR_AFTER(:tmp,:sep); idx := :idx + 1; END WHILE; genreArray[:idx] := :tmp; END FOR; genreList = UNNEST(:genreArray) AS ("GENRE"); SELECT "GENRE", count(1) FROM :genreList GROUP BY "GENRE";END;
练习4：列出每部电影包含的风格数目：
SELECT "MOVIEID" , "TITLE" , OCCURRENCES_REGEXPR("[|]" IN GENRES) + 1 "GENRE_COUNT" , "GENRES"FROM "MOVIELENS"."public.aa.movielens.hdb::data.MOVIES"ORDER BY "GENRE_COUNT" ASC;
练习5：罗列出每部电影的风格分布情况
SELECT "GENRE_COUNT" , COUNT(1)FROM ( SELECT OCCURRENCES_REGEXPR("[|]" IN "GENRES") + 1 "GENRE_COUNT" FROM "MOVIELENS"."public.aa.movielens.hdb::data.MOVIES")GROUP BY "GENRE_COUNT" ORDER BY "GENRE_COUNT";
比如至少拥有1个风格的电影，有2793部，2个风格的电影有3039部，等等。
练习6：计算movie的rating分布情况
SELECT DISTINCT MIN("RATING_COUNT") OVER( ) AS "MIN", MAX("RATING_COUNT") OVER( ) AS "MAX", AVG("RATING_COUNT") OVER( ) AS "AVG", SUM("RATING_COUNT") OVER( ) AS "SUM", MEDIAN("RATING_COUNT") OVER( ) AS "MEDIAN", STDDEV("RATING_COUNT") OVER( ) AS "STDDEV", COUNT(*) OVER( ) AS "CATEGORY_COUNT"FROM ( SELECT "MOVIEID", COUNT(1) as "RATING_COUNT" FROM "MOVIELENS"."public.aa.movielens.hdb::data.RATINGS" GROUP BY "MOVIEID")GROUP BY "RATING_COUNT";
明细情况：
SELECT "RATING_COUNT", COUNT(1) as "MOVIE_COUNT"FROM ( SELECT "MOVIEID", COUNT(1) as "RATING_COUNT" FROM "MOVIELENS"."public.aa.movielens.hdb::data.RATINGS" GROUP BY "MOVIEID")GROUP BY "RATING_COUNT" ORDER BY "RATING_COUNT" asc;
比如有397部电影的用户投票数为5票
SELECT "RATING_COUNT", COUNT(1) as "USER_COUNT"FROM ( SELECT "USERID", COUNT(1) as "RATING_COUNT" FROM "MOVIELENS"."public.aa.movielens.hdb::data.RATINGS" GROUP BY "USERID")GROUP BY "RATING_COUNT" ORDER BY 1 DESC;
SELECT "RATING", COUNT(1) as "RATING_COUNT"FROM "MOVIELENS"."public.aa.movielens.hdb::data.RATINGS"GROUP BY "RATING" ORDER BY 1 DESC;
有15095份用户投票，打的分数是5分
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