problems with AVG and LIMIT SQL

Question

So I am asked to find the query: Find average stars awarded by top 20 users in user reviews (most experienced). Compare it with the average stars awarded by bottom 20 users (least expereinced)

the schema is:

Restaurant (name)

Rank (how it was ranked by Trip Advisor, it is not known what their ranking function is!, it is NOT just SCORE)

Score (average stars awarded by reviewers)

User_Name - name of the reviewer/alias

Review_Stars - How many stars did this reviewer give to this restuarant

Review_Date - The date of the review

User_Reviews - Total number of reviews this reviewer gave over his/her history of reviewing for hotels/resturants etc

User_Restaurant_Reviews - Same but just for restaurants

User_Helpful - How many of the user reviews were marked as helpful by other reviewers

So my logic was to select distinct user_names, and their reviews, order that by reviews and limit it to 20. When I went to add the AVG(REVIEW_STARS) it completely changes the answers.

SELECT  DISTINCT USER_NAME, USER_REVIEWS
FROM TRIPADVISOR
ORDER BY USER_REVIEWS desc
limit 20;

RETURNS:

Dorian C 1329

Scipion-Paris 1219

Waddler 1213

CJTravelerTexas 937

runner-oh... 828

maurowave 818

gabrielct... 720

AJCG 667

pietro r 652

Albia-Newton 651

Jilllian 650

laserkid 648

pstiva 616

lucaheat 585

Chokk 511

Soracte 491

viaggiatr... 487

tornado9 479

Magorzata B 478

Marcello... 464

When I change the query to:

SELECT  DISTINCT USER_NAME, USER_REVIEWS, AVG(REVIEW_STARS)
FROM TRIPADVISOR
ORDER BY USER_REVIEWS desc
limit 20;

It returns: Iain U 60 4.3333

I think my logic must be off in the way I am going about this but if anyone can shed some light it would help

Answer 1

You are missing a GROUP BY statement, and you may want to remove the DISTINCT . You need something along the lines of:

SELECT USER_NAME, USER_REVIEWS, AVG(REVIEW_STARS)
FROM TRIPADVISOR
GROUP BY USER_NAME, USER_REVIEWS
ORDER BY USER_REVIEWS desc
limit 20;