[sql] Can I do a max(count(*)) in SQL?

Here's my code:

    select yr,count(*)  from movie
join casting on casting.movieid=movie.id
join actor on casting.actorid = actor.id
where actor.name = 'John Travolta'
group by yr

Here's the question

Which were the busiest years for 'John Travolta'. Show the number of movies he made for each year.

Here's the table structure:

movie(id, title, yr, score, votes, director)
actor(id, name)
casting(movieid, actorid, ord)

This is the output I am getting:

yr  count(*)
1976    1
1977    1
1978    1
1981    1
1994    1
etcetc

I need to get the rows for which count(*) is max.

How do I do this?

This question is related to sql

The answer is


Using max with a limit will only give you the first row, but if there are two or more rows with the same number of maximum movies, then you are going to miss some data. Below is a way to do it if you have the rank() function available.

SELECT
    total_final.yr,
    total_final.num_movies
    FROM
    ( SELECT 
        total.yr, 
        total.num_movies, 
        RANK() OVER (ORDER BY num_movies desc) rnk
        FROM (
               SELECT 
                      m.yr, 
                      COUNT(*) AS num_movies
               FROM MOVIE m
               JOIN CASTING c ON c.movieid = m.id
               JOIN ACTOR a ON a.id = c.actorid
               WHERE a.name = 'John Travolta'
               GROUP BY m.yr
             ) AS total
    ) AS total_final 
   WHERE rnk = 1

The following code gives you the answer. It essentially implements MAX(COUNT(*)) by using ALL. It has the advantage that it uses very basic commands and operations.

SELECT yr, COUNT(title)
FROM actor
JOIN casting ON actor.id = casting.actorid
JOIN movie ON casting.movieid = movie.id
WHERE name = 'John Travolta'
GROUP BY yr HAVING COUNT(title) >= ALL
  (SELECT COUNT(title)
   FROM actor
   JOIN casting ON actor.id = casting.actorid
   JOIN movie ON casting.movieid = movie.id
   WHERE name = 'John Travolta'
   GROUP BY yr)

Just order by count(*) desc and you'll get the highest (if you combine it with limit 1)


This question is old, but was referenced in a new question on dba.SE. I feel the best solutions haven't been provided, yet, so I am adding another one.

First off, assuming referential integrity (typically enforced with foreign key constraints) you do not need to join to the table movie at all. That's dead freight in your query. All answers so far fail to point that out.


Can I do a max(count(*)) in SQL?

To answer the question in the title: Yes, in Postgres 8.4 (released 2009-07-01, before this question was asked) or later you can achieve that by nesting an aggregate function in a window function:

SELECT c.yr, count(*) AS ct, max(count(*)) OVER () AS max_ct
FROM   actor   a
JOIN   casting c ON c.actorid = a.id
WHERE  a.name = 'John Travolta'
GROUP  BY c.yr;

Consider the sequence of events in a SELECT query:

The (possible) downside: window functions do not aggregate rows. You get all rows left after the aggregate step. Useful in some queries, but not ideal for this one.


To get one row with the highest count, you can use ORDER BY ct LIMIT 1 like @wolph hinted:

SELECT c.yr, count(*) AS ct
FROM   actor   a
JOIN   casting c ON c.actorid = a.id
WHERE  a.name = 'John Travolta'
GROUP  BY c.yr
ORDER  BY ct DESC
LIMIT  1;

Using only basic SQL features available in any halfway decent RDBMS - the LIMIT implementation varies:

Or you can get one row per group with the highest count with DISTINCT ON (only Postgres):


Answer

But you asked for:

... rows for which count(*) is max.

Possibly more than one. The most elegant solution is with the window function rank() in a subquery. Ryan provided a query but it can be simpler (details in my answer above):

SELECT yr, ct
FROM  (
   SELECT c.yr, count(*) AS ct, rank() OVER (ORDER BY count(*) DESC) AS rnk
   FROM   actor   a
   JOIN   casting c ON c.actorid = a.id
   WHERE  a.name = 'John Travolta'
   GROUP  BY c.yr
   ) sub
WHERE  rnk = 1;

All major RDBMS support window functions nowadays. Except MySQL and forks (MariaDB seems to have implemented them at last in version 10.2).


create view sal as
select yr,count(*) as ct from
(select title,yr from movie m, actor a, casting c
where a.name='JOHN'
and a.id=c.actorid
and c.movieid=m.id)group by yr

-----VIEW CREATED-----

select yr from sal
where ct =(select max(ct) from sal)

YR 2013


SELECT * from 
(
SELECT yr as YEAR, COUNT(title) as TCOUNT
FROM actor
JOIN casting ON actor.id = casting.actorid
JOIN movie ON casting.movieid = movie.id
WHERE name = 'John Travolta'
GROUP BY yr
order by TCOUNT desc
) res
where rownum < 2

Depending on which database you're using...

select yr, count(*) num from ...
order by num desc

Most of my experience is in Sybase, which uses some different syntax than other DBs. But in this case, you're naming your count column, so you can sort it, descending order. You can go a step further, and restrict your results to the first 10 rows (to find his 10 busiest years).


Thanks to the last answer

SELECT yr, COUNT(title)
FROM actor
JOIN casting ON actor.id = casting.actorid
JOIN movie ON casting.movieid = movie.id
WHERE name = 'John Travolta'
GROUP BY yr HAVING COUNT(title) >= ALL
  (SELECT COUNT(title)
   FROM actor
   JOIN casting ON actor.id = casting.actorid
   JOIN movie ON casting.movieid = movie.id
   WHERE name = 'John Travolta'
   GROUP BY yr)

I had the same problem: I needed to know just the records which their count match the maximus count (it could be one or several records).

I have to learn more about "ALL clause", and this is exactly the kind of simple solution that I was looking for.


it's from this site - http://sqlzoo.net/3.htm 2 possible solutions:

with TOP 1 a ORDER BY ... DESC:

SELECT yr, COUNT(title) 
FROM actor 
JOIN casting ON actor.id=actorid
JOIN movie ON movie.id=movieid
WHERE name = 'John Travolta'
GROUP BY yr
HAVING count(title)=(SELECT TOP 1 COUNT(title) 
FROM casting 
JOIN movie ON movieid=movie.id 
JOIN actor ON actor.id=actorid
WHERE name='John Travolta'
GROUP BY yr
ORDER BY count(title) desc)

with MAX:

SELECT yr, COUNT(title) 
FROM actor  
JOIN casting ON actor.id=actorid    
JOIN movie ON movie.id=movieid
WHERE name = 'John Travolta'
GROUP BY yr
HAVING 
    count(title)=
        (SELECT MAX(A.CNT) 
            FROM (SELECT COUNT(title) AS CNT FROM actor 
                JOIN casting ON actor.id=actorid
                JOIN movie ON movie.id=movieid
                    WHERE name = 'John Travolta'
                    GROUP BY (yr)) AS A)

     select top 1 yr,count(*)  from movie
join casting on casting.movieid=movie.id
join actor on casting.actorid = actor.id
where actor.name = 'John Travolta'
group by yr order by 2 desc