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mysqlsqlaggregate-functions
Mark
Mark

Reputation: 8431

MySQL: Select MAX() from sub-query with COUNT()

Before you mark this as duplicate please take a look at this SQLFiddle.

I have this schema:

CREATE TABLE book(book_id int,
                   book_name varchar(100),
                   author_id int,
                   editor_id varchar(100),
                   isbn varchar(100));
INSERT INTO book
VALUES
(1 , 'Book1 Title' ,  12  ,  'Editor1'   , '8000-9000' ),
(2 , 'Book2 Title' ,  98  ,  'Editor1'   , '8000-9001' ),
(1 , 'Book1 Title' ,  12  ,  'Editor1'   , '8000-9002' ),
(3 , 'Book3 Title' ,  3  ,  'Editor1'   , '8000-9003' );

CREATE TABLE author(author_id int,
                    fn varchar(100),
                    ln varchar(100));
INSERT INTO author
VALUES
(12, 'name1','lname1'),
(98,'name2','lname2'),
(3,'name3','lname3');

The sub-query:

  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id

has a result:

| AUTHOR_ID | BOOK_COUNT |
--------------------------
|         3 |          1 |
|        12 |          2 |
|        98 |          1 |

Now, the tricky part here is the result of this query:

SELECT MAX(book_count),a.* FROM
author a,(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
where a.author_id = b.author_id

is this:

| MAX(BOOK_COUNT) | AUTHOR_ID |    FN |     LN |
------------------------------------------------
|               2 |         3 | name3 | lname3 |

which should be like this:

| MAX(BOOK_COUNT) | AUTHOR_ID |    FN |     LN |
------------------------------------------------
|               2 |        12 | name1 | lname1 |

What do you think is wrong in the query?

Upvotes: 4

Views: 31089

Answers (3)

Ashutosh Arya
Ashutosh Arya

Reputation: 1168

This am wondering this query is running, you used aggregate function with out using group by. When you need to identity the user which has maximum nook count Please try

SELECT (book_count),b.author_id FROM
author a

INNER JOIN 
(
 SELECT c.author_id,COUNT(*) book_count FROM book c
 GROUP BY c.author_id) B
ON

 a.author_id = b.author_id
having book_count=MAX(book_count)

Let me know if it works.

Upvotes: 0

Himanshu
Himanshu

Reputation: 32602

Instead of MAX() you can simply use LIMIT for the same. Also use JOIN instead.

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
ON a.author_id = b.author_id
ORDER BY book_count DESC LIMIT 1

Output:

| BOOK_COUNT | AUTHOR_ID |    FN |     LN |
-------------------------------------------
|          2 |        12 | name1 | lname1 |

See this SQLFiddle

Edit:

If you want to use MAX() for that, you have to use sub-query like this:

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
ON a.author_id = b.author_id
WHERE book_count = 
        (SELECT MAX(book_count)
        FROM
        (
           SELECT c.author_id,COUNT(*) book_count FROM book c 
           GROUP BY c.author_id
        ) b )

See this SQLFiddle

Edit2:

Instead of using LIMIT in outer query you can simply use it in inner query too:

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
  ORDER BY COUNT(*) DESC LIMIT 1
) b 
ON a.author_id = b.author_id

See this SQLFiddle

Upvotes: 6

Mikhail Aksenov
Mikhail Aksenov

Reputation: 571

In fact, MySQL has a lack of support SQL's standard, because it allows use aggregate functions w/o GROUP BY clause and returns random data in result. You should avoid the usage of aggregates in that way.

EDIT: I mean, for example in MySQL you can execute query like this:

SELECT
  MAX(a), b, c
FROM
  table
GROUP BY
  b;

Which returns random data in c column, and that's terribly wrong.

Upvotes: 1

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