Select only rows by join tables max value

Question

I want to select only the row which has max b.enddate for u.classno, u.userno. But it doesn't work.

select u.classno, u.userno, b.enddate
from libUser u
join book b on b.id = u.bookid
group by u.classno, u.userno
having b.enddate=max(b.enddate) //doesn't works

Answer 1

Here is an excellent article in the official MySQL documentation, but only standard SQL is used there, so it can be applied to whatever RDBMS you are using.

The Rows Holding the Group-wise Maximum of a Certain Column

Task: For each article, find the dealer or dealers with the most expensive price.

This problem can be solved with a subquery like this one:

SELECT article, dealer, price FROM   shop s1 WHERE  price=(SELECT MAX(s2.price)               FROM shop s2               WHERE s1.article = s2.article); 

The preceding example uses a correlated subquery, which can be inefficient (see Section 13.2.10.7, “Correlated Subqueries”). Other possibilities for solving the problem are to use an uncorrelated subquery in the FROM clause or a LEFT JOIN.

Uncorrelated subquery:

SELECT s1.article, dealer, s1.price FROM shop s1 JOIN (   SELECT article, MAX(price) AS price   FROM shop   GROUP BY article) AS s2   ON s1.article = s2.article AND s1.price = s2.price; 

LEFT JOIN:

SELECT s1.article, s1.dealer, s1.price FROM shop s1 LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price WHERE s2.article IS NULL; 

The LEFT JOIN works on the basis that when s1.price is at its maximum value, there is no s2.price with a greater value and the s2 rows values will be NULL.

Answer 2

Whats wrong with:

select u.classno, u.userno, MAX(b.enddate) from libUser u join book b on b.id = u.bookid group by u.classno, u.userno