Question

Every once and a while I have a scenario like this, and can never come up with the most efficient query to pull in the information:

Let’s say we have a table with three columns (A int, B int, C int). My query needs to answer a question like this: “Tell me what the value of column C is for the largest value of column B where A = 5.” A real world scenario for something like this would be ‘A’ is your users, ‘B’ is the date something happened, and ‘C’ is the value, where you want the most recent entry for a specific user.

I always end up with a query like this:

SELECT
    C
FROM
    MyTable
WHERE
    A = 5
    AND B = (SELECT MAX(B) FROM MyTable WHERE A = 5)

What am I missing to do this in a single query (opposed to nesting them)? Some sort of ‘Having’ clause?

Answer 1

BoSchatzberg’s answer works when you only care about the 1 result where A=5. But I suspect this question is the result of a more general case. What if you want to list the top record for each distinct value of A?

SELECT t1.*
FROM MyTable t1
   INNER JOIN 
     (
       SELECT A, MAX(B)
       FROM MyTable
       GROUP BY A
     )  t2 ON t1.A = t2.A AND t1.B = t2.B