How to SELECT name, max(value from a table joined on name)? - mysql

Question

Summary: I have a forum with a forums table and a posts table. Every post has a unique ID (an auto-incrementing integer), and every post references a forums.id.
I'm trying to issue a SELECT query that retrieves all the forum names, all the forum IDs, and then the highest posts.id associated with that forum.
It's possible that there are no posts in a forum and in that case I want the max-posts-id to be 0.
Forums table:
| ID | Name |
|----|------|
| 1 | Dogs |
| 2 | Food |
| 3 | Work |
Posts table:
| ID | Forum_ID | Author | Text |
|----|----------|--------|------|
| 42 | 1 | Mr. S | foo |
| 43 | 3 | Mr. Y | bar |
| 44 | 1 | Ms. X | baz |
| 45 | 2 | Ms. A | foo |
| 46 | 1 | Mr. M | foo |
| 47 | 3 | Ms. A | bar |
| 48 | 2 | Mr. L | baz |
Desired result:
| Forum_ID | Name | Max_Posts_ID |
|----------|------|--------------|
| 1 | Dogs | 46 |
| 2 | Food | 48 |
| 3 | Work | 47 |
My attempt
SELECT
forums.id AS id,
forums.name AS name,
COALESCE(MAX(SELECT id FROM posts WHERE forums.id = ?), 0)
JOIN
posts ON forums.id = posts.forum_id;
But I don't think I can pass a parameter to my nested SELECT query, I don't think that's the right approach. What should I do instead?

Answer 1

You could use LEFT JOIN and aggregation:
SELECT f.id AS id,
f.name AS name,
COALESCE(MAX(p.id),0) AS Max_Posts_ID
FROM Forums f
LEFT JOIN Posts p
ON f.Id = p.forum_id
GROUP BY f.id, f.name
ORDER BY f.id;

Answer 2

Solution to your problem:
MySQL
SELECT fs.id AS Forum_ID ,
fs.name AS Name,
IFNULL(MAX(ps.ID),0) AS Max_Posts_ID
FROM forums fs
LEFT JOIN posts ps
ON fs.id = ps.forum_id
GROUP BY fs.id,fs.name;
Link To the MySQL Demo:
http://sqlfiddle.com/#!9/a18ab2/1
MSSQL
SELECT fs.id AS Forum_ID ,
fs.name AS Name,
ISNULL(MAX(ps.ID),0) AS Max_Posts_ID
FROM forums fs
LEFT JOIN posts ps
ON fs.id = ps.forum_id
GROUP BY fs.id,fs.name;
Link To the MSSQL Demo:
http://sqlfiddle.com/#!18/a18ab/2
OUTPUT:
Forum_ID Name Max_Posts_ID
1 Dogs 46
2 Food 48
3 Work 47

Answer 3

Let me correct your current attempt with correlation subquery
SELECT
id AS id,
name AS name,
(SELECT COALESCE(MAX(ID), 0) FROM Posts where forum_id = f.Id) AS Max_Posts_ID
FROM Forums f
Corrections :
Your outer query wasn't have from clause
Subquery wasn't referenced the outer query column id with Posts forum_id

Answer 4

Link to Code : http://tpcg.io/pI2HO5
BEGIN TRANSACTION;
/* Create a table called NAMES */
CREATE TABLE Forums (Id integer PRIMARY KEY, Name text);
/* Create few records in this table */
INSERT INTO Forums VALUES(1,'Dogs');
INSERT INTO Forums VALUES(2,'Food');
INSERT INTO Forums VALUES(3,'Work');
/* Create a table called NAMES */
CREATE TABLE Posts (Id integer PRIMARY KEY, forId integer);
/* Create few records in this table */
INSERT INTO Posts VALUES(42,1);
INSERT INTO Posts VALUES(43,3);
INSERT INTO Posts VALUES(64,1);
INSERT INTO Posts VALUES(45,2);
INSERT INTO Posts VALUES(46,1);
INSERT INTO Posts VALUES(47,3);
INSERT INTO Posts VALUES(48,2);
INSERT INTO Posts VALUES(51,2);
COMMIT;
/* Display all the records from the table */
SELECT Distinct forId as Forum_Id, Posts.id, (Select Name from Forums where forId == Forums.id) FROM Posts,Forums GROUP BY Posts.forId;
Output :
1|64|Dogs
2|51|Food
3|47|Work