INNER JOIN and GROUP BY to prevent duplicate results - mysql

Question

Context:
I'm working on a simple ORM (for PHP) that automatize most of queries, based on a static configuration.
Thus, from tables and entities definitions, the library handles joins automatically and generates appropriate fields/table alias... No problem for LEFT joins but INNER may result in duplicated results in case of relation One-to-Many.
My thought was to automatically add a GROUP BY clause (on the auto-increment key) if necessary.
The question
Is it correct to consider that I need to add a GROUP BY clause if (and only if) the join's ON and WHERE conditions doesn't match a unique key of the joined table ?
Example
A very simple example, where I want to select all events with (at least) an associated Showing.
If there is an other way to do it without INNER JOIN, I'm interested to know how :)
CREATE TABLE `Event` (
`Id` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
`Name` VARCHAR(255) NOT NULL
);
INSERT INTO `Event` (`Name`) VALUES ('My cool event');
CREATE TABLE `Showing` (
`Id` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
`EventId` INT UNSIGNED NOT NULL,
`Place` VARCHAR(50) NOT NULL,
FOREIGN KEY (`EventId`) REFERENCES `Event`(`Id`),
UNIQUE (`EventId`, `Place`)
);
INSERT INTO `Showing` (`EventId`, `Place`) VALUES (1, 'School');
INSERT INTO `Showing` (`EventId`, `Place`) VALUES (1, 'Park');
-- Correct queries
SELECT t.* FROM `Event` t INNER JOIN `Showing` t1 ON t.Id=t1.`EventId` WHERE t1.`PlaceId` = 'School';
SELECT t.* FROM `Event` t INNER JOIN `Showing` t1 ON t.Id=t1.`EventId` AND t1.`PlaceId` = 'School';
-- Query leading to duplicate values
SELECT t.* FROM `Event` t INNER JOIN `Showing` t1 ON t.Id=t1.`EventId`;
-- Group by query to prevent duplicate values
SELECT t.* FROM `Event` t INNER JOIN `Showing` t1 ON t.Id=t1.`EventId` GROUP BY t.`Id`;
Thanks !

Answer 1

(this should be a comment but its a bit long)
No problem for LEFT joins but INNER may result in duplicated results in case of relation One-to-Many
It's clear from that sentence that at least one of us is very confused about how a relational database works, and how object-relation mapping should work.
Query leading to duplicate values
The rows produced are not duplicates - you've written the query so it doesn't show you why they are different:
SELECT t1.place, t.*
FROM Event
INNER JOIN Showing
ON Event.Id=Showing.EventId;
If you're not interested in the data from 'showing' then why is it in your query? If you have events without related showing records then you should be using an 'EXISTS' - not a join (consider where you have a single event but 3 million showings)
SELECT t1.place, t.*
FROM `Event` t
WHERE EXISTS (SELECT 1
FROM Showing
WHERE Event.Id=Showing.EventId);
If you are strictly implementing ORM, then you probably shouldn't be writing queries with joins at all - but IMHO, the scenario is better served by using factories.

Answer 2

The data is saying that "My Cool Event" is happening at the park, and at the school. If you inner join the tables you will get more than one result.
Do this query to see what is going on:
Select t.*, t1.* FROM `Event` t INNER JOIN `Showing` t1 ON t.Id=t1.`EventId`;
That is the same query as your duplicate query, but selecting columns from both tables.
The first line of results says the event is happening at the park. The second line says that the same event is happening at the school.