multi to multi relational database not working - php

Question

I am studying relational databases on my own and after reading some articles I am having trouble implementing them.
Due to my lack of experience with relational databases I believe that my error can be in any of the following 3 (if not in all 3) steps:
Table Creaton
Value Insertion
Query call
I should point out that there aren't any syntax errors or something like that.
We are talking about an error in my understanding so I will approach the problem in detail.
The tables:
For the abstraction I have chosen to have people and the drinks which they like (1 person can like many drinks, one drink can be liked by many people.
Here is the code for the table People (It has 2 names, and a primary key):
CREATE TABLE `People`
(
`PeopleID` INT NOT NULL AUTO_INCREMENT,
`FirstName` VARCHAR(25),
`LastName` VARCHAR(25) NOT NULL,
PRIMARY KEY (`PeopleID`)
)
Here is the table Drinks (Name of drink, some number in menu or something like that for example and the primary key)
CREATE TABLE `Drinks`
(
`DrinksID` INT NOT NULL AUTO_INCREMENT,
`Code` VARCHAR(10) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL,
`Name` VARCHAR(100) NOT NULL,
PRIMARY KEY (`DrinksID`)
)
And lastly the table PeopleDrinks:
CREATE TABLE `PeopleDrinks`
(
`DrinksID` INT NOT NULL default 1,
`PeopleID` INT NOT NULL default 1,
PRIMARY KEY (`DrinksID`,`PeopleID`)
)
Note: from here I saw the example given for the third table and pretty much copied it.
Value insertion:
People input:
INSERT INTO People (FirstName, LastName) VALUES ('John', 'Smith')
INSERT INTO People (FirstName, LastName) VALUES ('Sam', 'Johnson')
INSERT INTO People (FirstName, LastName) VALUES ('Michael', 'Morgan')
Drinks input:
INSERT INTO Drinks (Code, Name)VALUES ('#543', 'Beer')
INSERT INTO Drinks (Code, Name) VALUES ('#132', 'Vodka')
INSERT INTO Drinks (Code, Name) VALUES ('#123', 'Wine')
PeopleDrinks input:
Note: The id of the drinks are 4, 5 and 6. Not 1, 2, 3.
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (1, 4)
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (2, 5)
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (3, 6)
Above we give as favorite one drink to every person. Below we will make one person like all the drinks:
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (1, 5)
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (1, 6)
And finally the query call:
//Consider $link an established, working connection to the database
$result = mysqli_query($link, "SELECT
`People`.*
FROM
`People`
JOIN
`PeopleDrinks` ON `People`.`PeopleID` = `PeopleDrinks`.`PeopleID`
WHERE
`PeopleDrinks`.`DrinksID` = 5");
while($row = mysqli_fetch_array($result))
{
echo $row['FirstName'] . " " . $row['LastName'];
echo "<br/>";
}
The above code, should print the name of anyone who likes the drink with id 5. I don't get an error, but I don't get any output.

Answer 1

You said the id of the drinks are 4,5,6
In the INsert you have used
INSERT INTO PeopleDrinks (DrinksID, PeopleID)VALUES (1, 4)
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (2, 5)
INSERT INTO PeopleDrinks (DrinksID, PeopleID) VALUES (3, 6)
The drink ids(first number) are 1,2,3 instead of 4,5,6 or something like that. I think your order is wrong.

Answer 2

Your many to many table needs foreigns key :
CREATE TABLE `PeopleDrinks` (
`DrinksID` INT NOT NULL,
`PeopleID` INT NOT NULL,
PRIMARY KEY (`DrinksID`,`PeopleID`),
FOREIGN KEY (`DrinksID`) REFERENCES `Drinks`(`DrinksID`),
FOREIGN KEY (`PeopleID`) REFERENCES `People`(`PeopleID`)
)
Remove the default values of DrinksID and PeopleID, this make no sense.
Then when you do your insert, you will be sure that the data you are trying to insert are right.
I think that your insert are false. When you say that your IDs are what they are, you are wrong. This is particulary true if your are doing a lot of inserts and deletes. Autoincrement will keep going forever, and you can't be sure. With Foreign Keys, the RDBMS will tell you if there is something wrong with your relations.
Anyway, your query looks great, so no problem for that.