Question

In a previous question I was told that if I want to store multiple languages for a user then it is best to have a separate table mapping languages and users together.
That makes sense because it is a classic many-to-many relationship so that is what I have done.

My question now is what query can I write that will concatenate all the language names for each user and then display that as a table?

I made the following (is it correct?):

CREATE TABLE language (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
name char(20) NOT NULL UNIQUE)

CREATE TABLE user (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
name char(20) NOT NULL UNIQUE)

CREATE TABLE user_language (
id_user INT REFERENCES user(id),
id_lang INT REFERENCES language(id) )

I then inserted 3 test users, 7 test languages and inserted 9 pairs into the user_language table. Now I want to print a concatenated list of the languages each user speaks next to the users names using only one MySQL query and I have no idea how to do that.
So for example I would like to see

User    |   Languages
--------|---------------
Bob     | English, French
Alex    | Spanish, Portuguese
Ivan    | Russian, English, German 

Any ideas? Am I even on the right track?

EDIT

I am aware of the JOIN function so printing the list out as a set of individual pairs is not my intention. I specifically want to concatenate the languages for each user without resorting to writing a loop in a script (it would be in PHP in my case).
I thought there might be some way of writing a subquery and using the CONCATENATE function that might be able to help me…? Am I way off?

Answer 1

You’ll want to take a look at using an INNER JOIN for this. Essentially, the JOIN is going to take a look at the IDs in each field and then match data together for entries that have matching IDs.

Consider the following SQL query:

SELECT user.*, language.name FROM user
INNER JOIN user_language ON user.id = user_language.id_user
INNER JOIN language ON user_language.id_lang = language.id;

Order of the INNER JOINs is important; the first thing we need to do is be able to reference language IDs relative to the user ID. The first INNER JOIN is going to give each user a list of language IDs which the user can speak. The second is going to map that language ID that was assigned to the user to a name.

However, the above will not return them in a comma-separated list. Instead, you’ll get something like this:

name    |    language
-------------------------
bob     |   English
bob     |   Dutch
Sally   |   English
Sally   |   Spanish
Sally   |   German

You’ll have to do some post processing to put the data into a comma-separated list. Doing so in almost any language should be fairly straightforward.