MySQL COUNT与GROUP AND JOIN

Question

I need help to SELECT some information from 3 tables with one SQL query. I will try to explain it as much as possible.

I have the following 3 tables:

Users

ID | FULL NAME | CATEGORY_ID
--------------------------
1  | JOHN DOE  | 3
2  | JOHN DOE  | 5
3  | JOHN DOE  | 2
4  | JOHN DOE  | 3
5  | JOHN DOE  | 3

Categories

ID | NAME
----------------------
2  | CATEGORY NAME 1
3  | CATEGORY NAME 2
5  | CATEGORY NAME 3

Registrations

ID | USER_ID
------------
1  | 1
2  | 2
3  | 4

Now I want the outcome of this SQL query to be an html table looking like:

Category | Number Of Registrations
CATEGORY NAME 2 | 3
CATEGORY NAME 3 | 3
CATEGORY NAME 1 | 1

So to sum it up, I have to select all user_id from registrations table, get the category_id for each user from users table and find the category name from categories table.

Is this doable?

Answer 1

The SQL tables form you're using doesn't feel right.

You should have a 'USERS' table as:

ID | FULL NAME

1 | JOHN DOE
2 | JOHN DOE
3 | JOHN DOE
4 | JOHN DOE
5 | JOHN DOE

'CATEGORIES' table as

ID | NAME

2 | CATEGORY NAME 1
3 | CATEGORY NAME 2
5 | CATEGORY NAME 3

'REGISTRATIONS' table as (I believe this table will store what user has registered to what category)

USER_ID | CATEGORY_ID

1 | 1
2 | 2
3 | 4

Then your query can be easily retrieved via:

SELECT c.name, count(*) as Number_of_Registrations
FROM categories c, registrations r 
WHERE c.id = r.category_id 
GROUP BY c.id;

Am I wrong about the registrations table? Lemme know then, I'll provide a new solution accordingly.

Answer 2

I think this should do it:

select cat.`category name`, count(reg.user_id) from registrations reg
inner join users on reg.user_id = users.ID
inner join categories cat on cat.ID = users.category_id
group by cat.category_id

Answer 3

You can try this

SELECT c.name, count(r.user_id) 
FROM categories c 
inner join users u 
    on  u.category_id = c.id 
inner join registrations r 
    on r.user_id = u.id 
group by c.id

Answer 4

 $sql1="SELECT * FROM CATEGORIES"; $query1=mysqli_query($con,$sql1); echo "<table>"; while($row1=mysqli_fetch_assoc($query1)){ echo"<tr><td>". $row1['NAME']."</td >"; $sql12="SELECT COUNT(CATEGORY_ID) AS NumberOfUsers FROM USERS WHERE CATEGORY_ID='".$row1['CATEGORY_ID']."'"; $query2=mysqli_query($con,$sql12); $row2=mysqli_fetch_assoc($query2); echo"<td>".$row2['NumberOfUsers']."</td></tr>" ; } echo"</table>"