从第一个表中选择所有结果而不与第二个表匹配

Question

I have 2 tables one is user table and another is rating table which have the mapping data of user rating

user table

user_id |  firstname | lastname
--------------------------------
1       | 1test      | 1test lname
2       | 2test      | 2test lname
3       | 3test      | 3test lname
4       | 4test      | 4test lname
5       | 5test      | 5test lname

rating table

user_id |  rating
------------------
1       |   4
2       |   3
3       |   5
1       |   4

In result i need avg value of rating with user info in one query

user_id |  firstname | lastname      | rating
---------------------------------------------
1       | 1test      | 1test lname   |  4
2       | 2test      | 2test lname   |  3
3       | 3test      | 3test lname   |  5
4       | 4test      | 4test lname   |  0
5       | 5test      | 5test lname   |  0

Answer 1

Here is your query,

SELECT ut.*,AVG(rt.rating)as rating 
FROM user_table ut
INNER JOIN rating_table rt ON ut.user_id = rt.user_id

Answer 2

This is query you can use this to find average of all rating of all user

SELECT u.*, IFNULL(AVG(rating),0) as rating 
FROM user u 
LEFT JOIN rating r
ON u.user_id = r.user_id
GROUP BY r.user_id;

Answer 3

Use LEFT JOIN to fetch all records from user table and matching records from rating table.

Try this:

SELECT u.user_id, 
       u.firstname, 
       u.lastname, 
       AVG(IFNULL(r.rating, 0)) AS rating
FROM `user` u 
LEFT OUTER JOIN rating r ON u.user_id = r.user_id 
GROUP BY u.user_id, u.firstname, u.lastname;