mysql错误：“子查询返回多于一行”，但我需要子查询多于一行

Question

I have two tables in my problem, one is the USER table with all details of user and a REQUEST table with friend requests from all users to all users. I want to list all the friends of one user who logs into my application.

The USER table attributes are,

id | uname   | passwd  | fname | e mail  | dob  | mobileno |  habbits | tastes | profession | image  
11 | nagaraj | *****   | naga  | ng@gml  | 3/94 | 998345   | singing  | sports | teacher  
12 | chiraag | ******* | chiru | ch@gml  | 2/93 | 894617   | writing  | music  | student

Similarly, the REQUEST table attributes are,

rqstTO | rqstFM  | fname   | e mail | mobile | status   
chirag | nagaraj | nagaraj | ng@gml | 99821  | accepted  

The tables look like this. Now I want to display the details of the friends( from USER table), only those friends who have accepted the request from the user who logged in. Output may be multiple users. The conditions to be satisfied are

1. all details of friends (from user table)

2. rqstFM should be the user logged in and rqstTO must be the friends to whom he sent requests (since request table has request details from all to all users)

3. status=accepted (they must have accepted the request)

The query I have written is

SELECT * 
FROM user 
WHERE habits='singing' AND uname = ( 
    SELECT rqstTO 
    FROM request 
    WHERE rqstFM ='"+username+"' AND status='Accepted'
);

(here +username+ is the uname of the person logged in obtained from HTML FORM tag). The error I keep getting is "subquery returns more than one row" but I do need multiple rows as logged in user may have sent requests to many ppl and among those all who accepted his request, their details need to be displayed from user table. Please, correct my query or provide an appropriate query for my problem

Answer 1

You need to use IN instead of = :

You can use as mentioned below:

SELECT * FROM user 
WHERE habits='singing' AND uname IN 
( SELECT rqstTO FROM request WHERE rqstFM ='"+username+"' AND status='Accepted');

However using subqueries are not recommended due to performance issues. You should rewrite your query by using JOIN

You can use 'JOIN' like below: (I haven`t tested it)

SELECT u.* FROM user u
JOIN request r ON u.uname = r.rqstTO 
WHERE u.habits='singing' AND r.rqstFM ='"+username+"' AND r.status='Accepted';

Answer 2

You want something like this (not tested it however):

SELECT user.uname, request.fname
FROM user
INNER JOIN request ON request.fname = user.fname
WHERE request.status = 'Accepted';

However, I would probably have a bit of a rethink of those tables, maybe this is personal preference but you would likely be better off having a join table that simply contains an id and the username, and then join on two other tables. One containing the requests and one containing the member details.