Nested query in From clause syntax and performance - mysql

Question

I'm having two tables: one for user information, the second for mapping some relation between users (two column table with two ids, from id to id relation)
I'm trying to find for a specific userid all his users' relations ids (inner select) and then get more info about them by joining to a table which has more info to show.
Given the following error:
Error: #1064 - You have an error in your SQL syntax; check the manual list that corresponds to your mySQL server version for the right syntax to use near ') AS i Limit 0,30' at line 6
What wrong with my query?
Is this query is okay in terms of performance, or there are other way to do so?
Query:
SELECT i.*
FROM
((SELECT uc.contactId
FROM tbl_users AS u
JOIN tbl_users_contacts AS uc ON u.Id = uc.userId
WHERE uc.userId =1) AS contacts_ids JOIN tbl_users AS u
ON contacts_ids.contactId = u.Id) AS i;
Edit: Fixed to:
SELECT *
FROM
((SELECT uc.contactId
FROM tbl_users AS u
JOIN tbl_users_contacts AS uc ON u.Id = uc.userId
WHERE uc.userId =1) AS contacts_ids JOIN tbl_users AS u
ON contacts_ids.contactId = u.Id);
Don't know why the final As i was a problem, so I ask for question 2 mainly for this post.

Answer 1

Consider the following
mysql> create table tbl_users ( iduser int,name varchar(100),email varchar(100));
Query OK, 0 rows affected (0.10 sec)
mysql> insert into tbl_users values
-> (1,'A','a#a.com'),
-> (2,'B','b#b.com'),
-> (3,'C','c#c.com'),
-> (4,'D','d#d.com'),
-> (5,'E','e#e.com');
Query OK, 5 rows affected (0.09 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> create table contacts (iduser int, contactid int );
Query OK, 0 rows affected (0.14 sec)
mysql> insert into contacts values
-> (1,2),(1,3),(1,5),(2,1),(2,5),(3,1),(3,4);
mysql> select * from tbl_users ;
+--------+------+---------+
| iduser | name | email |
+--------+------+---------+
| 1 | A | a#a.com |
| 2 | B | b#b.com |
| 3 | C | c#c.com |
| 4 | D | d#d.com |
| 5 | E | e#e.com |
+--------+------+---------+
5 rows in set (0.00 sec)
mysql> select * from contacts ;
+--------+-----------+
| iduser | contactid |
+--------+-----------+
| 1 | 2 |
| 1 | 3 |
| 1 | 5 |
| 2 | 1 |
| 2 | 5 |
| 3 | 1 |
| 3 | 4 |
+--------+-----------+
7 rows in set (0.00 sec)
Now as we can see userid = 1 has 3 contacts and we can get them as
select u.* from tbl_users u
join contacts c on c.contactid = u.iduser
where c.iduser = 1 ;
The output will be as
+--------+------+---------+
| iduser | name | email |
+--------+------+---------+
| 2 | B | b#b.com |
| 3 | C | c#c.com |
| 5 | E | e#e.com |
+--------+------+---------+
To boost up the performance you may add the following indexes
alter table tbl_users add index userid_idx(iduser);
alter table contacts add index cu_idx(iduser,contactid);
Change the table and column name into the query as per your need.