mysql joining two tables to get where certain column is a specific value

Question

I have to following tables and I want to join them where sequence is 1 in icd9 and each subject has one hadm_id in the admissions table. Each subject can have multiple hadm_id but I want those who only have 1 hadm_id. I also want subjects where the sequence value is 1.

icd9
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| subject_id  | int(11)      | NO   | MUL | NULL    |       |
| hadm_id     | int(11)      | NO   | MUL | NULL    |       |
| sequence    | int(11)      | NO   |     | NULL    |       |
| code        | varchar(100) | NO   |     | NULL    |       |
| description | varchar(255) | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+

admissions 
+------------+----------+------+-----+---------+-------+
| Field      | Type     | Null | Key | Default | Extra |
+------------+----------+------+-----+---------+-------+
| hadm_id    | int(11)  | NO   | PRI | NULL    |       |
| subject_id | int(11)  | NO   | MUL | NULL    |       |
| admit_dt   | datetime | NO   |     | NULL    |       |
| disch_dt   | datetime | NO   |     | NULL    |       |
+------------+----------+------+-----+---------+-------+

My query is as follows. When I run it I get the error below

select * from icd9 
  where sequence=1 as t1
inner join 
  (select * from 
    (select subject_id, count(hadm_id) 
      as n_admissions from admissions 
      group by subject_id
    ) as q1 
  where n_admissions = 1 
  order by subject_id limit 10) as q2
AS t2 
ON t1.subject_id = q2.subject_id;

I am not sure about how to fix this error. I tried several methods but I keep getting this.

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as t1
inner join 
  (select * from 
    (select subject_id, count(hadm_id) 
    ' at line 2

Answer 1

Try this:

select * 
from icd9 as t1
inner join 
    (
     select subject_id
     from admissions 
     group by subject_id
     having count(hadm_id) = 1
     order by subject_id 
     limit 10
    ) as q2
ON t1.subject_id = q2.subject_id and t1.sequence = 1;

That way, you are doing grouping, filter with HAVING, ordering and limiting in one subquery itself, and then joining with icd9.