sql MySQL error (1241) operand should contain 1 column(s) - mysql

Question

first I will like to state that am still a newbie on writing SQL Queries. I thoroughly searched for an answer on this Error and I got a good number of answers, but none seems to be helpful or I will say I don't really know how to apply the solutions to mine.
Here is my challenge, I have an application table, that stores applicants records with some unique columns e.g (dl_number,parent_id,person_id). The parent_id keeps tracks of individual applicant history records with the his/her first record and each applicant is meant to have a unique dl_number, but for some reasons, some applicants dl_number(s) are not unique, hence a need to identify the records with changing dl_number(s).
Below is the SQL Query, that am getting the [sql error (1241) operand should contain 1 column(s)] error on.
SELECT id,application_id,dl_number,surname,firstname,othername,birth_date,status_id,expiry_date,person_id,COUNT(DISTINCT(dl_number,parent_id,birth_date)) AS NumOccurrences
FROM tbl_dl_application
WHERE status_id > 1
GROUP BY dl_number,parent_id,birth_date
HAVING NumOccurrences > 1
Please any help on how to solve this, or a better way to solve this.
Sample table and expected result

Answer 1

DISTICT is not really a function to be used that way.
You can do SELECT DISTICT column1, column2 FROM table to get unique rows only, or similarly SELECT column, count(DISTINCT anothercolumn) FROM table GROUP BY column to get unique rows within a group.
Problem as I understand it: You look for duplicates in your table. Duplicates are defined as having identical values of these 3 columns: dl_n‌​umber, parent_id and birth‌​_date.
I'm also assuming that id is a primary key in your table. If not, replace the t2.id <> t.id condition with one that uniquely identify your row.
If you only wanted to know what are the duplicated groups, this should work:
SELECT dl_n‌​umber, parent_id, birth‌​_date, count(*) as NumOccurences -- You can only add aggregation functions here, not another column unless you group by it.
FROM tbl_dl_application t
WHERE status_id > 1 -- I don't know what this is but it should do no harm.
GROUP BY dl_n‌​umber, parent_id, birth‌​_date
HAVING count(*)>1
If, however, you want to know details of each duplicated row, this query will give you that:
SELECT *
FROM tbl_dl_application t
WHERE
status_id > 1 -- I don't know what this is but it should do no harm.
AND EXISTS (
SELECT 1
FROM tbl_dl_application t2
WHERE
t2.dl_number = t.dl_number
AND t2.parent_id = t.parent_id
AND t2.birth_date = t.birth_date
AND t2.id <> t.id
)
ORDER BY dl_n‌​umber, parent_id, birth‌​_date, id; -- So you have your duplicates nicely next to each other.
Please explain further if I misunderstood your objective, or ask if the solution is not clear enough.

Answer 2

**You have to use only one column while use to DISTINCT function. You used this three field dl_number,parent_id,birth_date. Just use 1 filed from these 3. Then query will run.**
For example.
SELECT id,application_id,dl_number,surname,firstname,othername,birth_date,status_id,expiry_date,person_id,COUNT(DISTINCT(parent_id)) AS NumOccurrences
FROM tbl_dl_application
WHERE status_id > 1
GROUP BY dl_number,parent_id,birth_date
HAVING NumOccurrences > 1