Date Difference and ORA-00937 - not a single-group group function - sql

Question

I am trying to add two tables and get the total days from two dates. But having the following simple issue: not a single-group group function.
This is what I've tried so far:
SELECT COUNT(status) AS "Present Days",
(SELECT TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY'))
FROM attendance m
INNER JOIN EMP_OFFICIAL k
ON k.EMPNO = m.EMPNO
WHERE m.empno='EMP00254'
AND m.status='P') AS "Total Days"
FROM attendance
WHERE empno = 'EMP00254'
AND status = 'P';
Can I get the days without using the DUAL?

Answer 1

First off, that subquery is most likely not scalar (it looks like you're assuming multiple rows can come from the attendance table) and second, it contains an unnecessary join..
What I would do instead is something like:
select count(*) as "Present Days",
to_date('01/10/2018', 'mm/dd/yyyy') - k.joining_date "Total Days"
from attendance a
inner join emp_official k on a.empno = k.empno
where a.empno = 'EMP00254'
and a.status = 'P'
group by to_date('01/10/2018', 'mm/dd/yyyy') - k.joining_date;
This does assume that the k.joining_date column is of DATE datatype and that 'k.empno' is a unique column.

Answer 2

If there is only one row in the EMP_OFFICIAL table for each employee then you could do:
SELECT COUNT(a.status) AS "Present Days",
TRUNC( SYSDATE ) - TRUNC( MIN( k.joining_date ) ) AS "Total Days"
FROM attendance a
RIGHT OUTER JOIN EMP_OFFICIAL k
ON ( k.EMPNO = a.EMPNO )
WHERE a.empno = 'EMP00254'
WHERE a.status = 'P';
or you could do the aggregation in a sub-query:
SELECT a."Present Days",
TRUNC( SYSDATE ) - TRUNC( k.joining_date ) AS "Total Days"
FROM (
SELECT EMPNO,
COUNT( status ) AS "Present Days"
FROM attendance
WHERE EMPNO = 'EMP00254'
AND status = 'P'
GROUP BY EMPNO
) a
RIGHT OUTER JOIN EMP_OFFICIAL k
ON ( k.EMPNO = a.EMPNO )
WHERE k.EMPNO = 'EMP00254';
Or you could use UNION ALL to query the two tables and have the result in two rows (rather than two columns):
SELECT 'Present Days' As type,
COUNT(status) AS Days
FROM attendance
WHERE empno = 'EMP00254'
WHERE status = 'P'
UNION ALL
SELECT 'Total Days',
TRUNC( SYSDATE ) - TRUNC( joining_date )
FROM EMP_OFFICIAL
WHERE empno = 'EMP00254';

Answer 3

Either remove COUNT(status) AS "Present Days" part and SELECT inside paranthesis :
SELECT (TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY')) "time difference" FROM attendance m
INNER JOIN EMP_OFFICIAL k ON k.EMPNO = m.EMPNO where m.empno='EMP00254' and m.status='P') AS "Total Days"
FROM attendance where empno = 'EMP00254' and status = 'P';
OR
use the following (add GROUP BY expression at the end of sql):
SELECT COUNT(status) AS "Present Days",
(TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY')) "time difference" FROM attendance m
INNER JOIN EMP_OFFICIAL k ON k.EMPNO = m.EMPNO where m.empno='EMP00254' and m.status='P') AS "Total Days"
FROM attendance where empno = 'EMP00254' and status = 'P'
GROUP BY (TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY'));
Since , groupped and non-groupped items can not be used together.
By the way, you don't need to use dual in your inline select statement like ( SELECT TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY') FROM DUAL ), just ( TRUNC(TO_DATE('01/10/2018', 'MM/DD/YYYY') - TO_DATE(k.JOINING_DATE, 'MM/DD/YYYY')) is enough.