How does the analytic function FIRST_VALUE work in SQL

I have just started diving in to SQL analytic functions and am a little hung up on FIRST_VALUE, maybe it is just specific to the example query from this site:

-- How many days after the first hire of each department were the next
-- employees hired?

SELECT empno, deptno, hiredate ? FIRST_VALUE(hiredate)
OVER (PARTITION BY deptno ORDER BY hiredate) DAY_GAP
FROM emp
WHERE deptno IN (20, 30)
ORDER BY deptno, DAY_GAP;

 EMPNO     DEPTNO    DAY_GAP
---------- ---------- ----------
  7369         20          0
  7566         20        106
  7902         20        351
  7788         20        722
  7876         20        756
  7499         30          0
  7521         30          2
  7698         30         70
  7844         30        200
  7654         30        220
  7900         30        286

I am struggling with two parts of this query. First, what is the '?' doing here? I have only seen it used for parameterized queries before. Second, how is the DAY_GAP actually being calculated? How does the query know to calculate the difference in days based on this query? Does it have something to do with the data type of hiredate?

Solution

First, what is the '?' doing here?

It's a mistake; it should be a - (minus sign). I'm guessing that Basu copied-and-pasted his Oracle transcript into a program that silently converted the ASCII minus sign into a Unicode en dash, and from there into a program that silently converted non-ASCII characters into ?.

Second, how is the DAY_GAP actually being calculated? How does the query know to calculate the difference in days based on this query? Does it have something to do with the data type of hiredate?

See previous; once you write it as hiredate - FIRST_VALUE(hiredate) OVER (...), it suddenly makes sense! :-)