如何从 Oracle 数据库中获取 MAX

Question

I am trying to get the MAX from a database but I am trying two methods, and need to be cleared if both methods are correct or not.

These are my Object types:

Emp_t (eno: number(4), ename: varchar2(15), edept: ref dept_t, salary: number(8,2))
Dept_t (dno: number(2), dname: varchar2(12), mgr ref emp_t)
Proj_t (pno: number(4), pname: varchar2(15), pdept ref dept_t, budget: number(10,2))

And these are my Tables:

Emp of Emp_t (eno primary key, edept references dept)
Dept of Dept_t (dno primary key, mgr references emp)
Proj of Proj_t (pno primary key, pdept references dept)

My question is:

Q.) Find the manager's name who is controlling the project with the largest budget

My first answer is:

SELECT p.pdept.mgr.ename
FROM proj p
WHERE p.budget IN (SELECT MAX(p.budget) FROM proj p)

And my second answer is:

SELECT p.pdept.mgr.ename, MAX(p.budget)
FROM proj p

I need to know which answer is correct. or both are right. If one is incorrect the reason for that?

Answer 1

You need join s. In the most recent versions of Oracle, a simple method uses order by and fetch :

select e.name
from proj_t p join
     emp_t e
     on p.mgr = e.eno join
     dept_t d
     on p.pdept = d.dno
order by p.budget desc
fetch first 1 row only;

The key point is that you need to use proper, explicit, standard JOIN syntax.

Answer 2

Neither of your queries are right.

Issue with 1st Query -

SELECT p.pdept.mgr.ename        ----- This is not the correct syntax
FROM proj p
WHERE p.budget IN (SELECT MAX(p.budget) FROM proj p)

The syntax is not correct for select clause. The syntax should be -

SELECT e.name
FROM proj_t p 
INNER JOIN emp_t e ON p.mgr = e.eno
INNER JOIN dept_t d ON p.pdept = d.dno
WHERE p.budget = (SELECT MAX(budget) FROM proj_t);

Issue with 2nd Query -

SELECT p.pdept.mgr.ename, MAX(p.budget)   -----  You cannot use other columns with aggregate function without group by clause.
FROM proj p

Please use the above query and you will get your desired result.

Answer 3

If you don't want to use MAX or limit the output by one row:

SELECT p.pdept.mgr.ename
FROM proj p
WHERE NOT EXISTS(
    SELECT *
    FROM proj subp
    WHERE subp.budget > p.budget
)

This will search for a higher budet in proj untill there is none.