Calling a function in select statement - oracle

Question

I have a function:
CREATE OR REPLACE FUNCTION numOfOrders(name VARCHAR2) RETURN sys_refcursor AS
test_cur sys_refcursor;
BEGIN
OPEN test_cur for
SELECT C_CustKey, C_Name, COUNT(*) AS Num_Of_Orders
FROM Customer c
INNER JOIN Orders o
ON o.O_CustKey = c.C_CustKey
WHERE C_Name = name
GROUP BY C_CustKey, C_Name;
RETURN test_cur;
END;
/
the screenshot of the code :
and i called it by typing
SELECT numOfOrders('john') FROM customer;
I get my intended output:
C_CustKey C_Name Num_Of_Orders
0001 john 10
but it gets repeated for the number of rows in my customer table, i can fix this by adding fetch first 1 rows only; but how do I prevent it from happening.
Link to output (I can assure you all 1214 rows are the same) :
Tables
Customer :
Orders :

Answer 1

To cut a long story short: this is wrong:
select numOfOrders('john') from customer;
--------
this
because you're selecting the same dataset for all rows in customer table. What would be better? This:
select numOfOrders('john') from dual;
----
this
Why dual? Because it contains only one row.
A simple example based on Scott's schema (I don't have your data): there are 14 employees in the whole emp table, some of them working in department 10.
Function, similar to yours:
SQL> create or replace function num_of_employees (p_deptno in dept.deptno%type)
2 return sys_refcursor
3 is
4 l_rc sys_refcursor;
5 begin
6 open l_rc for
7 select d.dname,
8 count(*) as num_of_employees
9 from dept d join emp e on e.deptno = d.deptno
10 where d.deptno = p_deptno
11 group by d.dname;
12 return l_rc;
13 end;
14 /
Function created.
This is what you should do - select from dual; the result is just a single line:
SQL> select num_of_employees(10) from dual;
NUM_OF_EMPLOYEES(10)
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DNAME NUM_OF_EMPLOYEES
-------------- ----------------
ACCOUNTING 3 --> this is the result
SQL>
But, if you select from emp (which has 14 rows), you'll get 14 duplicate rows as the result:
SQL> select num_of_employees(10) from emp;
NUM_OF_EMPLOYEES(10)
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DNAME NUM_OF_EMPLOYEES
-------------- ----------------
ACCOUNTING 3
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DNAME NUM_OF_EMPLOYEES
-------------- ----------------
ACCOUNTING 3
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DNAME NUM_OF_EMPLOYEES
-------------- ----------------
ACCOUNTING 3
<snip>
14 rows selected.
SQL>
You asked whether it'll work (the dual table) if function returns more than a single row: it will.
I modified the function so that - if no department is passed to the function - it returns number of employees in all departments (i.e. multiple rows):
SQL> create or replace function num_of_employees (p_deptno in dept.deptno%type)
2 return sys_refcursor
3 is
4 l_rc sys_refcursor;
5 begin
6 open l_rc for
7 select d.dname,
8 count(*) as num_of_employees
9 from dept d join emp e on e.deptno = d.deptno
10 where d.deptno = p_deptno or p_deptno is null --> modified WHERE clause
11 group by d.dname;
12 return l_rc;
13 end;
14 /
Function created.
Calling the function:
SQL> select num_of_employees(null) from dual;
NUM_OF_EMPLOYEES(NUL
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DNAME NUM_OF_EMPLOYEES
-------------- ----------------
ACCOUNTING 3
RESEARCH 5
SALES 6
SQL>