Need help in the query (oracle 11 g) - sql

Question

First of all, these are my create_table statements.
/*--- Base tables ---*/
CREATE TABLE CUSTOMER (
CUST_ID VARCHAR(10) NOT NULL,
CUST_FNAME VARCHAR(15),
CUST_LNAME VARCHAR(15),
CUST_HP NUMBER(10),
CUST_EMAIL VARCHAR(40),
primary key (CUST_ID)
);
CREATE TABLE STAFF(
STAFF_ID NUMBER(12) NOT NULL,
STAFF_FNAME VARCHAR(15),
STAFF_LNAME VARCHAR(15),
STAFF_DOB Date,
STAFF_AGE NUMBER(2),
STAFF_ADDRESS VARCHAR(70),
STAFF_HP NUMBER(10),
STAFF_SALARY NUMBER(6,2),
primary key (STAFF_ID)
);
CREATE TABLE SUPPLIER(
SUPP_ID VARCHAR(10) NOT NULL,
SUPP_NAME VARCHAR(30),
SUPP_TEL NUMBER(10),
SUPP_EMAIL VARCHAR(25),
SUPP_ADDRESS VARCHAR(80),
SUPP_CITY VARCHAR(20),
SUPP_STATE VARCHAR(10),
SUPP_ZIP NUMBER(5),
primary key (SUPP_ID)
);
CREATE TABLE PRODUCT(
PROD_ID VARCHAR(10) NOT NULL,
PROD_NAME VARCHAR(30),
PROD_DESC VARCHAR(70),
R_UNIT_PRICE NUMBER(3,2) DEFAULT 4.50,
L_UNIT_PRICE NUMBER(3,2) DEFAULT 5.00,
primary key (PROD_ID)
);
CREATE TABLE INGREDIENT(
ING_ID VARCHAR(10) NOT NULL,
ING_NAME VARCHAR(20),
ING_DESC VARCHAR(60),
primary key (ING_ID)
);
CREATE TABLE TOPPING(
TOP_ID VARCHAR(10) NOT NULL,
TOP_NAME VARCHAR(20),
TOP_DESC VARCHAR(40),
TOP_PRICE NUMBER(2,2),
primary key (TOP_ID)
);
/*--- Child tables ---*/
CREATE TABLE ORDERS(
ORDERS_ID VARCHAR(10) NOT NULL,
CUST_ID VARCHAR(10) NOT NULL,
STAFF_ID NUMBER(12) NOT NULL,
TOTAL_PRICE NUMBER(5,2),
ORDERS_DATE DATE,
primary key (ORDERS_ID),
foreign key (CUST_ID) references CUSTOMER(CUST_ID),
foreign key (STAFF_ID) references STAFF(STAFF_ID)
);
CREATE TABLE ORD_DETAIL(
SK1_ID NUMBER(4) NOT NULL,
ORDERS_ID VARCHAR(10) NOT NULL,
PROD_ID VARCHAR(10) NOT NULL,
TOP_ID VARCHAR(10) NOT NULL,
TEATYPE VARCHAR(8),
CUPSIZE CHAR(1),
QTY NUMBER(2),
CONSTRAINT CHK_CUPSIZE CHECK (CUPSIZE = 'R' OR CUPSIZE = 'L'),
CONSTRAINT CHK_TEATYPE CHECK (TEATYPE = 'Oolong' OR TEATYPE = 'Jasmine'),
primary key (SK1_ID),
foreign key (ORDERS_ID) references ORDERS(ORDERS_ID),
foreign key (PROD_ID) references PRODUCT(PROD_ID)，
foreign key (TOP_ID) references TOPPING(TOP_ID)
);
CREATE TABLE PROD_ING(
PROD_ID VARCHAR(10) NOT NULL,
ING_ID VARCHAR(10) NOT NULL,
primary key (PROD_ID , ING_ID),
foreign key (PROD_ID) references PRODUCT(PROD_ID),
foreign key (ING_ID) references INGREDIENT(ING_ID)
);
CREATE TABLE ING_SUPP(
ING_ID VARCHAR(10) NOT NULL,
SUPP_ID VARCHAR(10) NOT NULL,
SUPP_PRICE NUMBER(6,2),
primary key (ING_ID, SUPP_ID),
foreign key (ING_ID) references INGREDIENT(ING_ID),
foreign key (SUPP_ID) references SUPPLIER(SUPP_ID)
);
And my query is this:Which employee served the most number of customers?
But when I executed these statements:
SELECT MAX (COUNT (O.CUST_ID)) AS "Served customer"
FROM STAFF S, ORDERS O
WHERE (S.STAFF_ID = O.STAFF_ID)
GROUP BY O.STAFF_ID
HAVING COUNT (O.CUST_ID) > 0;
What I got was there was a column named "Served customer" with the maximum number only.
So, now my question is how to display the employee's ID, first name and last name along with
"Served customer" column.

Answer 1

The question is: "Which employee served the most number of customers? " Your query does not even return information about the employee, only about the number of customers.
The following aggregation returns for each staff member, the number of customers who were served:
SELECT O.STAFF_ID, COUNT(O.CUST_ID) AS "Served customer"
FROM STAFF SS JOIN
ORDERS O
ON S.STAFF_ID = O.STAFF_ID
GROUP BY O.STAFF_ID;
To get the maximum number we just need to order this by the count and take the first row. In Oracle, this uses a subquery:
SELECT so.*
FROM (SELECT O.STAFF_ID, COUNT(O.CUST_ID) AS "Served customer"
FROM STAFF SS JOIN
ORDERS O
ON S.STAFF_ID = O.STAFF_ID
GROUP BY O.STAFF_ID
ORDER BY COUNT(O.CUST_ID) DESC
) so
WHERE rownum = 1;