Create function with mysql - mysql

Question

Hello I have an assignment:
Consider an employee database with two relations:
employee(employee-name, street, city)
works(employee-name, company-name, salary)
where the primary keys are underlined. Write a query to find companies whose employees earn a lower salary, on average, than the average salary at “First Bank Corporation”.
Use user defined SQL functions (create function command) as appropriate to answer the above query, the function takes the company name as the input and returns the average salary of the given
company.
I created this function:
create function avg_salary (c_name varchar(30))
returns numeric(8,6)
begin
declare a_salary numeric(8,6);
select avg(salary) into a_salary
from works
where company.company_name = c_name
group by company_name;
return a_salary;
select company_name
from works
where a_salary<(select avg(salary) from works where company_name = ’ First Bank Corporation’);
end
But I get an error message:
Error 1415 (0A000) :Not allowed to return a result set from a function
I don't understand why I get this error.
Thank you for your help

Answer 1

You should read the mysql manual in conjunction with your tutorial to understand mysql specifics.
The error message is clear enough (every select returns a result set) so select company_name
from works is at fault.
Selects into and set = (select...) are fine.
In addition to the error the single quote around ’ First Bank Corporation’ is odd and errors when I try to compile the function.
Also the group by company_name; will be rejected in later versions of my sql because company_name is not included in the select list. see https://dev.mysql.com/doc/refman/8.0/en/group-by-handling.html - in this case it's not needed because you are avgerageing for a specified company.
Mysql also requires delimiters to be set before and after the create function where there are multiple statements in the function see https://dev.mysql.com/doc/refman/8.0/en/stored-programs-defining.html

Answer 2

Try this (example includes all code I used to create all objects and insert data):
DROP TABLE IF EXISTS employee;
CREATE TABLE IF NOT EXISTS employee (
`employee-name` varchar(100) NOT NULL,
`street` varchar(100) NOT NULL,
`city` varchar(100) NOT NULL,
PRIMARY KEY (`employee-name`)
);
DROP TABLE IF EXISTS works;
CREATE TABLE IF NOT EXISTS works (
`employee-name` varchar(100) NOT NULL,
`company-name` varchar(100) NOT NULL,
`salary` numeric(8,2) NOT NULL,
PRIMARY KEY (`employee-name`)
);
INSERT INTO
employee
(`employee-name`, `street`, `city`)
VALUES
('Employee 01', '1234 State ST.', 'Your City'),
('Employee 02', '1234 State ST.', 'Your City'),
('Employee 03', '1234 State ST.', 'Your City'),
('Employee 04', '1234 State ST.', 'Your City'),
('Employee 05', '1234 State ST.', 'Your City'),
('Employee 06', '1234 State ST.', 'Your City'),
('Employee 07', '1234 State ST.', 'Your City'),
('Employee 08', '1234 State ST.', 'Your City'),
('Employee 09', '1234 State ST.', 'Your City'),
('Employee 10', '1234 State ST.', 'Your City'),
('Employee 11', '1234 State ST.', 'Your City'),
('Employee 12', '1234 State ST.', 'Your City'),
('Employee 13', '1234 State ST.', 'Your City'),
('Employee 14', '1234 State ST.', 'Your City'),
('Employee 15', '1234 State ST.', 'Your City'),
('Employee 16', '1234 State ST.', 'Your City'),
('Employee 17', '1234 State ST.', 'Your City'),
('Employee 18', '1234 State ST.', 'Your City'),
('Employee 19', '1234 State ST.', 'Your City'),
('Employee 20', '1234 State ST.', 'Your City'),
('Employee 21', '1234 State ST.', 'Your City'),
('Employee 22', '1234 State ST.', 'Your City');
INSERT INTO
works
(`employee-name`, `company-name`, `salary`)
VALUES
('Employee 01', 'Company 01', 10000.00),
('Employee 02', 'Company 01', 10000.00),
('Employee 03', 'Company 01', 10000.00),
('Employee 04', 'Company 01', 10000.00),
('Employee 05', 'Company 01', 10000.00),
('Employee 06', 'Company 02', 15000.00),
('Employee 07', 'Company 02', 15000.00),
('Employee 08', 'Company 02', 15000.00),
('Employee 09', 'Company 03', 20000.00),
('Employee 10', 'Company 03', 20000.00),
('Employee 11', 'Company 03', 20000.00),
('Employee 12', 'Company 03', 20000.00),
('Employee 13', 'Company 03', 20000.00),
('Employee 14', 'Company 03', 20000.00),
('Employee 15', 'Company 03', 20000.00),
('Employee 16', 'Company 04', 60000.00),
('Employee 17', 'Company 04', 60000.00),
('Employee 18', 'Company 04', 60000.00),
('Employee 19', 'First Bank Corporation', 17000.00),
('Employee 20', 'First Bank Corporation', 17000.00),
('Employee 21', 'First Bank Corporation', 17000.00),
('Employee 22', 'First Bank Corporation', 17000.00);
DROP FUNCTION IF EXISTS avg_salary;
DELIMITER $$
CREATE FUNCTION IF NOT EXISTS avg_salary (c_name varchar(100))
RETURNS numeric(8,2)
BEGIN
DECLARE a_salary numeric(8,2);
SELECT avg(salary) INTO a_salary FROM works WHERE `company-name` = c_name GROUP BY `company-name`;
RETURN a_salary;
END$$
DELIMITER ;
SELECT
`company-name`
FROM
works
WHERE
avg_salary(`company-name`) < avg_salary('First Bank Corporation')
GROUP BY
`company-name`
It works perfectly for me, returning Company 01 and Company 02 as results.
ADDED after comment by original poster. Query now returns Avg Salary, as well as Company Name,
SELECT
`company-name` AS `Company Name`,
avg_salary(`company-name`) AS `AVG Salary`
FROM
works
WHERE
avg_salary(`company-name`) < avg_salary('First Bank Corporation')
GROUP BY
`company-name`

Answer 3

try something like below:
create function avg_salary (c_name varchar(30))
returns numeric(18,6)
begin
declare a_salary numeric(18,6);
set a_salary = select avg(salary) from works where company_name = c_name;
return (a_salary);
end
select company_name from works where a_salary<(select avg_salary('First Bank Corporation'));