如何处理带有总和的 SQL 子查询

Question

I am practicing queries on an example database in MySQL.

I have an employee table with a primary key of emp_id .

I have a works_with table with a composite key of emp_id and client_id . It also has a column of total_sales .

I am trying to write a query that returns the name of any employee who has sold over 100,000 total.

I was able to return the employee id and total for sums over 100,000 like so:

SELECT SUM(total_sales) AS total_sales, emp_id
FROM works_with 
WHERE total_sales > 100000
GROUP BY emp_id;

But I am unsure how to use this to also get employee name. I have tried nested queries but with no luck. For example when I try this:

SELECT first_name, last_name 
FROM employee
WHERE emp_id IN (
    SELECT SUM(total_sales) AS total_sales, emp_id
    FROM works_with WHERE total_sales > 100000
    GROUP BY emp_id
)

I get Error 1241: Operand should contain 1 column(s). I believe this is because I am selecting two columns in the nested query? So how would I handle this problem?

Answer 1

Just join :

select sum(w.total_sales) as total_sales, e.first_name, e.lastnmae
from works_with w
inner join employee e on e.emp_id = w.emp_id
group by e.emp_id
having sum(w.total_sales) > 10000;

Note that I used a having clause rather than the where clause: presumably, you want to sum all sales of each employee, and filter on that result. Your original queried sums only individual values that are greater than 100000.

Answer 2

Adding to GMB's solution.

Take your existing Select and wrap it in a Derived Table/CTE:

SELECT e.first_name, e.last_name, big_sales.total_sales
FROM employee as e
join 
 (
   SELECT SUM(total_sales) AS total_sales, emp_id
   FROM works_with
   GROUP BY emp_id
   HAVING total_sales > 100000
 ) as big_sales
on e.emp_id = big_sales.emp_id

Now you can show the total_sales plus employee details. Additionally this should be more efficient, because you aggregate & filter before the join.

If you only need to show the employee you can use a SubQuery (like the one you tried), but it must return a single column, ie remove the SUM from the Select list:

SELECT first_name, last_name 
FROM employee
WHERE emp_id IN (
    SELECT emp_id -- , SUM(total_sales) AS total_sales
    FROM works_with 
    GROUP BY emp_id
    HAVING SUM(total_sales) > 100000
)