How to reuse variables in the select statement of mysql - mysql

Question

I would like to use mysql variables to prevent same statements. In the following example i would like to sum the salary of an each employee and also sum it twice times. Of course the second column is wrong.
MariaDB [Messdaten]> select * from t;
+----+----------+--------+
| id | employee | salery |
+----+----------+--------+
| 1 | 10 | 1000 |
| 2 | 10 | 2000 |
| 3 | 20 | 3000 |
| 4 | 20 | 4000 |
+----+----------+--------+
4 rows in set (0.000 sec)
MariaDB [Messdaten]> select employee, #x:=sum(salery), 2*#x from t group by employee;
+----------+-----------------+-------+
| employee | #x:=sum(salery) | 2*#x |
+----------+-----------------+-------+
| 10 | 3000 | 14000 |
| 20 | 7000 | 14000 |
+----------+-----------------+-------+
2 rows in set (0.001 sec)
Of course i could use select employee, sum(salery), 2*sum(salery) but in my real use case the statements are very big and therefore bad readable.
What ist going wrong and if this is a gap of mysql are there some workarounds?

Answer 1

You can use a subquery like so to get the correct result while only summing (or executing a more complex statement) once
SELECT
employee,
totalSalary,
totalSalary*2 AS doubleSalary
FROM (
SELECT
employee,
sum(salary) AS totalSalary
FROM employees
GROUP BY employee
) AS employeeSalaries;
The unexpected variable behaviour is described in the MySQL docs here.
HAVING, GROUP BY, and ORDER BY, when referring to a variable that is assigned a value in the select expression list do not work as expected because the expression is evaluated on the client and thus can use stale column values from a previous row.