通过添加重复项来删除重复项[SQL]

Question

我有这样的查询：

select employee_id, salary
from salary
left join employee on salary.employee_id=employee.id_employee;

它向我返回这些结果

EMPLOYEE ID  | SALARY
-------------|-------
1            |     50
2            |     50
3            |     50
1            |     30

如何通过添加重复项来删除重复项，如下所示：

EMPLOYEE ID |  SALARY
------------|--------
1           |      80
2           |      50
3           |      50

Answer 1

没有理由将left join从salary到employee。大概employee_id中的每个salary都是有效的雇员。所以，这应该做您想要的：

select s.employee_id, sum(s.salary) as salary
from salary s
group by s.employee_id;

如果您要所有名员工，即使不是salary表中的员工，那么外部联接也是合适的，但是employee应该是第一位：

select e.id_employee, sum(s.salary) as salary
from employee e left join
     salary s
     on s.employee_id = e.id_employee
group by e.id_employee;

不在salary中的员工的值为NULL。

请注意，此查询中的group by条件位于employee中 first 表的left join上。

Answer 2

select employee_id, SUM(salary) as salary
from salary
left join employee on salary.employee_id=employee.id_employee
group by emplyee_id;

Answer 3

这正是group by子句的用途。您必须group by emplopyee_id并指定要如何汇总工资：

SELECT   employee_id, SUM(salary)
FROM     salary
GROUP BY employee_id