创建子查询时遇到问题

Question

我在使用子查询创建查询时遇到了一些问题。我必须使用子查询，因为这是学校的作业。

我有两张表，一张员工一张，一张部门一张。从这些表中，我必须 return 收入超过部门平均工资的员工。

这就是我目前所拥有的：

SELECT 
    ename,
    salary,
    (SELECT 
            department.depname
        FROM
            department
        WHERE
            department.depno = employee.depno) AS depname,
    (SELECT 
            AVG(salary)
        FROM
            employee
        WHERE
            employee.depno IN (SELECT 
                    department.depno
                FROM
                    department)) avgSalary
FROM
    employee
WHERE
    salary > (SELECT 
            AVG(salary)
        FROM
            employee
        WHERE
            employee.depno IN (SELECT 
                    department.depno
                FROM
                    department))

这个return秒：

'JONES', '2975.00', 'RESEARCH', '2073.214286'
'BLAKE', '2850.00', 'SALES', '2073.214286'
'CLARK', '2450.00', 'ACCOUNTING', '2073.214286'
'SCOTT', '3000.00', 'RESEARCH', '2073.214286'
'KING', '5000.00', 'ACCOUNTING', '2073.214286'
'FORD', '3000.00', 'RESEARCH', '2073.214286'

但平均工资 return 是所有工资的平均值。但我需要每个部门都这样做。

我知道如何通过执行以下查询来执行此操作：

SELECT 
    department.depname,
    (SELECT 
            AVG(salary)
        FROM
            employee
        WHERE
            department.depno = employee.depno) avgSalary
FROM
    department

其中 return 个：

ACCOUNTING  2916.666667
RESEARCH    2175.000000
SALES       1566.666667
OPERATIONS

但我不知道如何组合它们，或者如果不使用 join 是否可行。

任何帮助将不胜感激

编辑：

Table: employee
Columns:
empno decimal(4,0) PK 
ename varchar(10) 
efunction varchar(10) 
boss decimal(4,0) 
employed date 
salary decimal(7,2) 
commission decimal(7,2) 
depno decimal(2,0)

Table: department
Columns:
depno decimal(2,0) PK 
depname varchar(14) 
location varchar(13)

Answer 1

有点像这样...?

mysql> select   *
    -> from     employee e
    -> where    e.employee_salary > (
    ->             select   avg(employee_salary) avg
    ->             from     employee ee
    ->             -- this is how you join them without a join...
    ->             where    ee.department_id=e.department_id
    ->          );
+-------------+---------------+---------------+-----------------+
| employee_id | department_id | employee_name | employee_salary |
+-------------+---------------+---------------+-----------------+
|           2 |             1 | employee 2    |              40 |
|           6 |             1 | employee 6    |              50 |
+-------------+---------------+---------------+-----------------+
2 rows in set (0.04 sec)

哪里....

mysql> select * from employee;
+-------------+---------------+---------------+-----------------+
| employee_id | department_id | employee_name | employee_salary |
+-------------+---------------+---------------+-----------------+
|           1 |             1 | employee 1    |              20 |
|           2 |             1 | employee 2    |              40 |
|           3 |             1 | employee 3    |              30 |
|           4 |             1 | employee 4    |              30 |
|           5 |             1 | employee 5    |              30 |
|           6 |             1 | employee 6    |              50 |
+-------------+---------------+---------------+-----------------+
6 rows in set (0.00 sec)

没有加入...

mysql> select   e.*,
    ->
    ->          (select  department_name
    ->           from    department d
    ->           where   d.department_id=e.department_id) as department_name,
    ->
    ->          (select   avg(employee_salary) avg
    ->             from     employee ee
    ->             where    ee.department_id=e.department_id) as department_average
    ->
    -> from     employee e
    ->
    -> where    e.employee_salary > (
    ->             select   avg(employee_salary) avg
    ->             from     employee ee
    ->             where    ee.department_id=e.department_id
    ->          );
+-------------+---------------+---------------+-----------------+-----------------+--------------------+
| employee_id | department_id | employee_name | employee_salary | department_name | department_average |
+-------------+---------------+---------------+-----------------+-----------------+--------------------+
|           2 |             1 | employee 2    |              40 | department 1    | 33.333333333333336 |
|           6 |             1 | employee 6    |              50 | department 1    | 33.333333333333336 |
+-------------+---------------+---------------+-----------------+-----------------+--------------------+
2 rows in set (0.00 sec)

你真的应该避免像 select 那样的连接，除非真的有必要，因为在现实生活中最好只使用连接（对于 efficiency/performance 和更大的数据集）。不过，where 中的子查询是完全合适的。

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