Question

我在使用子查询创建查询时遇到了一些麻烦。我必须使用子查询，因为它是来自学校的作业。

我有两张桌子，一张是雇员，另一张是一张。从这些表中我必须返回收入超过部门平均工资的员工。

所以这就是我到目前为止：

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))

返回：

'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'

但是平均工资返回所有工资的平均值。但我需要按部门这样做。

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

返回：

ACCOUNTING  2916.666667
RESEARCH    2175.000000
SALES       1566.666667
OPERATIONS

但我不知道如何组合它们，或者如果不使用连接就可以。

非常感谢任何帮助

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)

其中....

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)

除非真的有必要，否则你应该真的避免在select中加入，因为在现实生活中使用连接更好（为了提高数据集的效率/性能）。但where中的子查询完全合适。

创建子查询的麻烦

1 个答案: