错误代码：1054。“字段列表”中的未知列“部门”0.000 秒

Question

我已经通过了：Error Code: 1248. Every derived table must have its own alias No solution found for query，但仍然不确定它期望的正确列名是什么？

案例一：

SELECT max(avg_sal), min(avg_sal) 
FROM (SELECT dept , avg(salary) as avg_sal from Employee e  GROUP BY dept HAVING dept in ('Admin','IT')); 

12:04:44 SELECT max(avg_sal), min(avg_sal) FROM (SELECT dept , avg(salary) as avg_sal from Employee e GROUP BY Dept HAVING dept in ('Admin','IT')) LIMIT 0 , 1000 错误代码: 1248. 每个派生表都必须有自己的别名 0.000 秒

案例：2

SELECT max(avg_sal), min(avg_sal) 
FROM (SELECT dept , avg(salary) as avg_sal from Employee e  GROUP BY dept HAVING dept in ('Admin','IT')) as dept; 

然后我得到

选择最大值（avg_sal），最小值（avg_sal） FROM (SELECT dept , avg(salary) as avg_sal from Employee e GROUP BY dept HAVING dept in ('Admin','IT')) as dept;

部门

@Entity
public class Dept {
    @Id
    @GeneratedValue
    private Integer id;
    private String name;
}

员工

@Entity
@NamedQuery(name = "Employee.findMaxSalariesByDept", 
            query = "SELECT e.dept, MAX(e.salary) FROM Employee e GROUP BY e.dept.name HAVING e.dept.name in ?1")
public class Employee {
    @Id
    @GeneratedValue
    private Long id;
    private String name;
    @ManyToOne(cascade = CascadeType.ALL)
    private Dept dept;
    private int salary;
}

显示表格：

CREATE TABLE `employee` (
   `id` bigint(20) NOT NULL AUTO_INCREMENT,
   `name` varchar(255) DEFAULT NULL,
   `salary` int(11) NOT NULL,
   `dept_id` int(11) DEFAULT NULL,
   PRIMARY KEY (`id`),
   KEY `FKfow2bhgypdy2ij4oyukrn6cqw` (`dept_id`)
 ) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

Answer 1

第一种情况，您需要 FROM() 子句的表别名，例如：FROM ( ) T

SELECT max(T.avg_sal)
    , min(T.avg_sal) 
FROM ( 
    SELECT dept_id dept, avg(salary) as avg_sal 
    from Employee e  
    GROUP BY dept_id
    WHERE dept_id  in ('Admin','IT')
) T  ; 

你应该使用 where 没有过滤器

第二种情况（表别名不要使用与列相同的名称 eg:dept 但使用 t_dept）

SELECT max(t_dept.avg_sal), min(t_dept.avg_sal) 
FROM ( 
  SELECT dept_id dept, avg(salary ) as avg_sal 
   from Employee e  
   WHERE  dept_id in ('Admin','IT')
  GROUP BY dept
) as t_dept; 

并且您可以使用 where for filter 来执行这些值未计算但在行内容中可用的事实