为什么这些SQL语句中只有一个有效？

Question

为什么第一个SQL语句不执行,第二个SQL语句不执行？

1.

select e.employee_id, e.last_name, e.salary as "SALARY"
from employees e
where employees.salary > (select avg(salary) from employees) 
order by SALARY;

2.

select e.employee_id, e.last_name, e.salary as "SALARY"
from employees e
where e.salary > (select avg(salary) from employees) 
order by SALARY;

Answer 1

因为您已经为表名定义了别名employee.

因此在WHERE子句中,您需要使用别名,而不是表名,因为它不再有效.

SELECT e.employee_id, e.last_name, e.salary as "SALARY" 
FROM   employees e 
WHERE  e.salary > (select avg(salary) from employees) 
    -- ^ ALIAS should be used, not the tableName
ORDER  BY SALARY;

这可能有点偏离主题,但这是一个额外的信息.

SQL操作顺序如下:

• FROM子句
• WHERE子句
• GROUP BY子句
• HAVING子句
• SELECT子句
• ORDER BY子句

由于您已在FROM子句上为表名提供了别名,因此操作顺序中的以下内容现在指的是给定的别名,而不是表名本身.