关于oracle LISTAGG函数的用法(以某个字段相同情况下，将对应的另一个字段连接起来)

SELECT department_id "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) "Employees"
  FROM employees
  GROUP BY department_id

--以上为主功能
  ORDER BY department_id;

Dept. Employees
------ ------------------------------------------------------------
    10 Whalen
    20 Hartstein; Fay
    30 Raphaely; Khoo; Tobias; Baida; Himuro; Colmenares
    40 Mavris
    50 Kaufling; Ladwig; Rajs; Sarchand; Bell; Mallin; Weiss; Davie

department_id相同情况下，将last_name以“;”连接起来，连接时以hire_date排序之后的顺序进行combine

在不使用group by的时候可以用over语法代替

SELECT department_id "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) "Employees" <span style="background-color: rgb(255, 0, 0);">OVER(PARTITION BY department_id)</span>
  FROM employees

over的用法可以解决一个问题:group by的时候select字段选择的时候字段必须在聚集函数中或者group by字段中。用over方法可以在select字段中加任意表中的字段。