SET 運算子

使用 SET 操作符注意事項

  在SELECT 列表中的列名和表示式在**數量和資料型別**上要相對應
  括號可以改變執行的順序
  ORDER BY 子句:
      只能在語句的最後出現
      可以使用第一個查詢中的列名, 別名或相對位置

SELECT department_id, TO_NUMBER(null) location, hire_date FROM employees UNION SELECT department_id, location_id, TO_DATE(null) FROM departments;
SELECT employee_id, job_id,salary FROM employees UNION SELECT employee_id, job_id,0 FROM job_history;

COLUMN a_dummy NOPRINT SELECT 'sing' AS "My dream", 3 a_dummy FROM dual UNION SELECT 'I"d like to teach', 1 FROM dual UNION SELECT 'the world to', 2 FROM dual ORDER BY 2;

子查詢

單列子查詢

SELECT select_list FROM table WHERE expr operator (SELECT select_list FROM table);

多列子查詢

1. 多列子查詢
成對比較
問題：查詢與141號或174號員工的manager_id和department_id相同的其他員工的employee_id, manager_id, department_id
SELECT employee_id, manager_id, department_id FROM EMPLOYEES WHERE ( manager_id, department_id ) in ( SELECT manager_id, department_id FROM employees WHERE employee_id IN (141,174) ) and EMPLOYEE_ID not in (141,174)

不成對比較
SELECT employee_id, manager_id, department_id FROM employees WHERE manager_id IN ( SELECT manager_id FROM employees WHERE employee_id IN ( 174, 141 )) AND department_id IN ( SELECT department_id FROM employees WHERE employee_id IN ( 174, 141 )) AND employee_id NOT IN ( 174, 141 );

2. FROM 子句中使用子查詢
問題：返回比本部門平均工資高的員工的last_name, department_id, salary及平均工資

 <font color=gray size=5 face="微軟雅黑">方法一</font>

select last_name,department_id,salary, (select avg(salary)from employees e3 where e1.department_id = e3.department_id group by department_id) avg_salary from employees e1 where salary > (select avg(salary) from employees e2 where e1.department_id = e2.department_id group by department_id )
方法二
SELECT a.last_name, a.salary, a.department_id, b.salavg FROM employees a, (SELECT department_id, AVG(salary) salavg FROM employees GROUP BY department_id) b WHERE a.department_id = b.department_id AND a.salary > b.salavg;

3. CASE 表示式中使用單列子查詢

4. ORDER BY 子句中使用單列子查詢

5. 相關子查詢

6. 相關更新

7. 相關刪除

EXISTS /NOT EXISTS 操作符

WITH 子句