ORCLA基础总结（二）

SET 运算符

使用 SET 操作符注意事项

  在SELECT 列表中的列名和表达式在**数量和数据类型**上要相对应
  括号可以改变执行的顺序
  ORDER BY 子句:
      只能在语句的最后出现
      可以使用第一个查询中的列名, 别名或相对位置

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;
使用相对位置排序举例

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;

子查询

单列子查询

子查询 (内查询) 在主查询之前一次执行完成。 子查询的结果被主查询(外查询)使用 。

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

多列子查询

1. 多列子查询
成对比较
问题：查询与141号或174号员工的manager_id和department_id相同的其他员工的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)

不成对比较
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 子句中使用子查询
问题：返回比本部门平均工资高的员工的last_name, department_id, salary及平均工资

方法一
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 表达式中使用单列子查询
问题：显式员工的employee_id,last_name和location。其中，若员工department_id与location_id为1800的department_id相同，则location为’Canada’,其余则为’USA’。
SELECT employee_id, last_name, ( CASE WHEN department_id = ( SELECT department_id FROM departments WHERE location_id = 1800 ) THEN 'Canada' ELSE 'USA' END ) location FROM employees;

4. ORDER BY 子句中使用单列子查询
问题：查询员工的employee_id,last_name,要求按照员工的department_name排序

SELECT employee_id, last_name FROM employeese ORDER BY ( SELECT department_name FROM departments d WHERE e.department_id = d.department_id );

5. 相关子查询
问题：查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id

问题：若employees表中employee_id与job_history表中employee_id相同的数目不小于2，输出这些相同id的员工的employee_id,last_name和其job_id
SELECT e.employee_id, last_name,e.job_id FROM employees e WHERE 2 <= (SELECT COUNT(*) FROM job_history WHERE employee_id = e.employee_id);

6. 相关更新

相关更新:使用相关子查询依据一个表中的数据更新另一个表的数据

7. 相关删除

相关删除:使用相关子查询依据一个表中的数据删除另一个表的数据

问题：删除表employees中，其与emp_history表皆有的数据
DELETE FROM employees E WHERE employee_id = (SELECT employee_id FROM emp_history WHERE employee_id = E.employee_id);

EXISTS /NOT EXISTS 操作符

问题：查询公司管理者的employee_id,last_name,job_id,department_id信息
SELECT employee_id, last_name, job_id, department_id FROM employees outer WHERE EXISTS ( SELECT 'X' FROM employees WHERE manager_id = outer.employee_id);
问题：查询departments表中，不存在于employees表中的部门的department_id和department_name
SELECT department_id, department_name FROM departments d WHERE NOT EXISTS (SELECT 'X' FROM employees WHERE department_id = d.department_id);

WITH 子句

1. 使用 WITH 子句, 可以避免在 SELECT 语句中重复书写相同的语句块
2. WITH 子句将该子句中的语句块执行一次并存储到用户的临时表空间中
3. 使用 WITH 子句可以提高查询效率

问题：查询公司中各部门的总工资大于公司中各部门的平均总工资的部门信息

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