1 --子查詢
  2 --子查詢 (內查詢) 在主查詢執行之前執行
  3 --主查詢(外查詢)使用子查詢的結果
  4 --問題：查詢工資大於149號員工工資的員工的資訊
  5 SELECT last_name
  6 FROM   employees
  7 WHERE  salary > 
  8                 (SELECT salary
  9                  FROM   employees
 10                  WHERE  employee_id = 149) ;
 11  
 12 --多利子查詢                
 

 13 --主查詢與子查詢返回的多個列進行比較
 14 --主查詢與子查詢返回的多個列進行比較
 15 --多列子查詢中的比較分為兩種:
 16 --成對比較
 17 --不成對比較
 18 
 19 --成對比較舉例
 20 --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  
 21 SELECT    employee_id, manager_id, department_id
 22 FROM    employees
 23 WHERE  (manager_id, department_id) IN
 

 24                       (SELECT manager_id, department_id
 25                        FROM   employees
 26                        WHERE  employee_id IN (141,174))
 27 AND    employee_id NOT IN (141,174);
 28 
 29 --不成對比較舉例
 30 SELECT  employee_id, manager_id, department_id
 31 FROM    employees
 32
 
 WHERE   manager_id IN                   (SELECT  manager_id
 33                    FROM    employees
 34                    WHERE   employee_id IN (174,141))
 35 AND     department_id IN                   (SELECT  department_id
 36                    FROM    employees
 37                    WHERE   employee_id IN (174,141))
 38 AND    employee_id NOT IN(174,141);
 39 
 40 --在 FROM 子句中使用子查詢
 41 --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
 42 --方法一
 43 select last_name,department_id,salary,
 44       (select avg(salary)from employees e3 
 45        where e1.department_id = e3.department_id 
 46        group by department_id) avg_salary
 47 from employees e1
 48 where salary > 
 49          (select avg(salary)
 50           from employees e2  
 51           where e1.department_id = e2.department_id
 52           group by department_id
 53           )
 54 
 55 --方法二
 56 SELECT  a.last_name, a.salary, 
 57         a.department_id, b.salavg
 58 FROM    employees a, (SELECT   department_id, 
 59                       AVG(salary) salavg
 60                       FROM     employees
 61                       GROUP BY department_id) b
 62 WHERE   a.department_id = b.department_id
 63 AND     a.salary > b.salavg;
 64 
 65 --單列子查詢表示式
 66 --單列子查詢表示式是在一行中只返回一列的子查詢
 67 --Oracle8i 只在下列情況下可以使用, 例如:
 68 --  SELECT 語句 (FROM 和 WHERE 子句)
 69 --  INSERT 語句中的VALUES列表中
 70 --Oracle9i中單列子查詢表示式可在下列情況下使用:
 71 --  DECODE  和 CASE
 72 --  SELECT 中除 GROUP BY 子句以外的所有子句中
 73 
 74 --在 CASE 表示式中使用單列子查詢
 75 --問題：顯式員工的employee_id,last_name和location。其中，若員工department_id與location_id為1800的department_id相同，則location為’Canada’,其餘則為’USA’。
 76 SELECT employee_id, last_name,
 77        (CASE
 78         WHEN department_id =(SELECT department_id FROM departments WHERE location_id = 1800)              
 79         THEN 'Canada' ELSE 'USA' END) location
 80 FROM   employees;
 81 
 82 --在 ORDER BY 子句中使用單列子查詢
 83 --問題：查詢員工的employee_id,last_name,要求按照員工的department_name排序
 84 SELECT   employee_id, last_name
 85 FROM     employees e
 86 ORDER BY (SELECT department_name
 87           FROM departments d
 88           WHERE e.department_id = d.department_id);
 89 
 90 --相關子查詢
 91 --相關子查詢按照一行接一行的順序執行，主查詢的每一行都執行一次子查詢
 92 --問題：查詢員工中工資大於本部門平均工資的員工的last_name,salary和其department_id
 93 SELECT last_name, salary, department_id
 94 FROM   employees outer
 95 WHERE  salary >(SELECT AVG(salary)
 96                 FROM   employees
 97                 WHERE  department_id =  outer.department_id) ;
 98 
 99 --問題：若employees表中employee_id與job_history表中employee_id相同的數目不小於2，輸出這些相同id的員工的employee_id,last_name和其job_id
100 SELECT e.employee_id, last_name,e.job_id
101 FROM   employees e 
102 WHERE  2 <= (SELECT COUNT(*)
103              FROM   job_history 
104              WHERE  employee_id = e.employee_id);
105 
106 --EXISTS 操作符
107 --EXISTS 操作符檢查在子查詢中是否存在滿足條件的行
108 --如果在子查詢中存在滿足條件的行:
109 --    不在子查詢中繼續查詢
110 --    條件返回 TRUE
111 --如果在子查詢中不存在滿足條件的行:
112 --    條件返回 FALSE
113 --    繼續在子查詢中查詢
114 --問題：查詢公司管理者的employee_id,last_name,job_id,department_id資訊
115 SELECT employee_id, last_name, job_id, department_id
116 FROM   employees outer
117 WHERE  EXISTS ( SELECT 'X'
118                  FROM   employees
119                  WHERE  manager_id = 
120                         outer.employee_id);
121                         
122 --NOT EXISTS 操作符應用舉例
123 --問題：查詢departments表中，不存在於employees表中的部門的department_id和department_name
124 SELECT department_id, department_name
125 FROM departments d
126 WHERE NOT EXISTS (SELECT 'X'
127                   FROM   employees
128                   WHERE  department_id 
129                          = d.department_id);
130 
131 --相關更新
132 --使用相關子查詢依據一個表中的資料更新另一個表的資料
133 --eg:1
134 ALTER TABLE employees
135 ADD(department_name VARCHAR2(14));
136 --eg:2
137 UPDATE employees e
138 SET    department_name = 
139               (SELECT department_name 
140                FROM   departments d
141                WHERE  e.department_id = d.department_id);
142 
143 --相關刪除
144 --問題：刪除表employees中，其與emp_history表皆有的資料
145 DELETE FROM employees E
146 WHERE employee_id =  
147            (SELECT employee_id
148             FROM   emp_history 
149             WHERE  employee_id = E.employee_id);
150 
151 --WITH 子句
152 --使用 WITH 子句, 可以避免在 SELECT 語句中重複書寫相同的語句塊
153 --WITH 子句將該子句中的語句塊執行一次並存儲到使用者的臨時表空間中
154 --使用 WITH 子句可以提高查詢效率
155 --問題：查詢公司中各部門的總工資大於公司中各部門的平均總工資的部門資訊
156 WITH 
157 dept_costs  AS (
158                 SELECT  d.department_name, SUM(e.salary) AS dept_total
159                 FROM    employees e, departments d
160                 WHERE   e.department_id = d.department_id
161                 GROUP BY d.department_name),
162 avg_cost    AS (
163                  SELECT SUM(dept_total)/COUNT(*) AS dept_avg
164                  FROM   dept_costs)
165 SELECT * 
166 FROM   dept_costs 
167 WHERE  dept_total >
168         (SELECT dept_avg 
169          FROM avg_cost)
170 ORDER BY department_name;

eg：

 1 --查詢員工的last_name, department_id, salary.其中員工的salary,department_id與有獎金的任何一個員工的salary,department_id相同即可
 2 select last_name, department_id, salary
 3 from employees 
 4 where (salary,department_id) in (
 5                                  select salary,department_id
 6                                  from employees
 7                                  where commission_pct is not null
 8                                     );
 9 
10 --選擇工資大於所有JOB_ID = 'SA_MAN'的員工的工資的員工的last_name, job_id, salary
11 select last_name, job_id, salary
12 from employees
13 where salary > all(
14                   select salary
15                   from employees
16                   where job_id = 'SA_MAN'
17                   );
18                   
19 --選擇所有沒有管理者的員工的last_name
20 select last_name
21 from employees e1
22 where not exists (
23               select 'A'
24               from employees e2
25               where e1.manager_id = e2.employee_id
26               );