①子查询
子查询是嵌套在 SQL 语句中的另一个SELECT 语句
子查询 (内查询) 在主查询执行之前执行
主查询(外查询)使用子查询的结果

②多列子查询
主查询与子查询返回的多个列进行比较

where (column1,column2) in (子查询)

例：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

多列子查询中的比较分为两种:
1)成对比较
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);

  
2)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);

③在 FROM 子句中使用子查询

例：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

1)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
          )

2)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;

④单列子查询表达式
单列子查询表达式是在一行中只返回一列的子查询
Oracle8i 只在下列情况下可以使用, 例如:
SELECT 语句 (FROM 和 WHERE 子句)
INSERT 语句中的VALUES列表中
Oracle9i中单列子查询表达式可在下列情况下使用:
DECODE  和 CASE
SELECT 中除 GROUP BY 子句以外的所有子句中

1)在 CASE 表达式中使用单列子查询
例：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;

2)在 ORDER BY 子句中使用单列子查询
例：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     employees e
ORDER BY (SELECT department_name
          FROM departments d
          WHERE e.department_id = d.department_id);

⑤相关子查询
 相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询

 SELECT column1, column2, ...
 FROM   table1 outer
 WHERE  column1 operator (SELECT  colum1, column2
                          FROM    table2
                          WHERE   expr1 = outer.expr2);

例：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
SELECT last_name, salary, department_id
FROM   employees outer
WHERE  salary >(SELECT AVG(salary)
   FROM   employees
   WHERE  department_id =  outer.department_id)

例：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);
⑥EXISTS 操作符
EXISTS 操作符检查在子查询中是否存在满足条件的行
1.如果在子查询中存在满足条件的行:
不在子查询中继续查找
条件返回 TRUE
2.如果在子查询中不存在满足条件的行:
条件返回 FALSE
继续在子查询中查找

例：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);

例：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);

⑦相关更新
UPDATE table1 alias1
SET    column = (SELECT expression
                 FROM   table2 alias2
                 WHERE  alias1.column =    
                        alias2.column);
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

⑧相关删除
DELETE FROM table1 alias1
 WHERE  column operator 
 (SELECT expression
   FROM   table2 alias2
   WHERE  alias1.column = alias2.column);
⑨WITH 子句
使用 WITH 子句, 可以避免在 SELECT 语句中重复书写相同的语句块
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
使用 WITH 子句可以提高查询效率

例：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
WITH dept_costs  AS (
   SELECT  d.department_name, SUM(e.salary) AS dept_total
   FROM    employees e, departments d
   WHERE   e.department_id = d.department_id
   GROUP BY d.department_name),
avg_cost    AS (
   SELECT SUM(dept_total)/COUNT(*) AS dept_avg
   FROM   dept_costs)
SELECT * 
FROM   dept_costs 
WHERE  dept_total >
        (SELECT dept_avg 
         FROM avg_cost)
ORDER BY department_name;