多列子查詢
where (manager_id, department_id) in
子查詢
100 90
102 60
124 50
主查詢的每行都與多行和多列的子查詢進行比較
列的比較
多列的比較，包含子查詢可以是：
不成對比較
成對比較
成對比較子查詢
1、顯示與員工名爲“John”同部門且同一個經理的其它員工信息
select employee_id, manager_id, department_id from empl_demo
where (manager_id, department_id) IN
(select manager_id, department_id from empl_demo
where first_name = 'John')
AND first_name <> 'John';
不成對比較
1、顯示名字不爲 “John”員工的經理ID和部門ID的員工號、經理號、部門號
select employee_id, manager_id, department_id
from empl_demo
where manager_id in
(select manager_id
from empl_demo
where first_name = 'john')
and department_id in
(select department_id
from empl_demo
where first_name = 'john')
and first_name <> 'john';
標量子查詢表達式
標量子查詢是從一行中返回一列的子查詢
標量子查詢可在下列情況下使用:
– DECODE 和 CASE 條件和表達式的一部分
– SELECT 中除 GROUP BY 子句以外的所有子句中
– UPDATE 語句的 SET 子句和 WHERE 子句
CASE 表達式中的標量子查詢：
select employee_id, last_name, department_id,
(case
when department_id =
(select department_id
from departments
where location_id = 1800)
then 'canada' else 'usa' end) location
from employees;
ORDER BY 子句中的標量子查詢：
select employee_id, last_name,department_id
from employees e
order by (select department_name
from departments d
where e.department_id = d.department_id);
相關子查詢
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
子查詢中使用主查詢中的列
select column1, column2, ...
from table1 Outer_table
where column1 operator
(selecT column1, column2
from table2
where expr1 = Outer_table.expr2);
2、查找所有的員工信息，誰的薪金超過其部門的平均工資
select last_name, salary, department_id
from employees outer_table
where salary >
(selecT AVG(salary)
from employees inner_table
where inner_table.department_id =
outer_table.department_id);
3、顯示哪些員工工作變更過至少兩次
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);
使用 EXISTS 運算符
EXISTS操作符檢查在子查詢中是否存在滿足條件的行。
如果在子查詢中存在滿足條件的行:
– 不在子查詢中繼續查找
– 條件返回 TRUE
如果在子查詢中不存在滿足條件的行:
– 條件返回 FALSE
– 繼續在子查詢中查找
1、使用 EXISTS 操作符查找領導
select employee_id, last_name, job_id, department_id
from employees outer
where exists ( select 'x'
from employees
where manager_id =
outer.employee_id);
查找沒有任何員工的部門
select department_id, department_name
from departments d
where not exists (select 'x'
from employees
where department_id = d.department_id);
相關UPDATE
使用相關子查詢依據一個表中的數據更新另一個表的數據。
update table1 alias1 set column = (select expression from table2 alias2
where alias1.column = alias2.column);
違反範式的表 EMPL6 添加字段存儲部門名稱（添加字段以後違反範式）
使用相關子更新填充表
alter table empl6 add(department_name varchar2(25));
update empl6 e
set department_name =
(select department_name
from departments d
where e.department_id = d.department_id);
相關DELETE
使用相關子查詢依據一個表中的數據刪除另一個表的數據
delete from table1 alias1
where column operator
(select expression
from table2 alias2
where alias1.column = alias2.column);
1、使用相關子查詢刪除EMPL6存在同時也存在於EMP_HISTORY表中的數據。
delete from empl6 e
where employee_id =
(select employee_id
from emp_history
where employee_id = e.employee_id);
WITH 子句
使用 WITH 子句, 可以避免在 SELECT 語句中重複書寫相同的語句塊
WITH 子句將該子句中的語句塊執行一次 並存儲到用戶的臨時表空間中
使用 WITH 子句可以提高查詢效率
1、使用WITH子句編寫一個查詢，來顯示部門名稱和這些部門員工的工資總額大於跨部門的平均工資的部門及工資總額
with
dept_costs as (
select d.department_name, sum(e.salary) as dept_total
from employees e join departments d
on 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;
遞歸 WITH 子句
遞歸WITH子句：
Enables formulation of recursive queries.
Creates query with a name, called the Recursive WITH element name
Contains two types of query blocks member: anchor and a recursive
Is ANSI-compatible
with reachable_from (source, destin, totalflighttime) as
(
select source, destin, flight_time
from flights
union all
select incoming.source, outgoing.destin,
incoming.totalflighttime+outgoing.flight_time
from reachable_from incoming, flights outgoing
where incoming.destin = outgoing.source
)
select source, destin, totalflighttime
from reachable_from;