01 SQL核心语句

在 sqlplus 中可以使用 ; 或 / 来执行一条 sql 语句, 但是 / 必须要换行, 个人感觉只有那些脚本啊什么的最后一般使用 / , 其他的都使用;

核心语句

SELECT

oracle 基于查询成本的优化器(cost-based optimizer, CBO) 用来产生实际的执行计划.  SQL 语句的执行顺序

5  select *

1  from tablename

2  where

3  group by

4  having

6  order by

INSERT

直接介绍多表插入

insert all
    when sum_orders < 10000 then
        into small_customers
    when sum_orders >= 10000 and sum_orders < 100000 then
        into medium_customers
    else
        into large_customers
select customer_id, sum(order_total) sum_orders
from oe.orders
group by customer_id;

-- 注意上例是仿照
insert into scott.bonus(ename, job, sal)
select ename, job, sal from scott.emp;

UPDATE

-- example 2 使用子查询作为修改的值

update employees2
set (salary, first_name) = ( select salary * 10, last_name
                                 from employees
                             where employees.employee_id = employees2.employee_id)
where department_id = 90;

DELETE

delete from (select * from employees2 where department_id = 90);

MERGE