sql初学者笔记 语法基础

• -- 很少支持
• 段落注释

SELECT

检索数据

排序检索数据

过滤数据

通配符搜索

创建计算字段

select prod_id ,quantity,item_price,quantity*item_price as expanded_price
from orderitems
where order_num = 20008;
#如上创建了一个expanded_price字段（quantity*item_price的结果的别名），其仅在此时有效而不会存放到表中。

使用函数

select vend_name, upper(vend_name) as vend_name_upcase
from vendors
#将vend_name列下的所有行以大写形式返回
select avg(prod_price) as avg_peice from products where vend_id ="1001"
#返回平均值
select count(*) as num_cust from customers
#返回长度（数目），也可对列表中特定值进行计数

分组

select  vend_id,count(*) as num_prods from  products #对vend_id每行进行计数
group by vend_id;#按照vend_id排序并分组

select cust_id,count(*) as orders
from orders 
group by cust_id 
having count(*)>=2#过滤分组中>=2的，having支持where的所有操作

select order_num,count(*) as items
from orderitems group by order_num
having count(*) >=3
order by items,order_num desc#对分组依照选定的列进行排序

子句查询

select cust_name,cust_contact from customers where cust_id =(select cust_id 
from orders
where order_num = (select order_num from orderitems where prod_id = "jp2000"));
#由内而外，哈哈
等效于：
select order_num 
from orderitems
where prod_id = "jp2000"；
select cust_id 
from orders
where order_num =20006
select cust_name,cust_contact from customers where cust_id =10003

联结（返回不在同一个表中的行）

select vend_name, prod_name,prod_price
from vendors,products
where vendors.vend_id=products.vend_id;#此处过滤联结条件。
#如没有联结条件过滤，将检索出“笛卡尔积”：表1行数*表2行数

select vend_name, prod_name,prod_price
from vendors inner join products
on vendors.vend_id=products.vend_id

自联结（比子查询更快）

select cust_id, cust_name, cust_contact
from customers
where cust_name = (select cust_name from customers where cust_contact ="jim jones");

select c1.cust_id, c1.cust_name,c1.cust_contact
from customers as c1,customers as c2#不以别名进行会引发错误
where c1.cust_name=c2.cust_name and c2.cust_contact="jim jones"#联结cust_name与c2.cust_name ，并过滤cust_contact="jim jones"的行

组合查询

select cust_name,cust_contact,cust_email,cust_state
from customers
where cust_state in("il","in","mi")
union  #组合上下select多个select之间需要多个union分隔，union默认排除重复，union all则不排除
select cust_name,cust_contact,cust_email,cust_state#union中每个查询必须包含相同的列、表达式、或聚集函数
from customers
where cust_name ="wascals"
order by cust_name;#不能分别对每条union指定不同的排序

INSERT

依赖于次序的插入

insert into customers
values("1000000006","toy land","123 any street","new york","ny", "11111","usa",null, null);
#依赖于次序的插入，必须为每一列提供一个值，如某列无添加则应写上null

提供列名的插入

insert into customers(cust_id,cust_contact,cust_email,cust_name,cust_address,cust_city,cust_state,cust_zip)
#必须为提供了列名的列给出一个值
values(null, null,"1000000006","toy land","123 any street","new york","ny", "11111");

从另一个表插入

insert into 表名（列名）
select 列名
from 表名
where 过滤

复制一个表

select * 
into custcopy 
from customers;

create table custcopy as
select * 
from customers;

UPDATE

更新单个列

update customers
set cust_email = "kim@@thetoystore.com"
where cust_id = "100000000005"#如不指定，将更新customers表cust_email列下的所有行

更新多个列

update customers
set cust_email = "kim@@thetoystore.com",cust_contact="sam roberts"
where cust_id = "100000000006"

DELLETE

delete from customers
where cust_id = "1000000006"#删除此行，不过滤则删除所有行
#update删除列
#truncate删除表

添加删除列&&表

添加表

create table orderitems
(order_num integer not null,
order_item integer not null,
prod_id char(10) not null,
quantity integer not null default 1,#设置quantity列下的行默认值为1
item_price decimal(8,2) not null);#not null即不允许填入null，默认可填入null，只有为 not null的列方可为主键及唯一标识

alter table vendors
add vend_phone char (20);

alter table vendors
drop column vend_phone;#此操作不可逆

drop table custcopy;#此操作不可逆

视图

create view#创建视图
drop view 视图名#删除视图

create view productcustomers as
select cust_name , cust_contact,prod_id
from customers,orders,orderitems
where customers.cust_id=orders.cust_id
and orderitems.order_num=orders.order_num;

select *
from productcustomers;

create view customeremaillist as
select cust_id, cust_name,cust_email
from customers
where cust_email is not null;#返回查询中所有cust_email不为空的，并将其添加到视图中

create view orderitemsexpanded as 
select order_num,prod_id,quantity,item_price,quantity*item_price,quantity*item_price as
expanded_price
from orderitems

事务管理

start transaction ;
-- 标识事务处理块,块中内容未执行完则整体撤销

savepoint delete1;#标识
rollback to delete1;返回标识delete1