字符串数据类型:
char(n):定长
varchar(n):按照实际长度存储
数据存储:
numeric(p,d):
p:整数和小数部分的数据长度
d:小数部分的精度
当定义约束条件为primary key 之后,不需要额外定义 unique 和 not null
alter table tableName add
alter table tableName drop A
属性不区分大小写
distinct + 属性 去重
all + 属性 保留重复
不写distinct && all 默认为all
between and 为闭区间
from borrower, loan 为笛卡尔乘积的运算
重命名:
当在select语句中使用as相当于为rename,但是在from子句中使用as, 相当于将原来的表重新拷贝了一份新的表在临时表空间中,当查询语句执行完时,此表将会被释放
string 模式匹配
%:任意长度字串
_:任意一个字符
转义字符
‘Main#%’ escape ‘#’
字符串的拼接:
||
e.g.
select name||male from student
为name 和 male 拼接在一起
排序:
order by name asc||desc
asc 可以省略
排序时注意排序额外空间的利用, 不能让结果集很大
并集
union all
交集
intersect
差集
except
e.g.
(select name from a)
except || union || intersect all
(select name form b)
聚集函数：忽略null值
avg()
min()
max()
sum()
count()
group by : group by 子句后面的列必须出现在select子句中
Having子句
类似于where子句，但是having对于使用聚集函数的列进行判断，也可以对于自然属性进行判断，但是where子句用于group by子句前，having子句在group by子句后判断
Null 值
e.g.
select number from loan where number is null
5+null return null
null>null
5<null
null = null
return null
not unkown = unknown
聚集函数中忽略空值，处理count(*)
在逻辑运算中，null视为false
子查询：
select distinct customer_name
from borrower, loan
where borrower.loan_number = loan.loan_number
and branch_name = 'P' and
(branch_name, customer_name) in
(select branch_name, customer_name
from depositer, account
where depositor.account_number = account.account.number)
因为一个表中的指针只有一个，所以可以通过as复制表结构到临时表中，从而在一个表中进行比较
还可以：
some:
= some和 in 等价
select branch_name from branch where assets > all (select assets from branch where branch_name = 'Brooklyn')
当比较的时候最好加上some||all，因为只能一对一，除非用没有group by 的聚集函数
Exists:
判断子查询的返回值是否空值
select customer_name from branch where customer_name exists
(select customer_name from branch where branch_name = 'Brooklyn')
外层查询的时候对于里层查询相当于常量、
except相当于两个集合做了差集
unique:
判断子查询返回的值是否有重复的值
select T.customer_name from depositor as T
where unique(
select R.customer_name from account, depositor as R
where T.customer_name = R.customer_name
and R.account_number = account.account_number
and account.branch_name = 'Perryridge'
)
with 子句：
相当于创建了一个新的临时表 简化查询
with max_balance(value) as
select max(balance) from account
select account_number from account, max_balance
where account.balance = max_balance.value
with branch_total(branch_name, value) as
select branch_name, sum(balance) from account
group by branch_name
with branch_total_avg(value) as
select avg(value)
from branch_total
select branch_name
from branch_total, branch_total_avg
where branch_total.value >= branch_total_avg.value
聚集函数不能嵌套
update
update account set balance = balance * 1.06 where balance >= 10000
update account set balance = balance * 1.05 where balance < 10000
或者
update account
set balance = case
when balance > 10000 then
balance*1.06
else
balance * 1.05
end
delete:
delete from depositor where account_number in
(select account_number from account where balance = 0)
select account_number from account where balance = 0
view
将一些常用的临时表存储在虚表中，实际存储的时查询语句
e.g.
create view all_customer as (
select branch_name, customer_name
from depositor, account
where depositor.account_number = account.account_number)
union
(select branch_name, customer_name
from borrower, account
where borrower.account_number = account.account_number)
# using view
select customer_name from all_customer
view insert
insert into all_acutomer values(
'John', '123')
可更新的视图：
哪些视图可以更新：
单表视图
视图中包含主键，不能使主键非空
不能包含聚集函数
insert
insert into account(branch_name)
values('P')
insert into depositor
select customer_name, loan_number
from loan, borrower
where branch_name = 'p'
and loan.account_number = borrower.account_number
内连接外连接
natural 相当于去掉了on 条件
