初学java 数据库相关学习

创建数据库：

主键: unsigned primary key auto_increment

外键: foreign key(xx) references table_name(xx)

字段:

类型: int ; tinyint ;char(20);varchar(255); date; datetime; text; float(5,2); double(10,2); long; decimal(15,10)

约束:primary key; foreign key; not null; unique; unsigned; default; check

备注: comment 'xxxxx'

java 复制代码

create table my_tab (
    id int unsigned primary key auto_increment
    username varchar(20) not null unique comment '用户名',
    age int CHECK (age > 0 AND age <= 200)
    gender tinyint unsigned not null comment '性别, 说明: 1 男, 2 女',
    image varchar(300)
    entrydate date
    column1 int,
    column2 int,
    CONSTRAINT fk_example1 FOREIGN KEY (column1) REFERENCES ref_table1(ref_column1),
    CONSTRAINT fk_example2 FOREIGN KEY (column2) REFERENCES ref_table2(ref_column2),
) comment '表';

表字段增删改：

java 复制代码

alter table tbname add qq varchar(11) comment 'QQ号码';
alter table tbname modify qq varchar(13) comment 'QQ号码';
alter table tbname change qq qq_num varchar(13) comment 'QQ号码';
alter table tbname drop qq;
rename table tbname to newtbname;

数据增删改查：

增: insert into table(...) value(...)

删: delete from table where(and ; or ; not ; is null)

改: update tabname set ... where ...

查: select [distinct] ... from where(条件) ... group by(分组)... having...(分组后条件)... order by(排序asc desc) ... limit(分页索引起始，查询总条数) ...

java 复制代码

insert into tbname(name, age) values ('张无忌', 10);
insert into tbname values('a',1),('b',2)...

delete from tabname
delete from tabname where not age=1
delete from tabname where age=1 and name!='a'
delete from tabname where (age=1 and name!='a') or name='c'

update tabname set area='wuhan',emp='fengxing'
update tabname set age=25,phone=123 where name='a'

select age from tb where area='wuhan'
select name,age,area from tb where name like '熊_龙%'
//聚合函数 count max min avg sum
select count(*) from tb where name like '%龙%'
select max(age) from tb
//分地区统计总数，年龄低于30，按照年龄分组，总数大于1个的，增序显示前3条数
SELECT age,area,count(*) FROM test WHERE age<30 GROUP BY age,area ORDER BY age ASC,area DESC LIMIT 100
//解析如下，按年龄地区统计总数，group by代表每行是一个满足条件（age<30）的聚合，优先按照age增序，area降序排列，显示100条:
//切记 select超过一个以上的列时如age,area，所有的列必须包含在group by 中,select查询的列应该是group by的子集，否则语意上有冲突，分组会出现分歧，比如使用group by age,按照age分组，但是你要求显示area列，那这个聚合可能有wuhan hangzhou shanghai，数据库不可能显示成
// 18 	wuhan,hangzhou,shanghai   32
//另外select必须有一个显示的列，不能是*，group by还需要筛选的可以在后面添加 having count(*)>5 / area!=shanghai 这种条件，对聚合后的列进行二次筛选，order by当然也必须要是group by内的列

函数和group by时配套使用的，
age		地区         count(*)
18		wuhan		  3
18		shanghai	  9
18		hangzhou      6
19		wuhan		  5
21		shanghai	  7
21		hangzhou	  2
26		hangzhou	  8
29		shanghai	  12