MySQL学习笔记五：数据类型

一、整数类型

1.1、tinyint [(M)][unsigned][zerofill]

1.2、smallint [(M)][unsigned][zerofill]

1.3、mediumint [(M)][unsigned][zerofill]

1.4、int [(M)][unsigned][zerofill]

1.5、bigint [(M)][unsigned][zerofill]

参数说明：

M：取值范围。

unsigned：无符号，控制是否有正负数。

zerofill：用来进行前导零填充，如tinyint的值为1，而其最长取值位数是3位，则填充后的结果会变成001。类型后面写了zerofill，默认就是unsigned无符号。

示例1：无参数控制

create table study01 (id01 tinyint,id02 int);
desc study01;
insert into study01 (id01,id02) values (100,101);
insert into study01 (id01,id02) values (-1,-2);
select * from study01;

示例2：有参数控制1

create table study02 (id01 tinyint(3) unsigned zerofill,id02 int(3) unsigned zerofill);
desc study02;
insert into study02 (id01,id02) values (1,1);
insert into study02 (id01,id02) values (12,1234);
select * from study02;

示例3：有参数控制2

create table study03 (id01 tinyint(3) unsigned,id02 int(3) unsigned);
desc study03;
insert into study03 (id01,id02) values (1,1);
insert into study03 (id01,id02) values (12,1234);
select * from study03;

二、小数类型

2.1、decimal (M,D)

2.2、float (M,D)

2.3、double (M,D)

参数说明：

zerofill

unsigned

示例1：无参数控制

create table studyf1 (f1 float,f2 double);
desc studyf1;
insert into studyf1 (f1,f2) values (-12.123,-1234.5678);
select * from studyf1;

示例2：有参数控制1

create table studyf2 (f1 float(5,2),f2 double(6,3) zerofill);
desc studyf2;
insert into studyf2 (f1,f2) values (12.34,12.34);
insert into studyf2 (f1,f2) values (1.1,1.2);
insert into studyf2 (f1,f2) values (123.45,123.456);
/*科学计数法(E)，小数点移动几位。*/
insert into studyf2 (f1,f2) values (0.1234E2,0.123456E3);
/*插入多了，就会四舍五入。*/
insert into studyf2 (f1,f2) values (12.126,12.34);
select * from studyf2;

示例3：有参数控制2

create table studyf3 (f1 float(10,4) unsigned zerofill);
desc studyf3;
insert into studyf3 (f1) values (12.345);
insert into studyf3 (f1) values (12.3456);
insert into studyf3 (f1) values (12.34567);
select * from studyf3;

三、日期类型

3.1、datetime(年月日时分秒)

create table studyd1 (mydate datetime);
insert into studyd1 (mydate) values ('20200902230130');
insert into studyd1 (mydate) values (20200902230130);
insert into studyd1 (mydate) values ('2020-09-02 23:01:30');
insert into studyd1 (mydate) values (null);
select * from studyd1;

3.2、timestamp(年月日时分秒/整数)

create table studyd2 (mytime timestamp);
insert into studyd2 (mytime) values ('20200902230130');
insert into studyd2 (mytime) values ('2020-09-02 23:01:30');
select * from studyd2;
/*+0 查看时间戳，显示整数。*/
select mytime+0 from studyd2;

3.3、date(年月日)

create table studyd3 (mydate date);
insert into studyd3 (mydate) values ('20200902');
insert into studyd3 (mydate) values ('2020-09-02');
select * from studyd3;

3.4、time(时分秒)

create table studyd4 (mytime time);
insert into studyd4 (mytime) values ('10:10:10');
/*D HH:MM:SS，D代表天，最大可以是34天，代表过去多少时间。*/
insert into studyd4 (mytime) values ('5 10:10:10');
select * from studyd4;

3.5、year

create table studyd5 (myyear year);
insert into studyd5 (myyear) values ('2020');
insert into studyd5 (myyear) values ('2021');
select * from studyd5;

四、字符类型

4.1、char(M)，M表示字符固定长度，最大为255字节。

create table studyz1 (mychar char(255));
insert into studyz1 (mychar) values ('YES');
insert into studyz1 (mychar) values ('NO');
insert into studyz1 (mychar) values ('Y ');
insert into studyz1 (mychar) values (' N');
select * from studyz1;
select mychar,length(mychar) `length` from studyz1;

4.2、varchar(M)，M表示字符可变长度，最大65535字节，需要1-2字节来保存信息，超过255的长度就用2个字节来保存。

utf8：一个字符占用3个字节 65535/3=21845 -1 -2=21844/21843

gbk：一个字符占用2个字节 65535/2=32767 -1 -2=32766/32765

最大长度是受最大65535字节和所使用的字符集有关。

create table studyz2 (myvarchar varchar(21844));
insert into studyz2 (myvarchar) values ('YES');
insert into studyz2 (myvarchar) values ('NO');
insert into studyz2 (myvarchar) values ('Y ');
insert into studyz2 (myvarchar) values (' N');
select * from studyz2;
select myvarchar,length(myvarchar) `length` from studyz2;

4.3、text

text和blob区别：blob用来保存二进制数据，text保存字符数据。

text和char/varchar区别：text不需要指定长度。

存储长度：1字节-4GB

/*tinytext：最大256 bytes，1字节开销，少于255个字符的，就比较好，比如：文章摘要。*/
create table text1 (id int,name tinytext);
/*text：最大64k，相当于65535个字符，2字节开销，比如：文章正文。*/
create table text2 (id int,name text);
/*mediumtext：最大16MB，相当于16777215个字符，3字节开销，存储相对大的文本数据，比如书籍文本，白皮书。*/
create table text3 (id int,name mediumtext);
/*longtext：最大4GB，4字节开销。*/
create table text4 (id int,name longtext);

4.4、blob

/*tinyblob：最大256 bytes*/
create table blob1 (id int,name tinyblob);
/*blob：最大64k*/
create table blob2 (id int,name blob);
/*mediumblob：最大16MB*/
create table blob3 (id int,name mediumblob);
/*longblob：最大4GB*/
create table blob4 (id int,name longblob);

4.5、enum：最多65535个枚举项，2字节开销，相于单项选择题。

create table studye1 (myenum enum('Y','N'));
insert into studye1 (myenum) values ('Y');
insert into studye1 (myenum) values ('N');
insert into studye1 (myenum) values ('1');
insert into studye1 (myenum) values ('2');
select * from studye1;
select myenum+0 from studye1;

4.6、set：集合，相当于多项选择题。

create table studys1 (myset set('A','B','C','D'));
insert into studys1 values ('A');
insert into studys1 values ('A,B');
insert into studys1 values ('C');
insert into studys1 values ('C,D');
select * from studys1;