mysql column type

Column type
Integer type tinyint, smallint, mediumint, int, bigint (optional parameter unsigned, (M, zerofill, only meaningful when used in combination))
Floating point type float (optional parameter D, M) decimal (optional parameters D, M)
String type char (M) varchar (M) text-text type
Date time type date, time, datatime, year

Give table Append a column, the column type is unsigned tinyint

alter table [vg] add [id] tinyint unsigned not null default 0;

The id of 255 can be stored in the table

insert into vg
(sname,id)
values
('test',255);

#Analyze M parameter

alter table vg add age1 tinyint(1) not null default 0;
insert into vg class (sname,age1) values ('M1',3);
insert into vg (sname,age1) values ('agin M',99);

#M is meaningless at this time

select * from vg;
+--------+-----+-------+------+------+
| sname | age | socre | test | age1 |
+--------+-----+-------+------+------+
| 刘备 | 28 | 0 | 0 | 0 |
| 张飞 | 0 | -1 | 0 | 0 |
| test | 0 | 0 | 255 | 0 |
| M1 | 0 | 0 | 0 | 3 |
| agin M | 0 | 0 | 0 | 99 |
+--------+-----+-------+------+------+

#M must cooperate with zerofill to make sense

Add column

alter table vg add snum smallint(5) zerofill not null default 0;
insert into vg (sname,snum) values ('吕布',1);
insert into vg (sname,snum) values ('廖化',15);

select * from vg;
+--------+-----+-------+------+------+-------+
| sname | age | socre | test | age1 | snum |
+--------+-----+-------+------+------+-------+
| 刘备 | 28 | 0 | 0 | 0 | 00000 |
| 张飞 | 0 | -1 | 0 | 0 | 00000 |
| test | 0 | 0 | 255 | 0 | 00000 |
| M1 | 0 | 0 | 0 | 3 | 00000 |
| agin M | 0 | 0 | 0 | 99 | 00000 |
| 吕布 | 0 | 0 | 0 | 0 | 00001 |
| 廖化 | 0 | 0 | 0 | 0 | 00015 |
+--------+-----+-------+------+------+-------+

#Compare A display that shows the meaning of the combination of M and zerofill
#Summary: M represents the width of 0 complement, which is meaningful only when used in combination with zerofill

char(M) defines the storage length of each column. It is fixed, M fixed codes (utf8, gbk) length (the query speed is relatively fast, causing a certain waste of resources)
Analogous to: No matter the distance, a one-yuan bus will cause waste for short distances

The length of varchar(M) can be (0-- Analogous to: segmented charging, you need to add a conductor to record the far and near charges

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