MYSQL——修改表，表字段类型及约束条件

一、修改表

create table t1(id int,name char);

alter table t1 rename tt1;


# 修改字段
alter table t1 modify id tinyint;

alter table t1 change id ID tinyint;

alter table t1 change id ID tinyint,change name NAME char(4);


# 增加字段
alter table t1 add gender char(4);
alter table t1 add gender char(4) first;
alter table t1 add level int after ID;


# 删除字段
alter table t1 drop gender;


# 复制表
create table t2 select user,host,password from mysql.user;

# 只复制表结构
create table t3 select user,host,password from mysql.user where 1!=1;

二、表字段的类型

# 1、=====================表字段类型之整型=======================
强调：整型的宽度是显示宽度，无需设置，存储宽度是固定死的
mysql> create table t5(id tinyint)

mysql> desc t4;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id    | tinyint(4) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql> insert t4 values(128);
ERROR 1264 (22003): Out of range value for column 'id' at row 1
mysql>
mysql>
mysql> insert t4 values(127);
Query OK, 1 row affected (0.05 sec)

mysql> select * from t4;
+------+
| id   |
+------+
|  127 |
+------+
1 row in set (0.00 sec)

mysql>

# 2、=====================表字段类型之浮点类型=======================
create table t7(x float(255,30),y double(255,30),z decimal(65,30));

insert t7 values
(1.111111111111111111111111111111,1.111111111111111111111111111111,1.111111111111111111111111111111);



# 3、=====================表字段类型之日期类型======================
year（1901/2155）

time 时:分:秒 （'-838:59:59'/'838:59:59'）

date 年:月:日 （1000-01-01/9999-12-31）

datetime 年:月:日 时:分:秒 1000-01-01 00:00:00/9999-12-31 23:59:59

timestamp 年:月:日 时:分:秒 1970-01-01 00:00:00/2037

create table t8(y year,t time,d date,dt datetime,ts timestamp);
insert t8 values(now(),now(),now(),now(),now());


create table student(
    id int,
    name char(10),
    born_year year,
    bitrh date,
    reg_time datetime
);


insert student values
(1,"wangjing","1911","1911-11-11","1911-11-11 11:11:11"),
(2,"lxx","1988","1988-11-11","1988-11-11 11:11:11");


insert student values
(3,"wangjing","1911","19111111","19111111111111");



# 注意：timestamp应该勇于记录更新时间
create table t9(
    id int,
    name varchar(16),
    -- update_time datetime not null default now() on update now(),
    update_time timestamp,
    reg_time datetime not null default now()
);


insert into t9(id,name) values(1,"egon");


# 测试效果
mysql> select * from t9;
+------+------+---------------------+---------------------+
| id   | name | update_time         | reg_time            |
+------+------+---------------------+---------------------+
|    1 | egon | 2020-09-01 16:45:51 | 2020-09-01 16:45:51 |
+------+------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> update t9 set name="EGON" where id=1;
Query OK, 1 row affected (0.06 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t9;
+------+------+---------------------+---------------------+
| id   | name | update_time         | reg_time            |
+------+------+---------------------+---------------------+
|    1 | EGON | 2020-09-01 16:46:50 | 2020-09-01 16:45:51 |
+------+------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql>



# 4、=====================表字段类型之字符类型======================

char 定长，不够则补全空格
    看起来特点：
        浪费空间
        读取速度快

varchar 变长，预留1-2bytes来存储真实数据的长度
    看起来特点：
        节省空间
        读取速度慢

ps：在存储的数据量刚好达到存储宽度限制时，其实varchar更费空间

总结：大多数情况下存储的数据量都达不到宽度限制，所以大多数情况下varchar更省空间
但省空间不是关键，关键是省空间 会带来io效率的提升，进而提升了查询效率

ab   |abc  |abcd |
1bytes+ab|1bytes+abc|1bytes+abcd|



===============验证
create table t11(x char(5));
create table t12(x varchar(5));

insert t11 values("我擦嘞 ");  -- "我擦嘞  "
insert t12 values("我擦嘞 ");  -- "我擦嘞 "

t11=>字符个数 5  字节个数 11
t12=>字符个数 4  字节个数 10

set sql_mode="pad_char_to_full_length";
select char_length(x) from t11;
select char_length(x) from t12;


select length(x) from t11;
select length(x) from t12;

# 5、=====================表字段类型之枚举类型与集合======================
枚举类型enum("a","b","c","d") 多选1
集合类型set("a","b","c","d") 多选


CREATE TABLE shirts (
    name VARCHAR(40),
    size ENUM('x-small', 'small', 'medium', 'large', 'x-large')
);

INSERT INTO shirts(name, size) VALUES ('dress shirt','large'), ('t-shirt','medium'),('polo shirt','small');

CREATE TABLE user (
    name VARCHAR(16),
    hobbies set("read","chou","drink","tang")
);
insert user values("lxx","tang,chou");
insert user values("hxx","tangchou");

三、约束条件

四、

五、

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