数据库之mysq修改表、表字段类型等相关内容-43
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;
2.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; --3
select char_length(x) from t12; --4
select length(x) from t11; --9
select length(x) from t12; --10
# 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");