notes:
Constraints:
|-- Primary key constraint (primary key)
|-- Foreign key constraint (foreign key)
|-- Non-null constraint (not null)
|-- Unique constraint (unique)
|-- Check constraint (check)
|-- Default
Operations on table contents:
In relational databases, the primary key is generally handed over to the database for maintenance (primary key self-increment):
|-- MySQL auto_increment
Increase (insert):
insert into t_name[(field 1 …)] values(v1 …);
Update:
modify the original data in the table to the data we need
update t_name set field 1 = new value, field 2 = new value... where condition
update t_hero set age = 18, gender = "female" where id = 7;
删除（delete）：
delete from tablename where 条件
delete from t_hero where id =11;
truncate statement (use with caution)
This statement is also used to delete the table and cannot be recovered
select
select * from 表名;
select id, username, age, gender, tel from t_hero;
select username from t_hero;
Table modification: modifications
to the table structure : alter table
|-- Add new fields
ALTER TABLE table name ADD column name column type constraints;
alter table t_hero add address varchar(255);
|-- Modify field type
ALTER TABLE table name MODIFY column name column type;
alter table t_hero modify id bigint;
|-- Modify field name
ALTER TABLE table name CHANGE old column name new column name column type;
alter table t_hero change id hero_id int;
|-- Modify the table name
ALTER TABLE table name RENAME new table name;
alter table t_hero rename hero;
-The second way of writing
rename table hero to t_hero;
Table replication:
|-- create table 新表名 like 源表
create table xiyou like t_hero;
|-- create table 新表名 select * from 源表
create table sanguo select * from xiyou;
Practice:
Practice on t_hero;
update:
delete:
|-- add a new field
ALTER TABLE table name ADD column name column type constraint;
|-- modify field type
ALTER TABLE table name MODIFY column name column type;
|-- modify field name
ALTER TABLE table name CHANGE old column name new column name column type;
|-- Modify table name
ALTER TABLE table name RENAME new table name;
-The second way of writing
rename table hero to t_hero;
table copy:
The second method:
