Table of contents
2. Delete the not-null constraint
1. Common constraints
not-null constraint
unique constraint
primary key constraint
foreign key constraints
2. Non-empty constraints
The not null constraint defines whether a specific column of a data row in a table can be null
1. Add a non-null constraint
When creating the table add
create table table name (field name field type...) not null;
Add when modifying the data type of the field in the table
alter table table name modify field name new data type not null;
2. Delete the not-null constraint
Delete when modifying the data type of the field in the table
alter table table name change column field name new data type;
alter table table name modify field name new data type;
3. Unique constraints
The unique constraint ensures that the combined values of one or more columns in the table are unique and prevents duplicate values from being entered. It is mainly used to ensure the integrity of non-primary key columns.
1. Add a unique constraint
1.1. Direct add
1.1.1 Table-level constraints
alter table table name add unique(field name);
1.1.2 Joint uniqueness constraints
alter table table name add unique(field name 1, field name 2);
The combination of field name 1 and field name 2 is unique
For example, in the code: the same id cannot exist in the same class;
2. delete
alter table table name drop index field name;
4. Primary key constraints
There can only be one primary key in a table. A primary key can consist of one or more columns.
A primary key constraint is equivalent to a combination of uniqueness and not-null constraints
Add both a unique constraint and a non-null constraint to a field
1. Add constraints
Add primary key constraints on creation
create table table name (field name field type primary key,...)
Add when alter table
alter table table name add primary key (field name);
2. Delete the primary key
alter table table name drop primary key;
5. Default value constraints
add on create
create table table name (field name field type default default value);
when modified
alter table table name modify field name field type default default value;
delete
6. Self-growth constraints
The auto-increment column must be a primary key column
If the auto-increment column specifies 0 and null, it will be auto-incremented based on the current maximum value
If the auto-increment column manually specifies a specific value, directly assign the value to the specific value
when creating the table
create table table name (field name field type primary key auto_increment);
When modifying the data type of a field in a table
alter table table name modify field name field type auto_increment;
7. Check constraints
Check if a field meets the requirements
alter table table name add check (field name between minimum and maximum);
