※? Primary key constraint: primary key

※? Unique key: uniqueness constraint

※? Foreign key constraints: foreign keys

※? Non-empty constraint: non-empty

※? Default constraint: default value

Second, the primary key constraint:

A table can only have one primary key. If you forget to set the primary key constraint when building a table, the query speed of columns set as primary keys will be very fast, so clustered indexes are generally used, which we will talk about later.

Add primary key constraint: set your age table as the primary key.

Grammar: alter? Add primary key to table name; (column name)? ※? You can have multiple column names.

Third, foreign key constraints:

What is a foreign key? When you need to use the primary key of another table as the primary key of this table when building a table, you need to set a foreign key. If you want to delete the data in the table after setting the outer room, it will be deleted in cascade or by default.

Add foreign key constraint: set the cno of this table as a foreign key.

Syntax: alter table table name add foreign key references? Name (column name) of the associated table;

Fourth, non-empty constraints:

When inserting new data, the corresponding column cannot be empty. Non-empty constraints are relative to the default constraints.

Add a non-empty constraint:

Syntax: alter table table name modify column name column type not null; ?

Verb (abbreviation for verb) default value constraint: default value:

If no value is inserted when inserting, the default value will be automatically inserted. Default value constraints are relative to non-empty constraints.

Add default constraint:

Syntax: alter table table name add column name column type not null default value' default value';

6. Uniqueness constraint:

The content in this column can only be unique and cannot be repeated.

Add a unique constraint:

? Syntax: alter table column name add unique? You can have multiple column names separated by commas. ※.