How can constraints be added to fields in MySQL?

MySQL allows you to add constraints to fields by using the `CONSTRAINT` keyword when creating a table. One common type of constraint is `NOT NULL`, which ensures that the value in the field is not empty.

CREATE TABLE table_name (
    column_name data_type NOT NULL
);

Unique constraint: ensures that the value in the field is unique within the table.

CREATE TABLE table_name (
    column_name data_type UNIQUE
);

3. A `PRIMARY KEY` constraint ensures that the values in a field are unique within a table and are used as the primary key for the table.

CREATE TABLE table_name (
    column_name data_type PRIMARY KEY
);

4. `FOREIGN KEY` constraint: ensures that the values of a field match the primary key or unique key of another table.

CREATE TABLE table_name1 (
    column_name1 data_type,
    column_name2 data_type,
    FOREIGN KEY (column_name1) REFERENCES table_name2(column_name2)
);

CHECK constraint: ensures that values in a field meet specified conditions.

CREATE TABLE table_name (
    column_name data_type,
    CHECK (condition)
);

6. `DEFAULT` constraint: Setting a default value for a field.

CREATE TABLE table_name (
    column_name data_type DEFAULT default_value
);

These constraints can also be added after table creation using the `ALTER TABLE` statement.

ALTER TABLE table_name
    ADD CONSTRAINT constraint_name constraint_type (column_name);