To define candidate keys in the relational schema,

candidate key, which is a combination of one or more attributes that uniquely identifies a row of data in the database, so it must have a UNIQUE constraint

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Candidate keys are properties in the relational schema that can be used as the identifier of the relationship. Its value is unique in each relational record and distinguishes the relational record from other relational records. Therefore, if you want to define candidate keys in the relational schema, it is most appropriate to use the UNIQUE constraint, because the UNIQUE constraint ensures that the value of one or more attributes is unique throughout the table, thus making the records in the table unique and satisfying the characteristics needed to define candidate keys.

For example, in a mysql database, if you want to define candidate keys in a table, you can use the following statement:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name);

SQL statement adds a UNIQUE constraint named constraint_name to the table table_name, which requires that the column_name field of the table have a unique value. This ensures that the field is a candidate key for the table.
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Candidate keys are the set of attributes that uniquely identify a tuple in relational mode, that is, the values of these attributes uniquely identify a tuple. The candidate bond has the characteristics of uniqueness, minimization and irreducibility. In relational databases, we typically use candidate keys as primary keys to ensure the uniqueness of each tuple in the relational schema.

In relational databases, to ensure the uniqueness of candidate keys, we can use the UNIQUE constraint, which can be achieved by defining a UNIQUE constraint on the property of the relational schema. The UNIQUE constraint requires that the value of the attribute be unique throughout the relational schema and is therefore suitable for defining candidate keys.

Whereas the CHECK constraint is used to limit the value range of attributes, the foreign key constraint is used to maintain referential integrity of the relational schema, and the DEFAULT constraint is used to set the default value when a new row is inserted, these constraints are not suitable for defining candidate keys.