In the context of relational databases:

A relation is a table with columns and rows that represents a set of entities and their attributes.

A relation schema is the structure of a relation, which includes the names of the columns and their data types.

A relation instance is a set of tuples (rows) that populate the relation.

The differences between relation schema and relation instance are:

A relation schema defines the structure of a relation, while a relation instance contains actual data.

A relation schema is fixed and does not change over time, while a relation instance can change as data is inserted, updated, or deleted.

Unary relation:

- A unary relation is a relation with only one attribute (column).

Degree of relation:

- The degree of a relation is the number of attributes (columns) in the relation.

Domain constraint:

- A domain constraint is a rule that specifies the allowable values for a particular attribute in a relation. For example, a domain constraint on a column that stores dates might specify that all dates must be in the format YYYY-MM-DD.

In summary, a relation is a table that represents a set of entities and their attributes. A relation schema defines the structure of a relation, while a relation instance contains actual data. A unary relation is a relation with only one attribute, and the degree of a relation is the number of attributes in the relation. A domain constraint is a rule that specifies the allowable values for a particular attribute in a relation.