UltraLite supports different types of indexes: unique keys, unique indexes and non-unique indexes. What differentiates one from the others is what is allowed in that index.
|Index characteristic||Unique keys||Unique indexes||Non-unique indexes|
|Allows duplicate index entries for rows that have the same values in indexed columns.||no||no||yes|
|Allows null values in index columns.||no||yes||yes|
You can create foreign keys to unique keys, but not to unique indexes.
Also, manually creating an index on a key column is not necessary and generally not recommended. UltraLite creates and maintains indexes for unique keys automatically.