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.
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