Which type of searches are unable to retrieve results for queries consisting of single digits, letters, or characters?

Keyword searches rely on the indexing and retrieval of specific terms within a document or database to return relevant results. When a query consists solely of single digits, letters, or characters, these searches may fail to retrieve any results because most search systems typically require longer terms or specific phrases to generate meaningful returns. Single letters or digits do not typically provide enough context or specificity for the system to locate relevant documents.

In contrast, other search types, like Boolean, phrase, and proximity searches, utilize different techniques that can sometimes accommodate broader or alternative matching criteria, thus potentially allowing more nuanced results even with simpler queries. However, for keyword searches, the requirement is for recognizable terms, making them less effective for minimalistic queries.