Final answer:
The three main approaches to indexing in a relational database are Clustered, Non-clustered, and Bitmap indexes, each having unique characteristics suitable for different types of data organization and retrieval.
Step-by-step explanation:
The three main approaches to indexing data in a relational database system are Clustered, Non-clustered, and Bitmap. A Clustered index reordersthe physical row order in the table and searches based on key values. Each table can have only one clustered index. A Non-clustered index, on the other hand, maintains a separate index structure from the data rows. This type of index can be more than one in a table and includes a pointer to the data rows containing the key value. The Bitmap index is a special type of index used primarily for columns with a low cardinality, meaning they have a small number of unique values. It uses bit arrays and is efficient for querying and data retrieval in such scenarios.