Indexes in SQL Server are used to speed up the retrieval of data from a table by providing a structured way to access rows quickly. Here are the main types of indexes:

1. Clustered Index:

   • Only one per table.

   • Sorts and stores the data rows in the table based on the key columns.

   • The table itself is organized according to the index.

   • Example:

   sql

   CREATE CLUSTERED INDEX idx_cl_mycolumn ON mytable(mycolumn);
   

2. Non-Clustered Index:

   • Multiple non-clustered indexes can be created per table.

   • Contains a sorted copy of the indexed columns and a pointer back to the actual data rows.

   • Example:

   sql

   CREATE NONCLUSTERED INDEX idx_ncl_mycolumn ON mytable(mycolumn);
   

3. Unique Index:

   • Ensures that no duplicate values exist in the indexed column(s).

   • Can be either clustered or non-clustered.

   • Example:

   sql

   CREATE UNIQUE INDEX idx_unique_mycolumn ON mytable(mycolumn);
   

4. Full-Text Index:

   • Used for full-text searches on large text columns.

   • Allows sophisticated word-based searches on textual data.

   • Example:

   sql

   CREATE FULLTEXT INDEX ON mytable(mycolumn) 
   KEY INDEX myprimarykey;
   

5. Columnstore Index:

   • Used for data warehousing and analytics.

   • Stores data in a columnar format which enhances query performance for large datasets.

   • Example:

   sql

   CREATE COLUMNSTORE INDEX idx_columnstore ON mytable(mycolumn);
   

6. Filtered Index:

   • Non-clustered index that includes a subset of rows in the table.

   • Useful for indexing a portion of the data.

   • Example:

   sql

   CREATE NONCLUSTERED INDEX idx_filtered ON mytable(mycolumn)
   WHERE mycolumn IS NOT NULL;
   

Indexes play a crucial role in optimizing the performance of database queries. Proper indexing can significantly improve query execution times, making your database operations much more efficient.

Would you like to delve deeper into any specific type of index or how to manage them?