In SQL, a non-clustered index is a type of database index used to improve the performance of queries by creating a separate structure that stores a subset of the table's data. Unlike a clustered index, which determines the physical order of the data in a table, a non-clustered index does not alter the physical storage order of the table.

Here's a more detailed explanation of non-clustered indexes in SQL:

1. Index Structure: A non-clustered index creates a separate structure, often implemented as a B-tree or a balanced tree, that contains the indexed columns' values and pointers to the corresponding rows in the table. This index structure allows for faster data retrieval and efficient query processing.

2. Improved Query Performance: Non-clustered indexes are designed to speed up data retrieval operations by providing a quick way to locate specific rows based on the indexed columns. When a query involves filtering, sorting, or joining data based on the indexed columns, the database engine can utilize the non-clustered index to efficiently locate and retrieve the required data.

3. Independent of Physical Data Order: Unlike a clustered index, which determines the physical order of the data in a table, a non-clustered index is independent of the table's physical data order. The non-clustered index structure is separate from the table's actual data pages, allowing for flexible organization and faster index operations.

4. Multiple Non-Clustered Indexes per Table: A SQL table can have multiple non-clustered indexes, each targeting different columns or combinations of columns. This allows for improved query performance in various scenarios, as different indexes can be used to optimize different types of queries.

5. Index Maintenance: When data is modified (inserted, updated, or deleted) in a table with non-clustered indexes, the indexes need to be updated to reflect the changes. This index maintenance operation incurs a small performance overhead during data modification operations but helps ensure the integrity and accuracy of the index data.

6. Selectivity and Index Usage: The selectivity of an index refers to the uniqueness or distinctness of the indexed values. Highly selective indexes, with a large number of unique values, tend to be more effective in improving query performance. The database engine utilizes statistics and query optimization techniques to determine the most efficient index to use based on the query's conditions and selectivity.

7. Index Considerations: While non-clustered indexes can significantly improve query performance, they come with some considerations. Creating too many indexes on a table can impact data modification operations, as each index needs to be updated when data changes. Additionally, indexes consume disk space, so it's essential to strike a balance between the benefits of improved query performance and the storage overhead of maintaining the indexes.

In summary, a non-clustered index in SQL is a separate data structure that stores a subset of table data to enhance query performance. It provides faster data retrieval by allowing efficient access to rows based on the indexed columns. Non-clustered indexes are independent of the table's physical data order, support multiple indexes per table, require maintenance during data modification operations, and their usage depends on selectivity and query optimization techniques. By obtaining SQL Certification, you can advance your career in the field of SQL Servers. With this Certification, you can demonstrate your expertise in working with SQL concepts, including querying data, security, and administrative privileges, among others. This can open up new job opportunities and enable you to take on leadership roles in your organization.