非聚集索引相对于主键（聚集索引）的优势是什么？
没有聚集索引时，插入数据速度快，因为数据不需要按特定顺序存储，只需追加到表的末尾。
然而，在关键列上进行索引搜索通常会较慢，因为搜索不能利用聚集索引的优势。
非聚集索引是一种独立于表数据物理存储顺序的索引结构。它通过单独的数据结构来保存索引键和指向表中相应数据行的指针，这些指针指向存储在磁盘上的实际数据行。在非聚集索引中，索引键的顺序与存储在磁盘上的数据行的逻辑顺序无关。
非聚集索引相对于聚集索引（主键）的优势在于，它可以提供更快的索引搜索性能。由于非聚集索引不会改变数据的物理存储顺序，它可以更好地适应各种查询模式。当查询需要根据非聚集索引的键进行搜索时，数据库引擎可以直接利用非聚集索引来定位所需的数据行，而不需要扫描整个表。
解决非聚集索引相对于聚集索引的优势问题的方法是，根据查询模式选择适当的索引类型。如果查询经常涉及到根据聚集索引键进行搜索，那么聚集索引可能更合适。如果查询经常涉及到根据非聚集索引键进行搜索，那么非聚集索引可能更合适。在某些情况下，可以同时使用聚集索引和非聚集索引来优化查询性能。
下面是一个使用非聚集索引的示例（使用SQL语言）：
-- 创建表
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(100),
age INT,
department VARCHAR(100)
);
-- 创建非聚集索引
CREATE INDEX idx_employees_name ON employees (name);
在上面的示例中，我们创建了一个名为"employees"的表，并在"name"列上创建了一个非聚集索引。这将提高根据"name"列进行搜索的查询性能。
优势之一：您可以在索引中“包含(include)”其他列。
这并不是非聚集索引（NCI）相对于聚集索引（CI）的优势，因为聚集索引会自动包含所有列。
The main advantage of a non-clustered index over a primary key (clustered index) is that you can include other columns in the index. While a primary key automatically includes all columns in the table in the index, a non-clustered index allows you to selectively include additional columns that are not part of the index key.
For example, let's say you have a table with columns for CustomerID, CustomerName, Email, and PhoneNumber. The primary key is defined on the CustomerID column, which means that the primary key index includes all columns in the table.
However, if you frequently need to search for customers by their email addresses, you can create a non-clustered index on the Email column. By including the CustomerName and PhoneNumber columns in the non-clustered index, you can improve the performance of queries that filter or sort data based on the email address.
To create a non-clustered index with included columns, you can use the following syntax:
CREATE NONCLUSTERED INDEX IX_Email
ON Customers (Email)
INCLUDE (CustomerName, PhoneNumber);
By including these additional columns in the non-clustered index, you can avoid the need for a separate lookup operation to retrieve the values of these columns from the table.
In summary, the ability to include additional columns in a non-clustered index provides flexibility and performance improvements for queries that involve filtering or sorting based on these columns. This is an advantage that non-clustered indexes have over primary keys (clustered indexes).
