Indexes
Indexes
In SQL, an index is a database object that provides a faster way to look up data in a table. Indexes optimize the retrieval of rows from a table based on the values in one or more columns.
They act like a reference or a pointer to the data, allowing the database engine to locate and retrieve the rows more efficiently.
Types of Indexes
The following are the two main types of indexes.
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Clustered Index:
The clustered Index determines the physical order of data rows in a table. A table can have only one clustered Index because the rows are stored in the order defined by it.
Example of creating a clustered index:
CREATE CLUSTERED INDEX idx_EmployeeID ON Employees (EmployeeID);In this example, a clustered index named
idx_EmployeeIDis created on theEmployeeIDcolumn of theEmployeestable. This means that the rows in theEmployeestable will be physically ordered based on the values in theEmployeeIDcolumn. -
Non-Clustered Index:
The non-Clustered Index does not affect the physical order of data in the table. Instead, it creates a separate structure, including indexed columns and a pointer to the corresponding rows.
Example of creating a non-clustered index:
CREATE INDEX idx_LastName ON Employees (LastName);In this example, a non-clustered index named
idx_LastNameis created on theLastNamecolumn of theEmployeestable. This Index will help speed up queries that involve filtering, sorting, or searching based on theLastNamecolumn.
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