Learn Now What are INDEXES in SQL

Indexes in SQL are data structures that can speed up the data retrieval process. They are used to optimize the performance of SQL queries by providing quick access to the data. An index is essentially a pointer to a row in a table, and it can be created on one or more columns of a table. When an index is created, it is stored in a separate data structure from the table, which is known as an index structure. In this way, the index can be searched more quickly than the table.

There are different types of indexes available in SQL, such as

clustered index
non-clustered index
unique index
composite index

The choice of the index depends on the specific requirements of the query.

Clustered Index: A clustered index is an index in which the physical order of the data in the table is the same as the logical order of the index. This means that the rows in the table are physically stored in the same order as the index. A table can have only one clustered index, and it is created automatically when a primary key is defined on a table. For example, consider the following CREATE TABLE statement:

CREATE TABLE employees (
  employee_id INT PRIMARY KEY,
  first_name VARCHAR(50),
  last_name VARCHAR(50),
  hire_date DATE
);

This statement creates a table called employees with four columns, including an employee_id column that is defined as the primary key. A clustered index is created on the employee_id column because it is the primary key.

Non-clustered Index: A non-clustered index is an index in which the physical order of the data in the table is different from the logical order of the index. This means that the rows in the table are physically stored in a different order than the index. A table can have multiple non-clustered indexes, and they are created explicitly using the CREATE INDEX statement. For example, consider the following CREATE INDEX statement:

CREATE INDEX idx_last_name ON employees (last_name);

This statement creates a non-clustered index on the last_name column of the employees table.

Unique Index: A unique index is an index that does not allow duplicate values to be stored in the indexed column(s). A table can have multiple unique indexes, and they are created using the CREATE UNIQUE INDEX statement. For example, consider the following CREATE UNIQUE INDEX statement:

CREATE UNIQUE INDEX idx_email ON employees (email);

This statement creates a unique index on the email column of the employees table.

Composite Index: A composite index is an index that is created on multiple columns of a table. It is useful for queries that use a combination of columns in the WHERE clause. A table can have multiple composite indexes, and they are created using the CREATE INDEX statement with multiple columns. For example, consider the following CREATE INDEX statement:

CREATE INDEX idx_name_hire_date ON employees (last_name, first_name, hire_date);

This statement creates a composite index on the last_name, first_name, and hire_date columns of the employees table.

In general, creating indexes on a table can significantly improve the performance of queries. However, creating too many indexes or inappropriate indexes can slow down the performance of queries. Therefore, it is important to carefully analyze the queries and data to determine the appropriate indexes.Learn SQL Now: Learn From Scratch to Advanced

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