Topic: Index in SQL
An index in SQL is essentially an data structure that improves the speed of data retrieval operations on a database table. It provides a quick lookup of data in a table based on the values within selected columns. Similar to the index of a book, which allows you to go directly to a page to find particular information instead of having to read the whole book.
There are several types of indexes in SQL, including unique index, clustered index, non-clustered index, and full-text index.
Clustered indexes sort and store rows of data in the table based on their key values. There can only be one clustered index per table.
Non-clustered indexes have a structure separate from the data rows, storing the non-clustered index key values each with a pointer to the data row that contains the key value.
A unique index ensures that no duplicate values can be inserted into the table, while a full-text index can include one or more character-based columns in the table and can speed up text queries on large amounts of data.
Indexes can be an efficient tool but must be used wisely as they can put off performance if not used appropriately. It may add some overhead on insert, update and delete operations as well as they will need to make changes to both the data pages and index pages.
Interview Questions:
1. What is an Index in SQL?
Answer: An Index in SQL is a data structure that improves the speed of data retrieval operations on a database table by providing quick lookup of data in a table based on the values within certain column(s).
2. What is the difference between Clustered and Non-Clustered indexes?
Answer: The fundamental difference between clustered and non-clustered indexes lies in the way they store information. Clustered indexes sort and store data rows in the table or view based on their key values, ensuring there’s only one clustered index per table. Non-Clustered indexes, however, maintain a separate record of data without altering the physical order of rows in a table, and a table can have multiple non-clustered indexes.
3. What is the purpose of Indexes in SQL?
Answer: The primary purpose of indexes in SQL is to enhance the speed and performance of database operations, particularly data retrieval or queries.
4. How do you create an index?
Answer: We can create an index in SQL with the CREATE INDEX statement. The syntax is: CREATE INDEX index_name ON table_name (column1, column2, …);
5. When should you avoid using indexes in SQL?
Answer: Indexing is not beneficial when the table is small or if the columns used are not often included in query conditions. Indexes can also slow down the performance of INSERT, UPDATE, and DELETE commands because they require additional updates on index keys and can consume physical storage. Therefore, these factors should be considered to avoid unnecessary indexing.
