Difference between Clustered and Nonclustered index in relational database is one of the most popular SQL interview questions almost as popular as difference between truncate and delete, primary key vs unique key and correlated vs noncorrelated subqueries. Indexes are very important concept, it makes your queries to run fast and if you compare a SELECT query which uses an indexed column to one who doesn't you will see big difference in performance. There can be two kinds on indexes in relational database Clustered and Nonclustered indexes. Clustered index determines physical sorting order of rows in a table similar to entries on yellow pages which are sorted on alphabetical order. Suppose you have a table Employee, which contains emp_id as primary key than clustered index which is created on primary key will sort the Employee table as per emp_id. That was a brief introduction of What is clustered index in SQL. On other hand Nonclustered index involves one extra step which point to physical location of record. In this SQL Interview question we will see some more difference between clustered and nonclustered index in point format.
Difference between Clustered and Nonclustered Indexes in SQL
1) Clustered Index physically sort all rows while Nonclustered Index doesn't.
2) In SQL one table can only have one Clustered Index but there is no such restriction on NonClustered Index.
3) In many relational databases Clustered Index is automatically created on primary key column.
That's all on difference between Clustered vs Nonclustered Index in SQL. Index are very critical to query performance but too many index can actually harm performance of INSERT and UPDATE queries because these requires rearrangement of indexes which can be time consuming based upon size of table or number of records on table. Always create Indexes on columns which is frequently used in SELECT query because there is huge performance difference between indexed and non indexed column in SQL queries.
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