Difference between clustered and nonclustered index [duplicate]

I need to add proper index to my tables and need some help.

I'm confused and need to clarify a few points:

• Should I use index for non-int columns? Why/why not

• I've read a lot about clustered and non-clustered index yet I still can't decide when to use one over the other. A good example would help me and a lot of other developers.

I know that I shouldn't use indexes for columns or tables that are often updated. What else should I be careful about and how can I know that it is all good before going to test phase?

A clustered index alters the way that the rows are stored. When you create a clustered index on a column (or a number of columns), SQL server sorts the table’s rows by that column(s). It is like a dictionary, where all words are sorted in alphabetical order in the entire book.

A non-clustered index, on the other hand, does not alter the way the rows are stored in the table. It creates a completely different object within the table that contains the column(s) selected for indexing and a pointer back to the table’s rows containing the data. It is like an index in the last pages of a book, where keywords are sorted and contain the page number to the material of the book for faster reference.

You really need to keep two issues apart:

1) the primary key is a logical construct - one of the candidate keys that uniquely and reliably identifies every row in your table. This can be anything, really - an INT, a GUID, a string - pick what makes most sense for your scenario.

2) the clustering key (the column or columns that define the "clustered index" on the table) - this is a physical storage-related thing, and here, a small, stable, ever-increasing data type is your best pick - INT or BIGINT as your default option.

By default, the primary key on a SQL Server table is also used as the clustering key - but that doesn't need to be that way!

One rule of thumb I would apply is this: any "regular" table (one that you use to store data in, that is a lookup table etc.) should have a clustering key. There's really no point not to have a clustering key. Actually, contrary to common believe, having a clustering key actually speeds up all the common operations - even inserts and deletes (since the table organization is different and usually better than with a heap - a table without a clustering key).

Kimberly Tripp, the Queen of Indexing has a great many excellent articles on the topic of why to have a clustering key, and what kind of columns to best use as your clustering key. Since you only get one per table, it's of utmost importance to pick the right clustering key - and not just any clustering key.

• GUIDs as PRIMARY KEY and/or clustered key
• The clustered index debate continues
• Ever-increasing clustering key - the Clustered Index Debate..........again!
• Disk space is cheap - that's not the point!

Marc