Question

Do Fields after “ORDER BY” or “WHERE” might have index (PRIMARY, UNIQUE, INDEX) in mysql?

Consider a table with the following columns:

ID | AddedDate | CatID | Title | Description | Status | Editor

In these queries, are ID, AddedDate and CatID might have index?

  SELECT * 
    FROM table WHERE ID = $id


 SELECT * 
    FROM table 
ORDER BY ID

  SELECT * 
    FROM table 
ORDER BY AddedDate

  SELECT * 
    FROM table 
ORDER BY CatID

Answer 1

You can order by any field. Please clarify our question if you want to know more / something else.

You might want to read ORDER BY optimization. There it says that fields with index might even improve the sorting as no extra has to be done (in the optimal case).

Update:

Yes, you can add an index if you want (if this is what you mean, it is still not clear as OMG Ponies points out). In general it is to say that you should add an index to those fields that you often use in WHERE clauses.