What is the difference between a clustered index and a non-clustered index

Question

Hi What is the difference between a clustered index and a non-clustered index ?

Vulpes · Answer

When data is clustered it is rearranged to match the ordering of the index - hence the expression 'clustered index'. Data which has not been rearranged but is still in its original order is described as 'unclustered' and any index on such data which imposes a specific ordering is therefore an 'unclustered index'. Clearly there can only be a single clustered index on clustered data. However there can be multiple unclustered indices on unclustered data.