A clustering index is an ordered file with two fields; the first field is of the same type as the clustering field of the data file, and the second field is a disk block pointer. There is one entry in the clustering index for each distinct value of the clustering field, and it contains the value and a pointer to the first block in the data file that has a record with that value for its clustering field.

Could any one explain to me how the clustering index have one entry for each distinct value of the clustering field While these below link to an illustrations image show a pointer to the same value 1 in the same block

Click Here

Hmm... The image is very confusing and contradict to the definition... It doesn't seem right in the image... Which book is this?

I understood it the image is correct 100% and that what it means

Cluster key value 1 appears for the first time and lies in the block number 1
Cluster key value 2 appears for the first time and lies in the block number 1
Cluster key value 3 appears for the first time and lies in the block number 2
.......

And when you read this part pointer to the first block in the data file that has a record with that value for its clustering field this seems reasonable

Ah that make sense ;)