Clustered indexes and nonclustered indexes

In fact, you can interpret an index as a special kind of directory. Microsoft's SQL Server provides two types of indexes: Clustered indexes (clustered index, also called clustered indexes, clustered indexes), and nonclustered indexes (nonclustered index, also called nonclustered indexes, non-clustered indexes). Let's take a look at the differences between clustered and nonclustered indexes, for example:
In fact, the body of our Chinese dictionary is itself a clustered index.

For example, we have to check the word "Ann", it will be very natural to open the first few pages of the dictionary, because "ann" Pinyin is "an", and alphabetical order of Chinese characters in the dictionary is the English letter "a" beginning and "Z", then the word "Ann" naturally ranked in the front of the dictionary. If you have turned over all the parts that begin with "a" and still cannot find the word, then it means that you do not have the word in your dictionary, and if you look up the word "Zhang", you will also turn your dictionary into the last part, because the pinyin of "Zhang" is "Zhang". That is, the body part of the dictionary is itself a directory, and you do not need to look up other directories to find what you need to find. We refer to this body of content itself as a directory of certain rules, called a "clustered index."
If you know a word, you can quickly check it out automatically. But you may also encounter the words you do not know, do not understand its pronunciation, at this time, you can not follow the method to find the word you want to check, and need to go to the "radicals" to find the word you are looking for, and then according to the page number after the word directly to a page to find the word you are looking for. But the sort of words you find in combination with the "radicals" and "gept" is not really the sort method of the body, for example, you check the word "Zhang", we can see in the Gept table after the Radicals "Zhang" page number is 672 pages, gept table "Zhang" above is "Chi" word, but the page number is 63 pages, "Zhang" below is "crossbow "Word, page is 390 pages. Obviously, these words are not really in the "Zhang" the word of the upper and lower side, now you see the continuous "Chi, Zhang, crossbow" three words is actually their order in the nonclustered index, is the dictionary body of words in the non-clustered index mapping. We can find the words you need in this way, but it takes two procedures to find the results in the catalog and then turn to the page numbers you need. We put this kind of directory purely as a directory, the body is purely the sort of body is called "nonclustered index".

Clustered indexes and nonclustered indexes