Re: Primary key - link to data ?

[Rick James]

In MySQL's MyISAM, no. The data is simply a file; the indexes (including PK) point into the data file. All indexes are BTree organized.

In MySQL's InnoDB, the PRIMARY KEY and the data are 'clustered' together, and organized in a BTree. That is, when you find a a particular PK (via drilling down the BTree), you are sitting at the one row that it belongs to. Secondary keys have the PK implicitly included; this is how they get to the data record. Again, all indexes are BTree.

In MySQL's MEMORY table engine, a Hash is used.

A PRIMARY KEY is necessarily UNIQUE.