It's sort of relational database philosophy/theory. A database table is in theory like a sparse array, indexed not by a number but by the possible values in its primary key. Each table is - in principle - infinite in size, although in reality only a certain number of array cells will ever be filled; and each cell - in principle - does not "have" a primary key as data, but the primary key is the location of the cell in a logical space.
With an array, you can't simply say "array element 5 is now array element 6". You can only move the data from one cell to another. Same goes for a table.
In principle.
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