Tape. SQL databases emerged when data was stored on tape.
join table1, table2 where table1.id = table2.customer_id
type operations would have a tape for table1 in one drive, and a tape for table2 in the other drive. Things like fixed length records emerged to make it possible to fast forward the tape a specific number of inches to the point where the next record would begin, facilitating non-linear access.
Once that model was completely baked into the tooling, it didn't go away when the data moved to HDs then SSDs. The paradigms have outlived the hardware.
It's a bit like the save icon still being a floppy disk.
join table1, table2 where table1.id = table2.customer_id
type operations would have a tape for table1 in one drive, and a tape for table2 in the other drive. Things like fixed length records emerged to make it possible to fast forward the tape a specific number of inches to the point where the next record would begin, facilitating non-linear access.
Once that model was completely baked into the tooling, it didn't go away when the data moved to HDs then SSDs. The paradigms have outlived the hardware.
It's a bit like the save icon still being a floppy disk.