Fairly simple example: your file structure is as follows:
table1(parent_id)-> table2 (id)
table2(parent_id)-> table3(id).
table 1 links to table2 and table2 links to table3
For whatever reason table3 has to go. But you want to update table1 with table3’s id.
so you’ve created a new column table3_id. (I know very bad naming convention but I’m hoping it aids the understanding of the solution)
update file1 set table3_id =
(select table3.id
from table1
inner join table2 on (table2.id = table1.parent_id )
inner join table3 on ( table3.id = table2.parent_id ) )
