SQL: aggregate table data in subgroups

fellow SQLers.

I have a simple yet complex problem that I need to solve with SQL (SQLite) for my FME-script to run efficiantly. I have data I want to aggregate but in subgroups - taking z-level into account. It's a geological profile where I want to simplify the incoming data before I create my profile.

Example data

x,y,z_from,z_to,value
1,2,1,2,A
1,2,2,3,A
1,2,3,4,B
1,2,4,5,A
1,2,5,6,A
1,2,6,7,A
1,2,7,8,C

With SQL I want to aggregate on value but only rows that are adjacent (sort of a dissolve but on non-spatial data). And I want to calculate (also in sql) the correct z_from- and z_to-values. So the result from the example data above should be:

x,y,z_from,z_to,value
1,2,1,3,A 
1,2,3,4,B
1,2,4,7,A
1,2,7,8,C 

Any help on this will be remembered for all eternity.

Peter