HANA linestring to segments - geospatial

Question

How can I divide a linestring to line segments? using SAP HANA DB
for example:
'LINESTRING(0 0, 2 2, 0 2, 0 5)'
will become:
'LINESTRING(0 0, 2 2) LINESTRING(2 2, 0 2) LINESTRING(0 2, 0 5)'

Answer 1

You can extract individual points with the function ST_PointN().
To call it for point 1, 2, 3, ... N
I use a temp table called TMP that would store counter values:
CREATE TABLE TMP (NUM INT);
select TOP 100 ROW_NUMBER() OVER () from PUBLIC.M_TABLES ;
Then I apply ST_PointN for every point using the counter table.
select NUM, LS.ST_PointN(NUM).ST_WKT() as CUR_POINT
from(
select NEW ST_LineString('LINESTRING(0 0, 2 2, 0 2, 0 5)') as LS, NUM
from TMP
)nested1
where NUM<=LS.ST_NumPoints()
This returns
NUM| CUR_POINT
1 |POINT (0 0)
2 |POINT (2 2)
3 |POINT (0 2)
4 |POINT (0 5)
You can easily concatenate those into a ST_Multipoint geometry with an aggregation using ST_UnionAggr():
select ST_UnionAggr(CUR_POINT).ST_AsWKT()
from(
select NUM, LS.ST_PointN(NUM) as CUR_POINT
from(
select NEW ST_LineString('LINESTRING(0 0, 2 2, 0 2, 0 5)') as LS, NUM
from TMP
)nested1
where NUM<=LS.ST_NumPoints()
)nested2
This returns
MULTIPOINT ((0 0),(2 2),(0 2),(0 5))
Note: we could do a loop instead of using a counter table.
Now for your exact question, we will build 3 ST_LineString() using a window function to combine the current point with the next one. There are multiple ways to write such a query, here's one:
select NEW ST_LineString('LineString ('||START_POINT.ST_X()||' '||START_POINT.ST_Y()||','||END_POINT.ST_X()||' '||END_POINT.ST_Y()||')').ST_AsWKT() as LS
from(
select CUR_POINT as START_POINT,
NEW ST_Point(FIRST_VALUE(CUR_POINT) OVER(order by NUM asc rows BETWEEN 1 following and 1 following) )as END_POINT
from(
select NUM, LS.ST_PointN(NUM) as CUR_POINT, LS.ST_NumPoints() as NB_POINTS
from(
select NEW ST_LineString('LINESTRING(0 0, 2 2, 0 2, 0 5)') as LS, NUM
from TMP
)nested1
where NUM<=LS.ST_NumPoints()
)nested2
)nested3
where END_POINT is not null
Tada:
LS
LINESTRING (0 0,2 2)
LINESTRING (2 2,0 2)
LINESTRING (0 2,0 5)