linestring created from coordinates POSTGIS - database

Question

Can someone help ?
Here is the part of my code (sql) which doesn't work :
SELECT ST_LENGTH(geom) into distance FROM
SELECT ST_GeographyFromText('srid=4326;linestring(lon_bus lat_bus, lon_stop lat_stop)') AS geom)
AS dis;
lon_bus, lat_bus, lon_stop and lat_stop are coordinates I get from my database. When I try this, I have an error of parsing. But when I replace these variable by numeric, it works. Can someone help me on it? I would like to keep these variables in my code.

Answer 1

It doesn't work because the WKT with variables is invalid. Remember, WKT is just regular text, so don't confuse WKT with SQL.
You can make a LineString from two point geometries, then cast it to ::geography.
SELECT ST_MakeLine(ST_MakePoint(lon_bus, lat_bus),
ST_MakePoint(lon_stop, lat_stop))::geography AS geog
FROM (
SELECT 1 AS lon_bus, 2 AS lat_bus, 3 AS lon_stop, 4 AS lat_stop
) AS data;
To get the geodesic length, use ST_Length on the geography.
Based on the usage, the question isn't about how to make a linestring, but how to calculated the distance between two geographic positions. There are several ways to do this:
SELECT
ST_Distance(bus, stop) AS cartesian_distance,
ST_Distance_Sphere(bus, stop) AS sphere_distance,
ST_Distance(bus::geography, stop::geography) AS geography_distance,
ST_Length(ST_MakeLine(bus, stop)::geography) AS geography_length
FROM (
SELECT ST_MakePoint(lon_bus, lat_bus) AS bus, ST_MakePoint(lon_stop, lat_stop) AS stop
FROM (SELECT 1 AS lon_bus, 2 AS lat_bus, 3 AS lon_stop, 4 AS lat_stop) AS data
) AS data;
-[ RECORD 1 ]------+-----------------
cartesian_distance | 2.82842712474619
sphere_distance | 314283.687770102
geography_distance | 313588.397192902
geography_length | 313588.397192902
The last two get the same result. If you don't need the linestring (e.g. to draw on a map), then the simplest method is used for geography_distance.