Question

I’m using PostgreSQL + PostGIS.

In table I have a point and line geometry in the same column of the same table, in different rows. To get the line I run:

SELECT the_geom
FROM filedata
WHERE id=3

If i want to take point I run:

SELECT the_geom
FROM filedata
WHERE id=4

I want take point and line together, like they’re shown in this WITH expression, but using a real query against the table instead:

WITH data AS (
SELECT 'LINESTRING (50 40, 40 60, 50 90, 30 140)'::geometry AS road,
       'POINT (60 110)'::geometry AS poi)

SELECT ST_AsText(
ST_Line_Interpolate_Point(road, ST_Line_Locate_Point(road, poi))) AS projected_poi

FROM data;

You see in this example data comes from a hand-created WITH expression. I want take it from my filedata table. My problem is i dont know how to work with data from two different rows of one table at the same time.

Answer 1

Use a self-join:

SELECT ST_AsText(
  ST_Line_Interpolate_Point(fd_road.the_geom, ST_Line_Locate_Point(
      fd_road.the_geom,
      fd_poi.the_geom
  )) AS projected_poi
FROM filedata fd_road, filedata fd_poi
WHERE fd_road.id = 3 AND fd_poi.id = 4;

Alternately use a subquery to fetch the other row, as Erwin pointed out.

The main options for using multiple rows from one table in a single expression are:

• Self-join the table with two different aliases as shown above, then filter the rows;
• Use a subquery expression to get a value for all but one of the rows, as Erwin’s answer shows;
• Use a window function like lag() and lead() to get a row relative to the current row within the query result; or
• JOIN on a subquery that returns a table

The latter two are more advanced options that solve problems that’re difficult or inefficient to solve with the simpler self-join or subquery expression.