[postgis-users] Select Point near Polyline Postgis

[anhtin]

hi all
I am developing a web application. i am using Mapserver and Postgis 
I have a question below
if i want to click one point on map to select the nearest point on the
nearest polyline from the point i clicked. How can i do?
Notes: The point I clicked is converted to X,Y coordinate (295149 2315499)

could you show me the sql command in postgis.
1. How can i select the nearest  polyline on the click point.
2. How can  i select the the nearest point on the selected nearest polyline.
:computer-user:
:rules:

Somebody suggest that I should search the polyline nearest on the click
point by the script below

SELECT * FROM mainroad WHERE GeomFromText('POINT(517651 2121421)', 42102) &&
the_geom 
 AND distance(the_geom,
GeomFromText('POINT(517651 2121421)', 42102)) < 200000

however,  sometime it has no result because the distance from point to
polyline is larger than 200000.
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