Aggregating attributes from line to point using QGIS Expression

Question

I have two feature classes: point and line. The purpose is to make geometrical relations between them. For instance, if points are on line then they automatically take attributes from a specific field from line layer (field-double).

But if i'm not giving any attribute it returns NULL (but it should aggregate from layer). What is wrong with expression?

if(NULL,
    aggregate(
        layer:='layer_you_want',
        aggregate:='concatenate',
        expression:=to_string("field_you_want"),
        concatenator:=', ',
        filter:=intersects($geometry, buffer(geometry(@parent), 0.0005))
        ),
    "remain_the_same_field"
    )

Answer 1

Use the following expression on the point layer and adapt it to your needs. It checks whether the point layer's attribute named value is 0:

• If not, the current value of value will be returned.
• if value = 0, the expression returns the value of the attribute called name of the line layer waterpipe - but only if the point is on the line (you should set a very small tolerance for the distance from the line, here 0.01). If points are not on the line, NULL will be returned

if (
    "value"=0,  -- set a condition
    overlay_nearest(
        'waterpipe',  -- name of the line layer
        name,  -- name of the attribute of the point layer
        max_distance:=0.01  -- small tolerance distance
    )[0],
    value
)

Red label is generated by the expression:

Answer 2

This one also worked:

if("diameter_valve" is NULL,
    aggregate(
        layer:='waterpipe_pipe',
        aggregate:='concatenate',
        expression:=to_string("diameter"),
        concatenator:=', ',
        filter:=intersects($geometry, buffer(geometry(@parent), 0.0005))
        ),
    "diameter_valve"
    )

Answer 3

This expression worked for me:

if("diameter_valve" is NULL,
    overlay_nearest('waterpipe', diameter_pipe, max_distance:=0.01)[0],
    "diameter_valve"
    )