/* Get the point of interest's geometry and save it in a common table expression */
WITH point_of_interest AS (
    SELECT
        geom
    FROM
        my_table
    WHERE
        id = 1
)

/* Select everything from all the other points if they are within 100m */
SELECT
    mt.*
FROM
    my_table AS mt,
    point_of_interest AS poi
WHERE
    id != 1 -- Don't return the point of interest itself
    AND
    ST_DWithin(poi.geom, mt.geom, 100) -- Return only the points within 100 m