Spatial SQL to return the id of the closest geometry from table_b to each geometry in table_a

nearest_neighbour_bigquery.sql

SELECT
  a.id,
  -- Note that on line 13 the results are grouped by a.id so if you want to return the geometry from table_a, 
  -- here you can use the function ANY_VALUE(a.geom) (as you cannot group by geometry in BigQuery) 
  ARRAY_AGG(b.id ORDER BY ST_Distance(a.geog, b.geog) LIMIT 1) [ORDINAL(1)] AS neighbor_id
  -- Here we return the id of the closest geometry from table_b to each geometry in table_a. 
  -- If you want to include more fields from table_b here you can use STRUCT(b.id, b.second_field, b.third_field) etc.
FROM
  table_a a
JOIN
  table_b b
ON
  ST_DWithin(a.geog, b.geom, 100) -- Here we use ST_DWithin to limit the results to within 100m. 
  -- The smaller the search radius, the faster the query will run, however the higher the liklihood 
  -- that we end up with geometries with no nearest neighbour. This can be optimised with prior knowledge of the dataset.
GROUP BY
  a.id