BigQuery SQL computing spatial statistics around multiple locations - sql

Question

Problem: I want to make a query that allows me to compute sociodemographic statistics within a given distance of multiple locations. I can only make a query that computes the statistics at a single location at a time.
My desired result would be a table where I can see the name of the library (title), P_60YMAS (some sociodemographic data near these libraries), and a geometry of the multipolygons within the buffer distance of this location as a GEOGRAPHY data type.
Context:
I have two tables:
'cis-sdhis.de.biblioteca' or library, that have points as GEOGRAPHY data type;
'cis-sdhis.inegi.resageburb' in which I have many sociodemographic data, including polygons as GEOGRAPHY data type (column name: 'GEOMETRY')
I want to make 1 Km Buffer around the libraries, make new multipolygons within this buffers and get some sociodemographic data and geometry from those multipolygons.
My first approach was with this query:
SELECT
SUM(P_60YMAS) AS age60_plus,
ST_UNION_AGG(GEOMETRY) AS geo
FROM
`cis-sdhis.inegi.resageburb`
WHERE
ST_WITHIN(GEOMETRY,
(
SELECT
ST_BUFFER(geography,
1000)
FROM
`cis-sdhis.de.biblioteca`
WHERE
id = 'bpm-mty3'))
As you can see, this query only gives me one library ('bpm-mty3'), an that's my problem: I want them all at once.
I thought that using OVER() would be one solution, but I don't really know where or how to use it.

Answer 1

OVER() could be a solution, but a more performant solution is JOIN.
You first need to join two tables on condition that the distance between them is less than 1000 meters, that gives you pairs of row, where libraries are paired with all relevant data, note we'll get multiple rows per library. The predicate to use is ST_DWithin(geo1, geo2, distance), alternative form is ST_Distance(geo1, geo2) < distance - in both cases you don't need buffer.
SELECT
*
FROM
`cis-sdhis.inegi.resageburb` data,
`cis-sdhis.de.biblioteca` lib
WHERE
ST_DWITHIN(lib.geometry, data.geography)
Then we need to compute stats per library, remember we have many rows per-library, and we need a single row for each library. For this we need to aggregate per library, let's do GROUP BY using id. When we need an info about the library itself the cheapest way to do it is to use ANY_VALUE aggregation function. So it will be something like
SELECT
lib.id, ANY_VALUE(lib.title) AS title,
SUM(P_60YMAS) AS age60_plus,
-- if you need to show the circle around library
ST_BUFFER(ANY_VALUE(lib.geometry)) AS buffered_location
FROM
`cis-sdhis.inegi.resageburb` data,
`cis-sdhis.de.biblioteca` lib
WHERE
ST_DWITHIN(lib.geometry, data.geography)
GROUP BY lib.id
One thing to note here is that ST_BUFFER(ANY_VALUE(...)) is much cheaper than ANY_VALUE(ST_BUFFER(...)) - it only computes buffer once per output row.