How to update table while joining on CTE results in BigQuery?

Question

I think it will be obvious to see what I\'m trying to do through this simplified example (works in PostgreSQL)....

with a as
(
    select 1 as id, 123.456 as val         


        

Answer 1

You can transform you update statement into below

#standardSQL
UPDATE 
  `project.dataset.mytable` mytable 
  SET value = new_value 
FROM (
  WITH a AS
  (
    SELECT 1 AS id, 123.456 AS value
  )  
  SELECT a1.id a1_id, a2.id a2_id, 
    COALESCE(a1.value, a2.value) new_value
  FROM 
    a AS a1, 
    a AS a2 
)
WHERE 
  a1_id = mytable.id 
OR 
  a2_id = mytable.id2

Answer 2

With BigQuery scripting (Beta now), CREATE TEMP TABLE can be an alternative way. See public documentation here.

You should test to see the performance impact of using a temp table, which may be better or worse. As you can image, with scripting, it is possible to put multiple UPDATE query in one script and sharing a temp table a among them, which increases the opportunity of having better performance.

CREATE TEMP TABLE a as
    select 1 as id, 123.456 as value;

update 
    mytable 
    set value = coalesce(a1.value, a2.value) 
from 
    a as a1, 
    a as a2 
where 
    a1.id = mytable.id 
or 
    a2.id = mytable.id2