Ensure at most a single BigQuery scheduled query is running at one time

bigquery_prevent_concurrent_runs.sql

DECLARE check_running_jobs DEFAULT ( 
  SELECT IF(COUNT(*) > 1, ERROR('query already running'), NULL) 
  FROM `region-name`.INFORMATION_SCHEMA.JOBS_BY_PROJECT
  WHERE state <> 'DONE'
);

Note: you can't use this if you use a destination table. In this case you should remove the destination table from the scheduled query definition and rewrite your query as

DECLARE check_running_jobs DEFAULT ( 
  SELECT IF(COUNT(*) > 1, ERROR('query already running'), NULL) 
  FROM `region-name`.INFORMATION_SCHEMA.JOBS_BY_PROJECT
  WHERE state <> 'DONE'
);
CREATE OR REPLACE TABLE `destination-table` AS (
  SELECT ...
);