BigQuery using Dynamic sql as the input source of a query - sql

Question

How can I use a dynamic query as an input source for a larger query?
There is a query I'm getting the union of values in different datasets/tables scattered around and the list is growing so I'm thinking of using the dynamic query than to write queries for each tables like this
SET QUERY = "";
SET tables = ["table1", "table2"...];
SET tables_size = ARRAY_LENGTH(tables);
WHILE i < tables_size DO
IF (i = tables_size -1) THEN
BEGIN
SET query = CONCAT(query, " SELECT id, name FROM ", tables[OFFSET(i)]);
BREAK;
END;
ELSE
SET query = CONCAT(query, " SELECT id, name FROM ", tables[OFFSET(i)], ' UNION ALL ');
END IF;
SET i = i + 1;
END WHILE;
EXECUTE IMMEDIATE query;
My goal is to use the output of the executed query as a FROM clause for a larger query.
It will be something like
Select A, B, C, D ... From *EXECUTE IMMEDIATE query* LEFT JOIN ... ON..
Is there a way to inject an output of a dynamic query as a table for another query?
I don't see TABLE as a variable type for bigquery so that was not my option.
I'm getting a bit tired of copy pasting table names to the exact query every time a new table is introduced to this logic.
SELECT id, name FROM table1 UNION ALL
SELECT id, name FROM table1 UNION ALL
SELECT id, name FROM table3...
If there is a simple way to do this? or maybe a reason to not use dynamic queries for performance reasons?

Answer 1

Hope one of these are helpful:
1. Wildcard tables
If tables you want to union have a common prefix, you can consider to use a wildcard table like below. I think this is more concise form rather than union-all:
-- Sample Tables
CREATE TABLE IF NOT EXISTS testset.table1 AS SELECT 1 AS id, 'aaa' AS name;
CREATE TABLE IF NOT EXISTS testset.table2 AS SELECT 2 AS id, 'bbb' AS name;
CREATE TABLE IF NOT EXISTS testset.table3 AS SELECT 3 AS id, 'ccc' AS name;
--- Wildcard tables
SELECT * FROM `testset.table*` WHERE _TABLE_SUFFIX IN ('1', '2', '3');
2. Dynamic SQL & Temp Table
You can't inject a dynamic SQL directly into another query but you can use a temp table to emulate it.
2.1 Dynamic SQL
More concise dynamic query to union all tables:
DECLARE tables DEFAULT ["testset.table1", "testset.table2", "testset.table3"];
SELECT ARRAY_TO_STRING(ARRAY_AGG(FORMAT('SELECT id, name FROM %s', t)), ' UNION ALL\n')
FROM UNNEST(tables) t;
2.2 Using a temp table
I thinks you can modify your larger query to use a dynamically generated temp table.
DECLARE tables DEFAULT ["testset.table1", "testset.table2", "testset.table3"];
CREATE TABLE IF NOT EXISTS testset.table1 AS SELECT 1 AS id, 'aaa' AS name;
CREATE TABLE IF NOT EXISTS testset.table2 AS SELECT 2 AS id, 'bbb' AS name;
CREATE TABLE IF NOT EXISTS testset.table3 AS SELECT 3 AS id, 'ccc' AS name;
EXECUTE IMMEDIATE (
SELECT "CREATE TEMP TABLE IF NOT EXISTS union_tables AS \n"
|| ARRAY_TO_STRING(ARRAY_AGG(FORMAT('SELECT id, name FROM %s', t)), ' UNION ALL\n') FROM UNNEST(tables) t
);
-- your larger query using a temp table
SELECT * FROM union_tables;
output: