Trying to create multiple temporary tables in a single query - sql

Question

I'd like to create 3-4 separate temporary tables within a single BigQuery query (all of the tables are based on different data sources) and then join them in various ways later on in the query.
I'm trying to do this by using multiple WITH statements, but it appears that you're only able to use one WITH statement in a query if you're not nesting them. Each time I've tried, I get an error saying that a 'SELECT' statement is expected.
Am I missing something? I'd prefer to do this all in one query if at all possible.

Answer 1

I don't know what you mean by "temporary tables", but I suspect you mean common table expressions (CTEs).
Of course you can have a query with multiple CTEs. You just need the right syntax:
with t1 as (
select . . .
),
t2 as (
select . . .
),
t3 as (
select . . .
)
select *
from t1 cross join t2 cross join t3;

Answer 2

bq mk --table --expiration [INTEGER] --description "[DESCRIPTION]"
--label [KEY:VALUE, KEY:VALUE] [PROJECT_ID]:[DATASET].[TABLE]
Expiration date will make your table temporary. You can create one table at the time with "bq mk" but you can use this in a script.
You can use the DDL, but here also you can only create one table at the time.
{CREATE TABLE | CREATE TABLE IF NOT EXISTS | CREATE OR REPLACE TABLE}
table_name [( column_name column_schema[, ...] )]
[PARTITION BY partition_expression] [OPTIONS(options_clause[, ...])]
[AS query_statement]
If by "temporary table" you meant "subqueries" this is the syntax you have to use:
WITH
subQ1 AS (
SELECT * FROM Roster
WHERE SchoolID = 52),
subQ2 AS (
SELECT SchoolID FROM subQ1)
SELECT DISTINCT * FROM subQ2;

Answer 3

You should be able to use common table expressions without issue. However, if you have large queries with a significant amount of common table expressions/subqueries you may run into resource issues in BQ specifically regarding the ability for BQ to create an execution plan. Temporary tables have helped me in these scenarios, but there are likely better practices.
CREATE TEMP TABLE T1 AS (
WITH
X as (query),
Y as (query),
Z as (query)
SELECT * FROM
(combination_of_above_queries)
);
CREATE TEMP TABLE T2 AS (
WITH
A as (query),
B as (query),
C as (query)
SELECT * FROM
(combination_of_above_queries)
);
CREATE TABLE new_table AS (
SELECT *
FROM (combination of T1 and T2)
)
Apologies, I just saw happened to come across this question and wanted to share what helped me... hope it is helpful for you.
https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#temporary_tables