In BigQuery, identify when columns do not match on UNION ALL

user_5573294 0
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with
  table1 as (
    select 'joe' as name, 17 as age, 25 as speed
  ),
  table2 as (
    select 'nick' as name, 21 as speed, 23 as strength
  )

select * from table1
union all
select * from table2

In Google BigQuery, this union all does not throw an error because both tables have the same number of columns (3 each). However I receive bad data output because the columns do not match. Rather than outputting a new table with 4 columns name, age, speed, strength with correct values + nulls for missing values (which would probably be preferred), the union all keeps the 3 columns from the top row.

Is there a good way to catch that the columns do not match, rather than the query silently returning bad data? Is there any way for this to return an error perhaps, as opposed to a successful table? I'm not sure how to check in SQL that the columns in the 2 tables match.

Edit: in this example it is clear to see that the columns do not match, however in our data we have 100+ columns and we want to avoid a situation where we make an error in a UNION ALL

google-bigquery union-all sql
2 Answers

Below is for BigQuery Standard SQL and using scripting feature of BQ

DECLARE statement STRING;
SET statement = (
  WITH  table1_columns AS (
    SELECT column FROM (SELECT * FROM `project.dataset.table1` LIMIT 1) t,
    UNNEST(REGEXP_EXTRACT_ALL(TRIM(TO_JSON_STRING(t), '{}'), r'"([^"]*)":')) column
  ), table2_columns AS (
    SELECT column FROM (SELECT * FROM `project.dataset.table2` LIMIT 1) t,
    UNNEST(REGEXP_EXTRACT_ALL(TRIM(TO_JSON_STRING(t), '{}'), r'"([^"]*)":')) column
  ), all_columns AS (
    SELECT column FROM table1_columns UNION DISTINCT SELECT column FROM table2_columns
  )
  SELECT (
      SELECT 'SELECT ' || STRING_AGG(IF(t.column IS NULL, 'NULL as ', '') || a.column, ', ') || ' FROM `project.dataset.table1` UNION ALL '
      FROM all_columns a LEFT JOIN table1_columns t USING(column)
    ) || (
      SELECT 'SELECT ' || STRING_AGG(IF(t.column IS NULL, 'NULL as ', '') || a.column, ', ') || ' FROM `project.dataset.table2`'
      FROM all_columns a LEFT JOIN table2_columns t USING(column)
    ) 
);
EXECUTE IMMEDIATE statement;

when applied to sample data from your question - output is

Row name    age     speed   strength     
1   joe     17      25      null     
2   nick    null    21      23
user_52219440

After saving table1 and table2 as 2 tables in a dataset in BigQuery, I then used the metadata using INFORMATION_SCHEMA to check that the columns matched.

SELECT *
FROM models.INFORMATION_SCHEMA.COLUMNS
where table_name = 'table1'

SELECT *
FROM models.INFORMATION_SCHEMA.COLUMNS
where table_name = 'table2'

INFORMATION_SCHEMA.COLUMNS returns information including the column names and their positioning. I can join these 2 tables together then to check that the names match...

user_55732940
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