title: bigquery type check operator? like typeof in Javascript; or workaround
body: <p>in a project where the BigQuery table's schema constantly evolves, I wonder is there a good way to write SQL code in generic?</p>
  
  <p>for example, a feature flag field in version 1 was a simple BOOLEAN, but later on evolved to a FLOAT to represent values between 0 (false) and 1 (true), then later changed to a STRUCT of multiple BOOLEANs,
  for every schema revision I changed table name as well, so now I have current table v3 and the old table v2 and v1 as well, the old tables have historical information is still useful sometimes, and volume is big not good to migrate all into v3 schema; since bigquery is mostly used as load-once and then append-only, or most cases read-only database, just query from old tables is good enough;
  with table name wildcards I can query all of the tables in a single query, but not sure how to handle the different input types, is there a dynamic type checking function to write the query SQL like this <code>typeof</code> operator in Javascript: ?</p>
  
  <pre><code>CASE typeof feature
    WHEN "BOOLEAN" THEN ... # handle v1
    WHEN "FLOAT"   THEN ... # handle v2
    WHEN "STRUCT"  THEN ... # handle v3
    WHEN ...
    ELSE
  END
  </code></pre>
  
  <p>or what would you suggest to work around? if the project's nature has an constantly evolving schema (because of fast moving requirement or many other common reasons)</p>