BigQuery has two SQL dialects for querying tables. They are legacy SQL, which was developed by Google, and standard SQL, which is compliant with SQL 2011 standards. Users can use either of the dialects—whichever is easy for them to learn and use. The built-in functions vary between legacy SQL and standard SQL.
If a query is written in the legacy SQL dialect, then it will be preceded with #legacySQL for your reference. If a query is written in standard SQL, then it will be preceded with #standardSQL. By default, the BigQuery console and BQ utility in Google Cloud SDK execute queries as legacy SQL.
SQL in legacy and standard mode is not case sensitive for keywords used in a query, but it is case sensitive for table names, project names, and dataset names in the query. Column names are not case sensitive.