At the beginning Google proposed a query language that called legacy SQL. This language was close to the classic SQL. Then Google decided to propose another language called standard SQL. This language is closer to SQL than legacy and allows generally more functionalities. Moreover this language is pushed by Google and Google won’t maintain 2 languages that’s why the first advice is use standard SQL
For going further I invite you to read this page : https://cloud.google.com/bigquery/docs/reference/standard-sql/migrating-from-legacy-sql
If you uncheck the box in yellow you activate standard SQL
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