To use custom expression, create a Custom Column (where the custom expression is used as a Field Formula to calculate values for the new column), or click on Filter or Summarize and select Custom Expression. Custom expressions to create filters, metrics, and custom columns You can also skip to the complete list of expressions. They are the power tools in the notebook editor of the query builder that allow you to ask more complicated questions. Custom expressions are like formulas in spreadsheet software like Excel, Google Sheets, and LibreOffice Calc.
