how to use sum(isnull(value),0) this sql query in powerbi

Try with ISBLANK()

Example:

=IF( ISBLANK('CalculatedMeasures'[PreviousYearTotalSales])  
   , 0  
   , ( 'CalculatedMeasures'[Total Sales]-'CalculatedMeasures'[PreviousYearTotalSales] )  
      /'CalculatedMeasures'[PreviousYearTotalSales])  

See more:

https://msdn.microsoft.com/query-bi/dax/isblank-function-dax