very useful functions: ISNULL and NULLIF -

this we can use when we want to replace null with your own value.

ISNULL

columnName - is null here
my value -- your custom value

select ISNULL(columnName ,'my value') as newvalue, *  from  table



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 NULLif

here function will display NULL if columnName mataches with yourValue
else it will display column's value from table

select NULLif( columnName,'yourValue') as NEWcurrencycode, * from  table