Need help on DAX function

Learner-PBI , try this dax
NewColumn =
IF(
ISBLANK('Table'[column1]) && ISBLANK('Table'[column2]),
"null",
IF(
ISBLANK('Table'[column1]),
'Table'[column2],
IF(
ISBLANK('Table'[column2]),
'Table'[column1],
'Table'[column1] & " - " & 'Table'[column2]
)
)
)

Hi Learner-PBI ,

NewColumn = IF('Table'[column1] = BLANK() && 'Table'[Column2] = BLANK(), "null", 'Table'[Column1] & "-" & 'Table'[Column2])

Or add a condition: connect with ‘-’ only if neither column is blank:

NewColumn = 
SWITCH(
    TRUE(),
    'Table'[Column1]=BLANK() && 'Table'[Column2]=BLANK(), "null",
    'Table'[Column1]=BLANK(), 'Table'[Column2],
    'Table'[Column2]=BLANK(), 'Table'[Column1],
    'Table'[Column1] & "-" & 'Table'[Column2]
)

The logic of the code provided by bhanu_gautam is fine, the problem should be in the emptystring with some features of ISBLANK():

ISBLANK – DAX Guide

Best Regards,
Gao
Community Support Team

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