ISNULL & COALESCE

ISNULL

ISNULL(someValue, '') -- if someValue is null, then replace it with '', and return ''

Note: it is considered bad practice to do ISNULL(colName, ‘’) because it blocks index seeks

it is better to do colName IS NULL

COALESCE

COALESCE(someValue, someValue2, 'Default') -- when both values are null, return 'Default'