Update Query

Hi All,

I want to Update column A1, when column A2 and A3both are NULL or have invalid data.

If A2 has Valid Data ignore next steps/do nothing ,if A2 it has NULL or INVALID data check column column A3, IF A3 has VALID DATA ignore next steps/do nothing , IF A3 has NULL or INVALID data update column A1 as BADDATA

syntax plz

Please help