if else if in expressions

[qlikvw2019]

I have 

column1 ,column2, column3

Below expression works fine : "If column1 & column2 are selected , then , else"

If(GetSelectedCount(column1)>=1 and GetSelectedCount(column2)>=1,MAX(PDB_SIZE),num(sum(DISTINCT(column2))/1024,'##.00'))

if i want to introduce more conditions  , please help with syntax

New condition: "If no columns selected, then"

[wandererroch · Accepted solution]

It would be

if(Some Comparison,Value if true,

       if(Another Comparison, Value if that's true,

              if(more comparisons,True again, The false value)

         )

  )

Basically the other expressions nest within each other in the section you hit when the condition isn't met.