Excel Power Query - Remove Column if it exists, otherwise don't try - powerquery

Question

I have a calculated Column Custom = Column1 + Column2 - Column3
After the calculation, i need to delete all columns except Custom
Problem is sometimes one of the columns [Column4] does not exist in the dataset
I can have the the Custom calculate properly with "try otherwise" as in:
#"Added Custom" = Table.AddColumn(#"Previous Step", "Custom", each [Column2]+[Column3]- (try [Column4] otherwise 0)),
#"Removed Columns7" = Table.RemoveColumns(#"Added Custom",{"Column2", "Column3", "Column4"}),
This works fine, however the second step fails if [column4] doesn't exist.
So i need a way to test if [Column4] exists and remove it if it does, otherwise don't try to.

Answer 1

how about
#"Added Custom" = Table.AddColumn(#"Previous Step", "Custom", each [Column2]+[Column3]- (try [Column4] otherwise 0)),
#"Removed Columns" = try Table.RemoveColumns(#"Added Custom,{"Column4"}) otherwise #"Added Custom"

Answer 2

One way to approach this is to select the columns you want to keep rather than removing columns you don't want. This is equivalent to removing all except the columns you specify.
Alternatively, you can intersect the all of table columns with your list.
Table.RemoveColumns(
#"Added Custom",
List.Intersect(
{
Table.ColumnNames(#"Added Custom"),
{"Column2", "Column3", "Column4"}
}
)
)

Answer 3

Another way:
add = Table.AddColumn(Source, "Custom", each List.Sum({[Column2],[Column3],-[Column4]?})),
del = Table.RemoveColumns(add,{"Column2", "Column3", "Column4"}, 1)

Answer 4

If the only column you want to retain is Custom, then just use Table.SelectColumns.
If there might be other columns you want to retain, you can select them also or you can generate a list of columns to remove.
From what you write, it seems you want to remove any columns whose name starts with Column. If that is the case, here is one method:
#"Removed Columns"= Table.RemoveColumns(#"Previous Step",
List.Select(Table.ColumnNames(#"Previous Step"), each Text.StartsWith(_,"Column")))