Replace Values from one table to another

Hi adoster ,

In Power Query:

let
    Source = Table.NestedJoin(#"SQL_Table A", {"ID"}, Excel_TableB, {"ID"}, "Excel_TableB", JoinKind.FullOuter),
    #"Expanded Excel_TableB" = Table.ExpandTableColumn(Source, "Excel_TableB", {"ID", "Code"}, {"Excel_TableB.ID", "Excel_TableB.Code"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Excel_TableB", "Custom", each if [Code] = "Error" then [Excel_TableB.Code] else [Code]),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Code", "Excel_TableB.ID", "Excel_TableB.Code"})
in
    #"Removed Columns"

Use DAX:

Column =
IF (
    'SQL_Table A'[Code] = "Error",
    LOOKUPVALUE ( Excel_TableB[Code], Excel_TableB[ID], 'SQL_Table A'[ID] ),
    'SQL_Table A'[Code]
)

Best Regards,

Jay