Database design help required
This doesn't call for any SCD design -- the state of an entity is not changing, slowly or otherwise. What you have is a series of events that you want to refer to the user associated with the event.

create table Executions(
    ID      int not null auto_increment,
    UserID  int not null,
    ExcDate date not null,
    &lt;any other data concerning the Execution event&gt;,
    constraint PK_Executions primary key( ID ),
    constraint FK_Execution_User foreign key( UserID )
        references Users( ID ),
);


Then remove any Execution related data (ExecutionID) from the Users table. Each execution event relates back to the associated user. So the Users table contains only user data and the Executions table contains only execution event data, including the user that performed the execution. And it maintains all such events not just the first and/or last.