Fluent nHibernate QueryOver SQL 'CASE' 等效项

使用Fluent nHibernate QueryOver来编写查询语句时，有时我们需要在查询中使用SQL的CASE语句。CASE语句可以根据条件返回不同的值，非常灵活实用。在Fluent nHibernate QueryOver中，我们可以使用等效项来实现CASE语句的功能。

csharp
var result = session.QueryOver()
    .SelectList(list => list
        .Select(x => x.Id)
        .Select(Projections.Conditional(
            Restrictions.Where(x => x.Value > 0),
            Projections.Constant("Positive"),
            Projections.Constant("Negative")
        ))
    )
    .List();

csharp
var result = session.QueryOver()
    .SelectList(list => list
        .Select(x => x.Id)
        .Select(Projections.Switch(
            Projections.Conditional(
                Restrictions.Where(x => x.Value > 0),
                Projections.Constant(1)
            ),
            Projections.Conditional(
                Restrictions.Where(x => x.Value < 0),
                Projections.Constant(-1)
            ),
            Projections.Constant(0)
        ))
    )
    .List();