在 Projection.Conditionals 中添加多个条件用于查询

Adding more than one condition in Projection.Conditionals for queryover(在 Projection.Conditionals 中添加多个条件用于查询)

问题描述

我正在尝试编写一个包含多个 when 子句的案例；像这样:

I am trying to write a case with more than one when clause; something like this:

...
case
    when 'starks' then 1
    when 'wildlings' then 2
    when 'lannisters' then 3
    Else 0
End
...

我之前已经用类似的东西做了一个条件

I've done a single conditional before with something like

.OrderBy(Projections.Conditional(
    Restrictions.Where<House>(r => r.Name.IsLike("starks")),
    Projections.Constant(0),
    Projections.Constant(1))).Asc();

但我不知道如何在其中添加额外的条件/when 子句:/我尝试添加额外的外部条件、额外的限制等，但总是以语法错误结束..

But I can't figure out how to add an extra condition / when clause in there :/ I've tried adding an extra outer conditional, extra restriction etc, but always end up with syntax error..

感谢您的帮助.

推荐答案

Projections.Conditional返回IProjection，其签名为:

/// <summary>
/// Conditionally return the true or false part, dependention on the criterion
/// </summary>
/// <param name="criterion">The criterion.</param><param name="whenTrue">The when true.
///    </param><param name="whenFalse">The when false.</param>
/// <returns/>
public static IProjection Conditional(ICriterion criterion
                                    , IProjection whenTrue
                                    , IProjection whenFalse);

这意味着，第三个参数也可以是这个条件投影:

And that means, that the third parameter can again be this Conditional projection:

.OrderBy
(
    Projections.Conditional(
        Restrictions.Where<House>(r => r.Name.IsLike("starks")),
        Projections.Constant(1),
        Projections.Conditional(
            Restrictions.Where<House>(r => r.Name.IsLike("wildlings")),
            Projections.Constant(2),
            Projections.Conditional(
                Restrictions.Where<House>(r => r.Name.IsLike("lannisters")),
                Projections.Constant(3),
                Projections.Constant(0)
                )
            )
        )
)
.Asc()

生成的 SQL 将如下所示:

The generated SQL will look like:

ORDER BY 
(case when this_.Name LIKE 'starks'     then 1 else 
(case when this_.Name LIKE 'wildlings'  then 2 else 
(case when this_.Name LIKE 'lannisters' then 3 else 0 end) end) end) asc

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