c# - SQL to LINQ - 案例语句

Question

我第一次将 SQL 语句重写到 LINQ。我不太确定如何正确写出我的案例陈述。运行时不喜欢我写的方式（我以这个链接为例）

我的目标是按照案例陈述组织我的数据。请让我知道我可以修改什么来实现我的目标。

我的错误说：

    InvalidOperationException: The LINQ expression 'DbSet<Table>
    .Where(c => c.ID ==(Nullable<int>)10)
    .OrderBy(c => c.column1)
    .ThenBy(c => _sortOrder_0.TryGetValue(
    key: c.column3,
    value:_order_1)?_order_1:_defaultOrder_2)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), ToList(), or ToListAsync()

原始 SQL 查询：

    SELECT column1, column2, column3 from table where ID = # order by column1, 
    Case column3 When 'X' Then 1 When 'Y' Then 2 When 'Z' Then 3 End

林克：

public async Task<IActionResult> Index()
{
    var sortOrder = new Dictionary<string, int>
        {
            {"X", 1 },
            {"Y", 2 },
            {"Z", 3 },
        };
    var defaultOrder = sortOrder.Max(x => x.Value) + 1;
    int order;
    var dataTest = _context.TableModel
        .Where(x =>
        (x.ID == 10)
        )
        .Select(x => new TableModel
        {
            column1 = x.column1,
            column2 = x.column2,
            column3 = x.column3,
        })
        .OrderBy(x => x.column1)
        .ThenBy(x => sortOrder.TryGetValue(x.column3, out order) ? order : defaultOrder)
        .AsNoTracking()
        .ToListAsync();
}

旁注：我使用 ID = 10 进行测试。感谢您的时间！

标签： c#sqllinqlinq-to-sqlentity-framework-core

Answer 1

ThenBy尝试对子句中的所有情况使用三元运算符：

 .ThenBy(x => x.column3 == "X" 
      ? 1 
      : x.column3 == "Y" 
          ? 2 
          : x.column3 == "Z" 
                ? 3 
                : defaultOrder)