我想创建一个等同于Left Join
我的表格设置如下:
Recipe
RecipeID
...
Instruction
RecipeID
StepID
SomeFlag
...
等效SQL:
SELECT *
FROM Recipe r
LEFT JOIN Instruction i
ON r.RecipeID = i.RecipeID
AND SomeFlag > 0
这是我到目前为止所做的:
var tmp = db.Recipe
.GroupJoin(
db.Instruction,
r => r.RecipeID,
i => i.RecipeID,
(r, i) => new {r, i},
???);
首先,GroupJoin
是这种操作的正确选择吗?据我所知,Join相当于SQL'Internal Join',而GroupJoin相当于'Left Join'。第二,获得我想要的结果的正确语法是什么?我一直在寻找一段时间,我似乎找不到合适的答案使用扩展方法。
答案 0 :(得分:5)
不要忘记阅读(GroupJoin
:MSDN http://msdn.microsoft.com/en-us/library/bb535047.aspx和Join
MSDN http://msdn.microsoft.com/fr-fr/library/bb534675.aspx)
GroupJoin
和Join
的最后一个参数是可选的(通过重载),通常不使用。
该功能允许您指定如何将r.RecipeID
与i.RecipeID
进行比较。由于RecipeID
必须是整数,因此使用默认比较器是一个不错的选择。所以请用:
var tmp = db.Recipe
.Join(db.Instruction,
r => r.RecipeID,
i => i.RecipeID,
(r, i) => new {r, i});
现在您要删除具有SomeFlag > 0
的所有说明。加入前为什么不这样做呢?
像这样:
var tmp = db.Recipe
.Join(db.Instruction.Where(instruction => instruction.SomeFlag > 0),
r => r.RecipeID,
i => i.RecipeID,
(r, i) => new {r, i});
@usr已经完美评论说Join
执行INNER JOIN。
正如您可能已经注意到的那样,LINQ没有针对INNER,OUTER,LEFT,RIGHT联接的不同方法。要了解特定SQL连接的等效LINQ,您可以在MSDN(http://msdn.microsoft.com/en-us/library/vstudio/bb397676.aspx)上找到帮助。
var tmp = from recipe in Recipes
join instruction in
from instruction in Instructions
where instruction.SomeFlag > 0
select instruction
on recipe.RecipeID equals instruction.RecipeID into gj
from instruction in gj.DefaultIfEmpty()
select new
{
recipe,
instruction
};
使用扩展方法这是一个丑陋的解决方案:
var tmp = Recipes.GroupJoin(Instructions.Where(instruction => instruction.SomeFlag > 0),
recipe => recipe.RecipeID,
instruction => instruction.RecipeID,
(recipe, gj) => new { recipe, gj })
.SelectMany(@t => @t.gj.DefaultIfEmpty(),
(@t, instruction) => new
{
@t.recipe,
instruction
});
答案 1 :(得分:1)
请告诉我,如果我不理解你,但这个扩展方法返回的结果与你在sql中提供的结果相同。
public static IEnumerable<ResultType> GetLeftJoinWith(this IEnumerable<Recipe>, IEnumerable<Instructions> ins)
{
var filteredInstructions = ins.Where(x => x.SomeFlag > 0);
var res = from r in rec
join tmpIns in filteredInstructions on r.RecipeID equals t.RecipeID into instructions
from instruction in instructions.DefaultIfEmpty()
select new { r, instruction };
return res;
}
答案 2 :(得分:0)
尝试
var model = db.Recipe
.GroupJoin(db.Instructions.Where(instruction => instruction.SomeFlag > 0),r => r.RecipeID,i => i.RecipeID, (r, i) => new { Recipe = r, Instructions = i })
.SelectMany(t => t.Instructions.DefaultIfEmpty(),(t, Instructions) => new
{
Recipe = t.Recipe,
Instructions = Instructions
});