聯接是指將乙個資料來源物件與另乙個資料來源物件進行關聯或者聯合的操作。這兩個資料來源物件通過乙個共同的值或者屬性進行關聯。
linq有兩個聯接操作符：join和groupjoin。
join操作符類似於t-sql中的inner join，它將兩個資料來源相聯接，根據兩個資料來源中相等的值進行匹配。例如，可以將產品表與產品類別表相聯接，得到產品名稱和與其相對應的類別名稱。以下的**演示了這一點：
//查詢語法
var query =
(from p in db.products
join c in db.categories on p.categoryid equals c.categoryid
where p.categoryid == 1
select new ).tolist();
//方法語法
var q =
db.products
.join
(db.categories,
p => p.categoryid,
c => c.categoryid,
(p, c) => new
).where(p => p.categoryid == 1)
.tolist();
//查詢語法
var query =
(from c in db.categories
join p in db.products on c.categoryid equals p.categoryid into r
select new
).tolist();
//方法語法
var q =
db.categories
.groupjoin
(db.products,
c => c.categoryid,
p => p.categoryid,
(c, p) => new
).tolist();
生成的sql:
select
[project1].[categoryid] as [categoryid],
[project1].[categoryname] as [categoryname],
[project1].[c1] as [c1],
[project1].[productid] as [productid],
[project1].[productname] as [productname],
[project1].[supplierid] as [supplierid],
[project1].[categoryid1] as [categoryid1],
[project1].[quantityperunit] as [quantityperunit],
[project1].[unitprice] as [unitprice],
[project1].[unitsinstock] as [unitsinstock],
[project1].[unitsonorder] as [unitsonorder],
[project1].[reorderlevel] as [reorderlevel],
[project1].[discontinued] as [discontinued]
from ( select
[extent1].[categoryid] as [categoryid],
[extent1].[categoryname] as [categoryname],
[extent2].[productid] as [productid],
[extent2].[productname] as [productname],
[extent2].[supplierid] as [supplierid],
[extent2].[categoryid] as [categoryid1],
[extent2].[quantityperunit] as [quantityperunit],
[extent2].[unitprice] as [unitprice],
[extent2].[unitsinstock] as [unitsinstock],
[extent2].[unitsonorder] as [unitsonorder],
[extent2].[reorderlevel] as [reorderlevel],
[extent2].[discontinued] as [discontinued],
case when ([extent2].[productid] is null) then cast(null as int) else 1 end as [c1]
from [dbo].[categories] as [extent1]
left outer join [dbo].[products] as [extent2] on [extent1].[categoryid] = [extent2].[categoryid]
) as [project1]
order by [project1].[categoryid] asc, [project1].[c1] asc
下面的示例中使用了產品表，以categoryid作為分組關鍵值，按照產品類別對產品進行了分組。
//查詢語法
var query =
(from p in db.products
group p by p.categoryid).tolist();
//方法語法
var q =
db.products
.groupby(p => p.categoryid)
.tolist();
串聯是乙個將兩個集合聯接在一起的過程。在linq中，這個過程通過concat操作符來實現。
在下面的示例中，將會把類別名稱串聯在產品名稱之後：
//方法語法
var q =
db.products
.select(p => p.productname)
.concat
(db.categories.select(c => c.categoryname)
).tolist();
生成的sql:
select
[unionall1].[productname] as [c1]
from (select
[extent1].[productname] as [productname]
from [dbo].[products] as [extent1]
union all
select
[extent2].[categoryname] as [categoryname]
from [dbo].[categories] as [extent2]) as [unionall1]
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