Linq Join

Linq的Join 相當於 SQL的 Join

目前Linq有兩種方法

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(

    this IEnumerable<TOuter> outer, //被Join的資料

    IEnumerable<TInner> inner, //要Join的資料

    Func<TOuter, TKey> outerKeySelector, //與inner相同的資料 

    Func<TInner, TKey> innerKeySelector, //與outer相同的資料

    Func<TOuter, TInner, TResult> resultSelector); //select 後的資料

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(

    this IEnumerable<TOuter> outer,

    IEnumerable<TInner> inner,

    Func<TOuter, TKey> outerKeySelector,

    Func<TInner, TKey> innerKeySelector,

    Func<TOuter, TInner, TResult> resultSelector,

    IEqualityComparer<TKey> comparer); //客製比較器

from c in Customers

join o in Orders on c.CustomerID equals o.CustomerID

select c.ContactName

可以反寫成

Customers

   .Join (

      Orders,

      c => c.CustomerID,

      o => o.CustomerID,

      (c, o) => c.ContactName

   )

又可以變成 Customers.Join(Orders , Customer => c , Orders => o ,(c , o ) => c.ContactName);

在SQL中可以表現是

SELECT c.ContactName

From Customers as c

Join

Orders as o

On c.CustomerID = o.CustomerID

而 Linq的Join 是 Inner Join

差距大概在這邊，因此假如要做資料一定要有值 沒值就Null了 記得要加Null判斷

參考:https://ithelp.ithome.com.tw/articles/10196394?sc=iThelpR

https://vector.cool/mysql-%E5%9C%96%E8%A7%A3-left-join-inner-joinright-join-%E7%9A%84%E5%B7%AE%E7%95%B0%E5%8F%8A%E7%AF%84%E4%BE%8B/