var query = from k in DBContext.KCYD join i in DBContext.ITEM on k.SPXXID equals i.ITEM_ID into g from gc in g.DefaultIfEmpty() join s in DBContext.SPKC on k.SpxxId equals s.SPXXID into g1 from gc1 in g1.DefaultIfEmpty() where k.Ckid == gc1.CKID select new { ... ... };
但是生成的SQL语句，KCYD和ITEM表是LEFT OUTER JOIN的，但是联SPKC表却变成了INNER JOIN，这是为啥呢，经过一番折腾下来，发现问题出在where k.Ckid == gc1.CKID，如果把这个条件去掉的话，那就成了LEFT OUTER JOIN了，然后我就在想这个条件应该放在哪呢，LINQ里面到底支不支持联合主键的问题呢，在网上搜了半天，发现可以用 on new {字段1,字段2} equals new {字段1,字段2} into g的方法，于是修改代码如下：
var query = from k in DBContext.KCYD join i in DBContext.ITEM on k.SPXXID equals i.ITEM_ID into g from gc in g.DefaultIfEmpty() join s in DBContext.SPKC on new {k.SpxxId,k.Ckid} equals new {s.SPXXID,s.CKID} into g1 from gc1 in g1.DefaultIfEmpty() select new { ... ... };
但是很不给力的是这样居然提示错误：The type arguments cannot be inferred from the query. 简直就是杯具，难道LINQ不支持这样搞？唉，在我绝望的时候同事为我看出了端倪，原来equals两边的参数字段名的大小写必须完全匹配。即完整代码如下：
var query = from k in DBContext.KCYD join i in DBContext.ITEM on k.SPXXID equals i.ITEM_ID into g from gc in g.DefaultIfEmpty() join s in DBContext.SPKC on new {SPXXID=k.SpxxId,CKID=k.Ckid} equals new {s.SPXXID,s.CKID} into g1 from gc1 in g1.DefaultIfEmpty() select new { ... ... };
