EF 查询表数据，如果查询外键表，且这个外键有可能为空，但主表资料也就查不出来的解决方法

var items = from x in ctx.MES_Entity_RegisterMeasureES

join y in ctx.MES_Code_TypesFieldProduct on x.ProductTypeCode equals y.ProductTypeCode

into temp

from tt in temp.DefaultIfEmpty()

join z in ctx.MES_Entity_RegisterProductionUnits on x.ProductionUnitID equals z.ProductionUnitID

into temp1

from bb in temp1.DefaultIfEmpty()

select new RegisterMeasureESModel() { ProductionUnitName = bb.ProductionUnitName ,

ProductType=tt.ProductType };