EntityFrameworkCore

left join

var query = from log in logRepository.Table.Include(a => a.User)
                        join
                        user in logRepository.Table on log.UserID equals user.ID into lu
                        from lu1 in lu.DefaultIfEmpty()
                        select new { log.Title,log.Description,log.User.Name};

生成的sql

SELECT  [log].[ID], [log].[Name], [log].[CompanyName], [log.User].[Password]
FROM [Log] AS [log]
LEFT JOIN [User] AS [log.User] ON [log].[UserID] = [log.User].[ID]
LEFT JOIN [User] AS [user] ON [log].[UserID] = [user].[ID]

log.UserID 为可空类型，否则生成的sql中会是inner jion  

延迟加载

建议不要使用

每次用到延迟加载属性，都会查一次数据库；

比如列表中(100行)访问延迟加载属性，会多查100次数据库；

比如序列化的时候，也会根据延迟加载属性查询数据库；