极客教程

FreeSql在查询数据下足了功能，链式查询语法、多表查询、表达式函数支持得非常到位。

static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
    .UseConnectionString(FreeSql.DataType.MySql, "Data Source=127.0.0.1;Port=3306;User ID=root;Password=root;Initial Catalog=cccddd;Charset=utf8;SslMode=none;Max pool size=10")
    .Build(); //请务必定义成 Singleton 单例模式class Topic {
    [Column(IsIdentity = true)]    public int Id { get; set; }    public string Title { get; set; }    public int Clicks { get; set; }    public DateTime CreateTime { get; set; }    public int CategoryId { get; set; }    public Category Category { get; set; }
}class Category {
    [Column(IsIdentity = true)]    public int Id { get; set; }    public string Name { get; set; }    public int ParentId { get; set; }    public CategoryType Parent { get; set; }    public List<Topic> Topics { get; set; }
}class CategoryType {    public int Id { get; set; }    public string Name { get; set; }
}

1、导航属性联表

fsql.Select<Topic>()
  .LeftJoin(a => a.Category.Id == a.CategoryId)
  .LeftJoin(a => a.Category.Parent.Id == a.Category.ParentId)
  .Where(a => a.Category.Parent.Id > 0)
  .ToList();//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`, a__Category.`Id` as6, a__Category.`Name`, a__Category.`ParentId`//FROM `Topic` a//LEFT JOIN `Category` a__Category ON a__Category.`Id` = a.`CategoryId`//LEFT JOIN `CategoryType` a__Category__Parent ON a__Category__Parent.`Id` = a__Category.`ParentId`

提示：正确配置导航关系后，不再需要手工调用 LeftJoin

2、复杂联表

fsql.Select<Topic, Category, CategoryType>()
  .LeftJoin((a,b,c) => a.CategoryId == b.Id)
  .LeftJoin((a,b,c) => b.ParentId == c.Id)
  .Where((a,b,c) => c.Id > 0)
  .ToList((a,b,c) => new { a,b,c });//或者
fsql.Select<Topic>().From<Category, CategoryType>((s, b, c) => s
  .LeftJoin(a => a.CategoryId == b.Id)
  .LeftJoin(a => b.ParentId == c.Id))
  .Where((a,b,c) => c.Id > 0)
  .ToList((a,b,c) => new { a,b,c });//SELECT ...//FROM `Topic` a//LEFT JOIN `Category` b ON a.`CategoryId` = b.`Id`//LEFT JOIN `CategoryType` c ON b.`ParentId` = c.`Id`//WHERE c. `Id` > 0

3、WithSql

fsql.Select<Topic, Category, CategoryType>()
  .WithSql(      "select * from Topic where id=@id1",      "select * from Category where id=@id2",      null, //不设置 CategoryType 对应的 SQL      new { id1 = 10, id2 = 11, id3 = 13 }
  )
  .LeftJoin((a,b,c) => a.CategoryId == b.Id)
  .LeftJoin((a,b,c) => b.ParentId == c.Id)
  .ToList();//SELECT ...//FROM ( select * from Topic where id=@id1 ) a//LEFT JOIN ( select * from Category where id=@id2 ) b ON a.`CategoryId` = b.`Id`//LEFT JOIN `CategoryType` c ON b.`ParentId` = c.`Id`

4、SQL联表

fsql.Select<Topic>()
  .LeftJoin("Category b on b.Id = a.CategoryId and b.Name = @bname", new { bname = "xxx" })
  .ToList();//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`//FROM `Topic` a//LEFT JOIN Category b on b.Id = a.CategoryId and b.Name = @bname

延伸问题：SQL联表 b 表的字段如何在 ToList 中指定？

.ToList(a => new
{
  bid = Convert.ToInt32("b.Id"),
  bName = "b.Name"
})