FreeSql （二十三）分组、聚合

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

[Table(Name = "tb_topic")]
class Topic {
    [Column(IsIdentity = true, IsPrimary = true)]
    public int Id { get; set; }
    public int Clicks { get; set; }
    public string Title { get; set; }
    public DateTime CreateTime { get; set; }
}

分组聚合

var groupby = fsql.Select()
    .GroupBy(a => new { tt2 = a.Title.Substring(0, 2), mod4 = a.Id % 4 })
    .Having(a => a.Count() > 0 && a.Avg(a.Key.mod4) > 0 && a.Max(a.Key.mod4) > 0)
    .Having(a => a.Count() < 300 || a.Avg(a.Key.mod4) < 100)
    .OrderBy(a => a.Key.tt2)
    .OrderByDescending(a => a.Count())
    .ToList(a => new { a.Key.tt2, cou1 = a.Count(), arg1 = a.Avg(a.Key.mod4) });
//SELECT substr(a.`Title`, 1, 2) as1, count(1) as2, avg((a.`Id` % 4)) as3 
//FROM `xxx` a 
//GROUP BY substr(a.`Title`, 1, 2), (a.`Id` % 4) 
//HAVING (count(1) > 0 AND avg((a.`Id` % 4)) > 0 AND max((a.`Id` % 4)) > 0) AND (count(1) < 300 OR avg((a.`Id` % 4)) < 100) 
//ORDER BY substr(a.`Title`, 1, 2), count(1) DESC

不分组求聚合值，请使用 ToAggregate 替代 ToList

导航属性分组

假如 Topic 有导航属性 Category，Category 又有导航属性 Area，导航属性分组代码如下：

var list = fsql.Select()
    .GroupBy(a => new { a.Clicks, a.Category })
    .ToList(a => new { a.Key.Category.Area.Name });

注意：如上这样编写，会报错无法解析 a.Key.Category.Area.Name，解决办法使用 Include：

var list = fsql.Select()
    .Include(a => a.Category.Area)
    //必须添加此行，否则只分组 Category 而不包含它的下级导航属性 Area

    .GroupBy(a => new { a.Clicks, a.Category })
    .ToList(a => new { a.Key.Category.Area.Name });

但是，你还可以这样解决：

var list = fsql.Select()
    .GroupBy(a => new { a.Clicks, a.Category, a.Category.Area })
    .ToList(a => new { a.Key.Area.Name });

API

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