Group Aggregation Query

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static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
    .UseConnectionString(FreeSql.DataType.MySql, connectionString)
     //Be sure to define as singleton mode
    .Build(); 

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; }
}

Group Aggregation

var list = fsql.Select<Topic>()
    .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, cou1 = a.Count(), arg1 = a.Avg(a.Value.Clicks) });

//SELECT substr(a.`Title`, 1, 2) as1, count(1) as2, avg(a.`Id`) as3 
//FROM `Topic` 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

To find the aggregate value without grouping, please use ToAggregate instead of ToList

Navigation Property Grouping

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<Topic>()
    .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<Topic>()
    .Include(a => a.Category.Area)
    //This line must be added, 
    //otherwise only the Category will be grouped without its sub-navigation property Area

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

However, you can also solve it like this:

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

Multi-table Grouping

var list = fsql.Select<Topic, Category, Area>()
    .GroupBy((a, b, c) => new { a.Title, c.Name })
    .Having(g => g.Count() < 300 || g.Avg(g.Value.Item1.Clicks) < 100)
    .ToList(g => new { count = g.Count(), Name = g.Key.Name });

• g.Value.Item1 corresponds to Topic
• g.Value.Item2 corresponds to Category
• g.Value.Item3 corresponds to Area

Aggregate

var list = fsql.Select<Topic>()
    .Aggregate(a => Convert.ToInt32("count(distinct title)"), out var count)
    .ToList();

API

Method	Return	Parameter	Description
ToSql	string		返回即将执行的SQL语句
ToList<T>	List<T>	Lambda	执行SQL查询，返回指定字段的记录，记录不存在时返回 Count 为 0 的列表
ToList<T>	List<T>	string field	执行SQL查询，返回 field 指定字段的记录，并以元组或基础类型(int,string,long)接收，记录不存在时返回 Count 为 0 的列表
ToAggregate<T>	List<T>	Lambda	执行SQL查询，返回指定字段的聚合结果（适合不需要 GroupBy 的场景）
Sum	T	Lambda	指定一个列求和
Min	T	Lambda	指定一个列求最小值
Max	T	Lambda	指定一个列求最大值
Avg	T	Lambda	指定一个列求平均值
【Grouping】
GroupBy	<this>	Lambda	按选择的列分组，GroupBy(a => a.Name)
GroupBy	<this>	string, parms	按原生sql语法分组，GroupBy("concat(name, @cc)", new { cc = 1 })
Having	<this>	string, parms	按原生sql语法聚合条件过滤，Having("count(name) = @cc", new { cc = 1 })
【Members】
Key			返回 GroupBy 选择的对象
Value			返回主表 或 From<T2,T3....> 的字段选择器

Reference

• 《多表查询》
• 《返回数据》
• 《LinqToSql》
• 《仓储层Repository》
• 《过滤器、全局过滤器》
• 《优化之：延时加载》
• 《优化之：贪婪加载》
• 《Expression 表达式函数》
• 《性能》
• 《租户》