FreeSql （十九）多表查询 - 好文

多表查询，常用的有联表 LeftJoin/InnerJoin/RightJoin ，这三个方法在上篇文章
<https://www.cnblogs.com/FreeSql/p/11531346.html>已经介绍过。

除了联表，还有子查询 Where Exists，和 Select 子表：
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();
[Table(Name = "tb_topic")] class Topic { [Column(IsIdentity = true, IsPrimary =
true)] public int Id { get; set; } public int Clicks { get; set; } public int
TestTypeInfoGuid { get; set; } public string Title { get; set; } public
DateTime CreateTime { get; set; } }
子表 Exists
var sql2222 = fsql.Select<Topic>().Where(a => fsql.Select<Topic>().Where(b =>
b.Id == a.Id).Any()).ToList(); // SELECT a.`Id`, a.`TypeGuid`, a.`Title`,
a.`CreateTime` // FROM `xxx` a // WHERE (exists(SELECT 1 // FROM `xxx` b //
WHERE (b.`Id` = a.`Id`))) //两级相同的子表查询 sql2222 = fsql.Select<Topic>().Where(a =>
fsql.Select<Topic>().Where(b => b.Id == a.Id && fsql.Select<Topic>().Where(c =>
c.Id == b.Id).Where(d => d.Id == a.Id).Where(e => e.Id == b.Id) .Offset(a.Id)
.Any() ).Any() ).ToList(); // SELECT a.`Id`, a.`TypeGuid`, a.`Title`,
a.`CreateTime` // FROM `xxx` a // WHERE (exists(SELECT 1 // FROM `xxx` b //
WHERE (b.`Id` = a.`Id` AND exists(SELECT 1 // FROM `xxx` c // WHERE (c.`Id` =
b.`Id`) AND (c.`Id` = a.`Id`) AND (c.`Id` = b.`Id`) // limit 0,1)) // limit
0,1))
子表 First/Count/Sum/Max/Min/Avg
var subquery = fsql.Select<Topic>().ToSql(a => new { all = a, first =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).First(b => b.Id), count =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).Count(), sum =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).Sum(b => b.Score), max =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).Max(b => b.Score), min =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).Min(b => b.Score), avg =
fsql.Select<Child>().Where(b => b.ParentId == a.Id).Avg(b => b.Score) });
系列文章导航

*
（一）入门 <https://www.cnblogs.com/FreeSql/p/11531300.html>

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（二）自动迁移实体 <https://www.cnblogs.com/FreeSql/p/11531301.html>

*
（三）实体特性 <https://www.cnblogs.com/FreeSql/p/11531302.html>

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（四）实体特性 Fluent Api <https://www.cnblogs.com/FreeSql/p/11531304.html>

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（五）插入数据 <https://www.cnblogs.com/FreeSql/p/11531306.html>

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（六）批量插入数据 <https://www.cnblogs.com/FreeSql/p/11531309.html>

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（七）插入数据时忽略列 <https://www.cnblogs.com/FreeSql/p/11531316.html>

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（八）插入数据时指定列 <https://www.cnblogs.com/FreeSql/p/11531318.html>

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（九）删除数据 <https://www.cnblogs.com/FreeSql/p/11531320.html>

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（十）更新数据 <https://www.cnblogs.com/FreeSql/p/11531321.html>

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（十一）更新数据 Where <https://www.cnblogs.com/FreeSql/p/11531324.html>

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（十二）更新数据时指定列 <https://www.cnblogs.com/FreeSql/p/11531327.html>

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（十三）更新数据时忽略列 <https://www.cnblogs.com/FreeSql/p/11531334.html>

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（十四）批量更新数据 <https://www.cnblogs.com/FreeSql/p/11531335.html>

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（十五）查询数据 <https://www.cnblogs.com/FreeSql/p/11531339.html>

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（十六）分页查询 <https://www.cnblogs.com/FreeSql/p/11531341.html>

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（十七）联表查询 <https://www.cnblogs.com/FreeSql/p/11531346.html>

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（十八）导航属性 <https://www.cnblogs.com/FreeSql/p/11531352.html>

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（十九）多表查询

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（二十）多表查询 WhereCascade <https://www.cnblogs.com/FreeSql/p/11531372.html>

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（二十一）查询返回数据 <https://www.cnblogs.com/FreeSql/p/11531376.html>

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（二十二）Dto 映射查询 <https://www.cnblogs.com/FreeSql/p/11531381.html>

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（二十三）分组、聚合 <https://www.cnblogs.com/FreeSql/p/11531384.html>

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（二十四）Linq To Sql 语法使用介绍 <https://www.cnblogs.com/FreeSql/p/11531392.html>

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（二十五）延时加载 <https://www.cnblogs.com/FreeSql/p/11531395.html>

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（二十六）贪婪加载 Include、IncludeMany、Dto、ToList
<https://www.cnblogs.com/FreeSql/p/11531404.html>

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（二十七）将已写好的 SQL 语句，与实体类映射进行二次查询
<https://www.cnblogs.com/FreeSql/p/11531416.html>

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（二十八）事务 <https://www.cnblogs.com/FreeSql/p/11531423.html>

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（二十九）Lambda 表达式 <https://www.cnblogs.com/FreeSql/p/11531425.html>

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（三十）读写分离 <https://www.cnblogs.com/FreeSql/p/11531430.html>

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（三十一）分区分表 <https://www.cnblogs.com/FreeSql/p/11531435.html>

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（三十二）Aop <https://www.cnblogs.com/FreeSql/p/11531471.html>

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（三十三）CodeFirst 类型映射 <https://www.cnblogs.com/FreeSql/p/11531543.html>

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（三十四）CodeFirst 迁移说明 <https://www.cnblogs.com/FreeSql/p/11531550.html>

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（三十五）CodeFirst 自定义特性 <https://www.cnblogs.com/FreeSql/p/11531576.html>

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