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Created Query from Multi-table (markdown)
AlexLEWIS
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Query-from-Multi-table.md
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237
Query-from-Multi-table.md
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[中文](%e5%a4%9a%e8%a1%a8%e6%9f%a5%e8%af%a2) | **English**
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```csharp
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static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
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.UseConnectionString(FreeSql.DataType.MySql, connectionString)
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.Build(); //请务必定义成 Singleton 单例模式
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class Topic {
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[Column(IsIdentity = true)]
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public int Id { get; set; }
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public string Title { get; set; }
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public int Clicks { get; set; }
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public DateTime CreateTime { get; set; }
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public int CategoryId { get; set; }
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public Category Category { get; set; }
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}
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class Category {
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[Column(IsIdentity = true)]
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public int Id { get; set; }
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public string Name { get; set; }
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public int ParentId { get; set; }
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public CategoryType Parent { get; set; }
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public List<Topic> Topics { get; set; }
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}
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class CategoryType {
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public int Id { get; set; }
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public string Name { get; set; }
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}
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```
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## 1、导航属性联表
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```csharp
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fsql.Select<Topic>()
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.LeftJoin(a => a.Category.Id == a.CategoryId)
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.LeftJoin(a => a.Category.Parent.Id == a.Category.ParentId)
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.Where(a => a.Category.Parent.Id > 0)
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.ToList();
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//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`, a__Category.`Id` as6, a__Category.`Name`, a__Category.`ParentId`
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//FROM `Topic` a
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//LEFT JOIN `Category` a__Category ON a__Category.`Id` = a.`CategoryId`
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//LEFT JOIN `CategoryType` a__Category__Parent ON a__Category__Parent.`Id` = a__Category.`ParentId`
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```
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> 提示：正确配置[导航关系](%e5%ae%9e%e4%bd%93%e5%85%b3%e7%b3%bb)后，不再需要手工调用 LeftJoin
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## 2、复杂联表
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```csharp
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fsql.Select<Topic, Category, CategoryType>()
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.LeftJoin((a,b,c) => a.CategoryId == b.Id)
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.LeftJoin((a,b,c) => b.ParentId == c.Id)
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.Where((a,b,c) => c.Id > 0)
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.ToList((a,b,c) => new { a,b,c });
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//或者
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fsql.Select<Topic>().From<Category, CategoryType>((s, b, c) => s
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.LeftJoin(a => a.CategoryId == b.Id)
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.LeftJoin(a => b.ParentId == c.Id))
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.Where((a,b,c) => c.Id > 0)
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.ToList((a,b,c) => new { a,b,c });
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//减少定义 a,b,c 写法
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fsql.Select<Topic, Category, CategoryType>()
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.LeftJoin(w => w.t1.CategoryId == w.t2.Id)
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.LeftJoin(w => w.t2.ParentId == w.t3.Id)
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.Where(w => w.t3.Id > 0)
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.ToList(w => new { w.t1,w.t2,w.t3 });
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//SELECT ...
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//FROM `Topic` a
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//LEFT JOIN `Category` b ON a.`CategoryId` = b.`Id`
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//LEFT JOIN `CategoryType` c ON b.`ParentId` = c.`Id`
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//WHERE c. `Id` > 0
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```
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> 经验：[一对多，分表只取最后一条记录](https://github.com/dotnetcore/FreeSql/issues/430)
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## 3、WithSql
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```csharp
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fsql.Select<Topic, Category, CategoryType>()
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.WithSql(
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"select * from Topic where id=@id1",
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"select * from Category where id=@id2",
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null, //不设置 CategoryType 对应的 SQL
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new { id1 = 10, id2 = 11, id3 = 13 }
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)
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.LeftJoin((a,b,c) => a.CategoryId == b.Id)
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.LeftJoin((a,b,c) => b.ParentId == c.Id)
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.ToList();
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//SELECT ...
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//FROM ( select * from Topic where id=@id1 ) a
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//LEFT JOIN ( select * from Category where id=@id2 ) b ON a.`CategoryId` = b.`Id`
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//LEFT JOIN `CategoryType` c ON b.`ParentId` = c.`Id`
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```
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> 提示：ISelect.ToSql 可与 WithSql 配合使用
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## 4、SQL联表
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```csharp
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fsql.Select<Topic>()
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.LeftJoin("Category b on b.Id = a.CategoryId and b.Name = @bname", new { bname = "xxx" })
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.ToList();
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//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`
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//FROM `Topic` a
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//LEFT JOIN Category b on b.Id = a.CategoryId and b.Name = @bname
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```
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延伸问题：SQL联表 b 表的字段如何在 ToList 中指定？
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```csharp
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.ToList(a => new {
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bid = Convert.ToInt32("b.Id"),
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bName = "b.Name"
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})
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```
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## 5、子表Exists
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```csharp
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fsql.Select<Topic>()
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.Where(a => fsql.Select<Topic>().As("b").Where(b => b.Id == a.Id).Any())
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.ToList();
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//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`
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//FROM `Topic` a
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//WHERE (exists(SELECT 1
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// FROM `Topic` b
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// WHERE (b.`Id` = a.`Id`)
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// limit 0,1))
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```
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> 提示：由于子查询的实体类与上层相同，使用 As("b") 指明别名，以便区分
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## 6、子表In
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```csharp
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fsql.Select<Topic>()
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.Where(a => fsql.Select<Topic>().As("b").ToList(b => b.Id).Contains(a.Id))
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.ToList();
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//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`
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//FROM `Topic` a
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//WHERE (((a.`Id`) in (SELECT b.`Id`
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// FROM `Topic` b)))
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```
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## 7、子表Join
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v1.8.0+ string.Join + ToList 实现将子查询的多行结果，拼接为一个字符串，如："1,2,3,4"
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```csharp
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fsql.Select<Topic>().ToList(a => new {
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id = a.Id,
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concat = string.Join(",", fsql.Select<StringJoin01>().ToList(b => b.Id))
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});
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//SELECT a.`Id`, (SELECT group_concat(b.`Id` separator ',')
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// FROM `StringJoin01` b)
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//FROM `Topic` a
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```
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> 提示：子查询 string.Join + ToList 适配了 sqlserver/pgsql/oracle/mysql/sqlite/firebird/达梦/金仓/翰高 [#405](https://github.com/dotnetcore/FreeSql/issues/405)
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## 8、子表First/Count/Sum/Max/Min/Avg
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```csharp
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fsql.Select<Category>().ToList(a => new {
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all = a,
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first = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).First(b => b.Id),
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count = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Count(),
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sum = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Sum(b => b.Clicks),
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max = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Max(b => b.Clicks),
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min = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Min(b => b.Clicks),
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avg = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Avg(b => b.Clicks)
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});
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```
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## 9、AsSelect
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```csharp
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fsql.Select<Category>()
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.Where(a => a.Topics.AsSelect().Any(b => b.Title.Contains("xx")))
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.ToList();
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```
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效果等同于：
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```csharp
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fsql.Select<Category>()
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.Where(a => fsql.Select<Topic>().Any(b => b.Title.Contains("xx")))
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.ToList();
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```
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将集合属性快速转换为 ISelect 进行子查询操作。
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## 10、WhereCascade
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多表查询时，像isdeleted每个表都给条件，挺麻烦的。WhereCascade使用后生成sql时，所有表都附上这个条件。
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如：
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```csharp
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fsql.Select<t1>()
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.LeftJoin<t2>(...)
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.WhereCascade(x => x.IsDeleted == false)
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.ToList();
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```
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得到的 SQL：
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```sql
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SELECT ...
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FROM t1
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LEFT JOIN t2 on ... AND (t2.IsDeleted = 0)
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WHERE t1.IsDeleted = 0
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```
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实体可附加表达式时才生效，支持子表查询。单次查询使用的表数目越多收益越大。
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可应用范围：
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- 子查询，一对多、多对多、自定义的子查询；
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- Join 查询，导航属性、自定义的Join查询；
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- Include/[IncludeMany](%e8%b4%aa%e5%a9%aa%e5%8a%a0%e8%bd%bd#3%E5%AF%BC%E8%88%AA%E5%B1%9E%E6%80%A7-onetomanymanytomany) 的子集合查询；
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> 暂时不支持【延时属性】的广播；
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> 此功能和【过滤器】不同，用于单次多表查询条件的传播；
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## 参考资料
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- [《返回数据》](%e8%bf%94%e5%9b%9e%e6%95%b0%e6%8d%ae)
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- [《仓储层Repository》](Repository)
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- [《LinqToSql》](LinqToSql)
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- [《优化之：延时加载》](%e5%bb%b6%e6%97%b6%e5%8a%a0%e8%bd%bd)
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- [《优化之：贪婪加载》](%e8%b4%aa%e5%a9%aa%e5%8a%a0%e8%bd%bd)
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- [《Expression 表达式函数》](%e8%a1%a8%e8%be%be%e5%bc%8f%e5%87%bd%e6%95%b0)
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- [《性能》](%e6%80%a7%e8%83%bd)
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- [《分表、分库》](%e5%88%86%e8%a1%a8%e5%88%86%e5%ba%93)
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- [《租户》](%e7%a7%9f%e6%88%b7)
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