From fbcbf224e8f78e430cbd01d2a815d8af821097cc Mon Sep 17 00:00:00 2001
From: AlexLEWIS <liuyis@live.com>
Date: Fri, 6 Aug 2021 11:39:52 +0800
Subject: [PATCH] Created Delete Data (markdown)

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+[中文](%e5%88%a0%e9%99%a4) | **English**
+
+Deleting data is a very dangerous operation. FreeSql does not support deletion very powerfully. By default, it only supports single-table and conditional deletion methods.
+
+If it is executed when the `Where` condition is empty, only `0` or the `default` value is returned, and no real SQL delete operation is performed.
+
+```csharp
+static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
+    .UseConnectionString(FreeSql.DataType.MySql, connectionString)
+    .UseAutoSyncStructure(true) //Automatically synchronize the entity structure to the database
+    .Build(); //Be sure to define as singleton mode
+
+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; }
+}
+```
+
+## Dynamic Conditions
+```csharp
+fsql.Delete<Topic>(object dywhere)
+```
+`dywhere` supports:
+
+* Primary key value
+* `new[] { 主键值1, 主键值2 }`
+* Topic Object
+* `new[] { TopicObject1, TopicObject2 }`
+* `new { id = 1 }`
+
+```csharp
+var t1 = fsql.Delete<Topic>(new[] { 1, 2 }).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1 OR `Id` = 2)
+
+var t2 = fsql.Delete<Topic>(new Topic { Id = 1, Title = "test" }).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1)
+
+var t3 = fsql.Delete<Topic>(new[] { new Topic { Id = 1, Title = "test" }, new Topic { Id = 2, Title = "test" } }).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1 OR `Id` = 2)
+
+var t4 = fsql.Delete<Topic>(new { id = 1 }).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1)
+```
+
+## Delete Conditions
+
+> For safety reasons, when there are no conditions, the delete action will not be executed to avoid deleting the entire table data by mistake. Delete the entire table data: `fsql.Delete<T>().Where("1=1").ExecuteAffrows()`
+
+```csharp
+var t5 = fsql.Delete<Topic>().Where(a => a.Id == 1).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1)
+
+var t6 = fsql.Delete<Topic>().Where("id = @id", new { id = 1 }).ToSql();
+//DELETE FROM `Topic` WHERE (id = @id)
+
+var item = new Topic { Id = 1, Title = "newtitle" };
+var t7 = fsql.Delete<Topic>().Where(item).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` = 1)
+
+var items = new List<Topic>();
+for (var a = 0; a < 10; a++) items.Add(new Topic { Id = a + 1, Title = $"newtitle{a}", Clicks = a * 100 });
+var t8 = fsql.Delete<Topic>().Where(items).ToSql();
+//DELETE FROM `Topic` WHERE (`Id` IN (1,2,3,4,5,6,7,8,9,10))
+```
+
+## `ISelect.ToDelete` Advanced Delete
+
+`IDelete` does not support navigation objects, multi-table association, etc. By default, `ISelect.ToDelete` can convert the query to `IDelete` so that the navigation object can be used to delete data:
+
+```csharp
+fsql.Select<T1>().Where(a => a.Options.xxx == 1).ToDelete().ExecuteAffrows();
+```
+Note: This method is not to query the data to the memory and delete it cyclically. The above code generates and executes the following SQL:
+
+```sql
+DELETE FROM `T1` WHERE id in (select a.id from T1 a left join Options b on b.t1id = a.id where b.xxx = 1)
+```
+
+The benefits of using this method for complex deletion:
+
+- Preview data before deleting to prevent mistaken deletion operations;
+- Support complex deletion operations, for example: Use `Limit(10)` on `ISelect` to delete the first 10 records that meet the conditions;
+
+
+## Reference
+
+- [《Database Transaction》](Database-Transaction)
+- [《FreeSql 101, Part 1: Insert Data》](Insert-Data)
+- [《FreeSql 101, Part 3: Update Data》](Update-Data)
+- [《FreeSql 101, Part 4: Query Data》](Query-Data)
+- [《Repository Layer》](Repository-Layer)
+- [《Tenant》](Tenant)
+
+## API
+
+| Methods | Return | Parameters | Description |
+| - | - | - | - |
+| Where | \<this\> | Lambda | Expression conditions, only support entity basic members (not including navigation objects) |
+| Where | \<this\> | string, parms | Raw SQL syntax conditions, `Where("id = @id", new { id = 1 })` |
+| Where | \<this\> | T1 \| IEnumerable\<T1\> | Pass in the entity or collection, and use its primary key as the condition |
+| WhereExists | \<this\> | ISelect | 子查询是否存在 |
+| CommandTimeout | \<this\> | int | Command timeout setting (seconds) |
+| WithTransaction | \<this\> | DbTransaction | Set transaction object |
+| WithConnection | \<this\> | DbConnection | Set the connection object |
+| ToSql | string | | Returns the SQL statement to be executed. |
+| ExecuteAffrows | long | | Execute SQL and return the number of rows affected. |
+| ExecuteDeleted | List\<T1\> | | Execute SQL and return the deleted records. |
+