如何清空（Truncate）带外键（FK）的表？

使用 `TRUNCATE` 清空 TABLE 时报如下错误：

> 无法截断表 'TABLE_NAME'，因为该表正由 FOREIGN KEY 约束引用。

## SQL Server 对策

### 案1

1. 删除数据；
2. 重置自增的种子序列（如果有自增字段的话）；

```sql
DELETE FROM [TABLE_NAME]
DBCC CHECKIDENT ('TABLE_NAME', RESEED, 0)
```

### 案2

1. 删除外键约束；
2. 执行 `TRUNCATE`；
3. 增加外键约束；

```sql
BEGIN TRAN
ALTER TABLE [TABLE_NAME] DROP CONSTRAINT [FK_KEY_NAME]
TRUNCATE TABLE [TABLE_NAME]
ALTER TABLE [TABLE_NAME]  WITH NOCHECK ADD CONSTRAINT [FK_KEY_NAME] FOREIGN KEY([COLUMN_NAME]) REFERENCES [ANOTHER_TABLE_NAME] ([COLUMN_NAME])
COMMIT
```

## MySQL 对策

### 案1

1. 禁用外键检查；
2. 执行 `TRUNCATE`；
3. 恢复外键检查；

```sql
SET FOREIGN_KEY_CHECKS = 0;

TRUNCATE TABLE_NAME;
TRUNCATE ANOTHER_TABLE_NAME;

SET FOREIGN_KEY_CHECKS = 1;
```

### 案2

1. 删除数据；
2. 重置自增的种子序列（如果有自增字段的话）；

```sql
DELETE FROM TABLE_NAME;
ALTER TABLE TABLE_NAME AUTO_INCREMENT = 1;
```

### 案3

1. 删除外键约束；
2. 执行 `TRUNCATE`；
3. 增加外键约束；

```sql
START TRANSACTION;
ALTER TABLE <Table Name> DROP FOREIGN KEY <Foreign Key Name>;
TRUNCATE <Table Name>;
TRUNCATE <Another Table Name>;
ALTER TABLE <Table Name> ADD CONSTRAINT <Foreign Key Name> FOREIGN KEY <Foreign Key Name>(<Field Name>) REFERENCES <Another Table Name>(<Field Name>);
COMMIT;
```

## 参考

1. [Cannot truncate table because it is being referenced by a FOREIGN KEY constraint?](https://stackoverflow.com/questions/253849/cannot-truncate-table-because-it-is-being-referenced-by-a-foreign-key-constraint)
1. [How to truncate a foreign key constrained table?](https://stackoverflow.com/questions/5452760/how-to-truncate-a-foreign-key-constrained-table)
1. [Alter table to add the foreign](http://www.java2s.com/Tutorial/MySQL/0080__Table/Altertabletoaddtheforeignkey.htm)

如何清空（Truncate）带外键（FK）的表？

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