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sql: add switch for check constraint #14320

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Jun 28, 2023
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sql: add switch for check constraint #14320

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9 changes: 9 additions & 0 deletions constraints.md
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Expand Up @@ -51,6 +51,10 @@ Query OK, 1 row affected (0.03 sec)

## `CHECK` 约束

> **注意：**
>
> `CHECK` 约束功能默认关闭，需要将变量 [`tidb_enable_check_constraint`](/system-variables.md#tidb_enable_check_constraint-从-v720-版本开始引入) 设置为 `ON` 后才能开启。

`CHECK` 约束用于限制表中某个字段的值必须满足指定条件。当为表添加 `CHECK` 约束后，在插入或者更新表的数据时，TiDB 会检查约束条件是否满足，如果不满足，则会报错。

TiDB 中 `CHECK` 约束的语法如下，与 MySQL 中一致：
Expand Down Expand Up @@ -128,6 +132,11 @@ ALTER TABLE t DROP CONSTRAINT t_chk_1;
ALTER TABLE t ALTER CONSTRAINT c1 NOT ENFORCED;
```

### 与 MySQL 的兼容性

- 不支持在添加列的同时添加 `CHECK` 约束（例如，`ALTER TABLE t ADD COLUMN a CHECK(a > 0)`)），否则只有列会被添加成功，TiDB 会忽略 `CHECK` 约束但不会报错。
- 不支持使用 `ALTER TABLE t CHANGE a b int CHECK(b > 0)` 添加 `CHECK` 约束，使用该语句时 TiDB 会报错。

## 唯一约束

唯一约束是指唯一索引和主键列中所有的非空值都是唯一的。
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8 changes: 8 additions & 0 deletions system-variables.md
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Expand Up @@ -1450,6 +1450,14 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
- 默认值：`OFF`
- 这个变量用于控制是否开启 cascades planner。

### `tidb_enable_check_constraint` <span class="version-mark">从 v7.2.0 版本开始引入</span>

- 作用域：GLOBAL
- 是否持久化到集群：是
- 类型：布尔型
- 默认值：`OFF`
- 这个变量用于控制是否启用 [`CHECK` 约束](/constraints.md#check-约束)。

### `tidb_enable_chunk_rpc` <span class="version-mark">从 v4.0 版本开始引入</span>

- 作用域：SESSION
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