Add docs for information_schema.CHECK_CONSTRAINTS (pingcap#15090)

TOC.md

@@ -914,6 +914,7 @@
       - INFORMATION_SCHEMA
         - [Overview](/information-schema/information-schema.md)
         - [`ANALYZE_STATUS`](/information-schema/information-schema-analyze-status.md)
+        - [`CHECK_CONSTRAINTS`](/information-schema/information-schema-check-constraints.md)
         - [`CLIENT_ERRORS_SUMMARY_BY_HOST`](/information-schema/client-errors-summary-by-host.md)
         - [`CLIENT_ERRORS_SUMMARY_BY_USER`](/information-schema/client-errors-summary-by-user.md)
         - [`CLIENT_ERRORS_SUMMARY_GLOBAL`](/information-schema/client-errors-summary-global.md)

information-schema/information-schema-check-constraints.md

@@ -0,0 +1,52 @@
+---
+title: CHECK_CONSTRAINTS
+summary: 了解 INFORMATION_SCHEMA 表 `CHECK_CONSTRAINTS`。
+---
+
+# CHECK\_CONSTRAINTS
+
+`CHECK_CONSTRAINTS` 表提供关于表上 [`CHECK` 约束](/constraints.md#check-约束)的信息。
+
+```sql
+USE INFORMATION_SCHEMA;
+DESC CHECK_CONSTRAINTS;
+```
+
+命令输出如下：
+
+```sql
++--------------------+-------------+------+-----+---------+-------+
+| Field              | Type        | Null | Key | Default | Extra |
++--------------------+-------------+------+-----+---------+-------+
+| CONSTRAINT_CATALOG | varchar(64) | NO   |     | NULL    |       |
+| CONSTRAINT_SCHEMA  | varchar(64) | NO   |     | NULL    |       |
+| CONSTRAINT_NAME    | varchar(64) | NO   |     | NULL    |       |
+| CHECK_CLAUSE       | longtext    | NO   |     | NULL    |       |
++--------------------+-------------+------+-----+---------+-------+
+4 rows in set (0.00 sec)
+```
+
+下述示例使用 `CREATE TABLE` 语句添加 `CHECK` 约束：
+
+```sql
+CREATE TABLE test.t1 (id INT PRIMARY KEY, CHECK (id%2 = 0));
+SELECT * FROM CHECK_CONSTRAINTS\G
+```
+
+命令输出如下：
+
+```sql
+*************************** 1. row ***************************
+CONSTRAINT_CATALOG: def
+ CONSTRAINT_SCHEMA: test
+   CONSTRAINT_NAME: t1_chk_1
+      CHECK_CLAUSE: (`id` % 2 = 0)
+1 row in set (0.00 sec)
+```
+
+`CHECK_CONSTRAINTS` 表的字段描述如下：
+
+* `CONSTRAINT_CATALOG`：约束的目录，始终为 `def`。
+* `CONSTRAINT_SCHEMA`：约束的库名。
+* `CONSTRAINT_NAME`：约束的名字。
+* `CHECK_CLAUSE`：检查约束的子句。