TABLE_CONSTRAINTS

TABLE_CONSTRAINTS 表描述了哪些表具有约束（constraint）以及相关信息。

DESC INFORMATION_SCHEMA.table_constraints;

+--------------------+--------+------+------+---------+---------------+
| Column             | Type   | Key  | Null | Default | Semantic Type |
+--------------------+--------+------+------+---------+---------------+
| constraint_catalog | String |      | NO   |         | FIELD         |
| constraint_schema  | String |      | NO   |         | FIELD         |
| constraint_name    | String |      | NO   |         | FIELD         |
| table_schema       | String |      | NO   |         | FIELD         |
| table_name         | String |      | NO   |         | FIELD         |
| constraint_type    | String |      | NO   |         | FIELD         |
| enforced           | String |      | NO   |         | FIELD         |
+--------------------+--------+------+------+---------+---------------+

表中的列：

• CONSTRAINT_CATALOG: 约束所属 catalog 的名称。此值始终为 def。
• CONSTRAINT_SCHEMA: 约束所属数据库的名称。
• CONSTRAINT_NAME: 约束的名称，可以是 TIME INDEX 或 PRIMARY。
• TABLE_NAME: 表的名称。
• CONSTRAINT_TYPE: 约束的类型。值可以是 TIME INDEX 或 PRIMARY KEY。TIME INDEX 和 PRIMARY KEY 信息类似于 SHOW INDEX 语句的执行结果。
• enforced: 不支持 CHECK 约束，此值始终为 YES。

select * from INFORMATION_SCHEMA.table_constraints WHERE table_name = 'monitor'\G;

输出结果：

*************************** 1. row ***************************
constraint_catalog: def
 constraint_schema: public
   constraint_name: TIME INDEX
      table_schema: public
        table_name: monitor
   constraint_type: TIME INDEX
          enforced: YES
*************************** 2. row ***************************
constraint_catalog: def
 constraint_schema: public
   constraint_name: PRIMARY
      table_schema: public
        table_name: monitor
   constraint_type: PRIMARY KEY
          enforced: YES