# Unique Key 和索引

__*UNIQUE KEY*__

+ 唯一键具有以下特点:
  + 唯一标识表中的单个行
  + 接受 `NULL`
  + 每张表允许多个唯一键
```
tidb> CREATE TABLE test.dropme (
   ->   id BIGINT AUTO_RANDOM,
   ->   name CHAR(35) NOT NULL,
   ->   PRIMARY KEY (id),
   ->   ~!@UNIQUE KEY@!~ (name)
   ->   );
tidb> SELECT key_name, column_name, index_id FROM information_schema.tidb_indexes 
   -> WHERE table_schema='test' AND table_name='dropme';
+----------+-------------+----------+
| key_name | column_name | index_id |
+----------+-------------+----------+
| PRIMARY  | id          |        0 |
| ~!@name@!~     | name        |        1 |
+----------+-------------+----------+
```
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