在實際的開發中，我們肯定經常要查看錶結構的，特別是遇到自己不是最初開發的專案的時候，通過表結構，大概就能看出表裡存什麼資料，每個欄位代表什麼意思。實際上有很多查看錶結構的方式，下面就拿rails裡面的表schema_migrations介紹一下我自己常用的命令：

desc table_name;

mysql> desc schema_migrations;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| version | varchar(255) | NO   | PRI | NULL    |       |
+---------+--------------+------+-----+---------+-------+
1 row in set (0.00 sec)
 

它是我們最常用的命令，這個命令也是describe schema_migrations的縮寫。

show fields from table_name;

mysql> show fields from schema_migrations;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| version | varchar(255) | NO   | PRI | NULL    |       |
+---------+--------------+------+-----+---------+-------+
1 row in set (0.01 sec)
 

和describe命令達到的效果一樣。 在它的基礎上就有一個更強大的命令：

show full fields from table_name

mysql> show full fields from schema_migrations;
+---------+--------------+-----------------+------+-----+---------+-------+---------------------------------+---------+
| Field   | Type         | Collation       | Null | Key | Default | Extra | Privileges                      | Comment |
+---------+--------------+-----------------+------+-----+---------+-------+---------------------------------+---------+
| version | varchar(255) | utf8_general_ci | NO   | PRI | NULL    |       | select,insert,update,references |         |
+---------+--------------+-----------------+------+-----+---------+-------+---------------------------------+---------+
1 row in set (0.01 sec)
 

每個欄位的詳細資訊也就出來了，但是遺憾的是索引沒有處理，說到檢視索引，很明顯就會想到下面的命令：

show index from table_name

mysql> show index from schema_migrations;
+-------------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table             | Non_unique | Key_name                 | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| schema_migrations |          0 | unique_schema_migrations |            1 | version     | A         |          21 |     NULL | NULL   |      | BTREE      |         |               |
+-------------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

這個命令和show keys from table_name得到的效果一樣。 實際上有個終極命令，既可以看到本身的欄位又可以看到索引等其他資訊，還能得到完整的建立表結構的命令，那就是：

show create table table_name

mysql> show create table schema_migrations;
+-------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table             | Create Table                                                                                                                                                   |
+-------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
| schema_migrations | CREATE TABLE `schema_migrations` (
  `version` varchar(255) NOT NULL,
  UNIQUE KEY `unique_schema_migrations` (`version`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

通過命令執行的結果可以看到建立表的sql語句，欄位名，索引，搜尋引擎，編碼方式等資訊，是不是很強大。