Primary key 主鍵

1. 登入 mariadb。

   [root@kvm3 ~]# mycli -S /var/lib/mysql/mysql.sock -uroot -p123qwe
   mariadb 10.3.11-MariaDB
   mycli 1.20.1
   Chat: https://gitter.im/dbcli/mycli
   Mail: https://groups.google.com/forum/#!forum/mycli-users
   Home: http://mycli.net
   Thanks to the contributor - Lewis Peckover
   

2. 使用資料庫 dyw。

   mariadb root@localhost:(none)> use dyw;                                         
   You are now connected to database "dyw" as user "root"
   Time: 0.001s
   

3. 查看資料表。

   mariadb root@localhost:dyw> show tables;                                        
   +---------------+
   | Tables_in_dyw |
   +---------------+
   | dcount        |
   | tbl           |
   +---------------+
   2 rows in set
   Time: 0.022s
   

4. 查詢資料表 dcount 沒有索引。

   mariadb root@localhost:dyw> show index from dcount\G                            
   
   mariadb root@localhost:dyw>
   

5. 於資料表 dcount 欄位 author 加入主鍵

   mariadb root@localhost:dyw> alter table dcount add primary key(author);         
   You're about to run a destructive command.
   Do you want to proceed? (y/n): y
   Your call!
   Query OK, 0 rows affected
   Time: 0.038s
   

6. 查詢資料表 dcount 的索引，其中 Key_name: PRIMARY 表示主鍵，Non_qunique: 0 表示不重複。 Collation: How the column is sorted in the index. In MySQL, this can have values “A” (Ascending) or NULL (Not sorted).
   Cardinality: An estimate of the number of unique values in the index.

   mariadb root@localhost:dyw> show index from dcount\G                            
   ***************************[ 1. row ]***************************
   Table         | dcount
   Non_unique    | 0
   Key_name      | PRIMARY
   Seq_in_index  | 1
   Column_name   | author
   Collation     | A
   Cardinality   | 6
   Sub_part      | <null>
   Packed        | <null>
   Null          | 
   Index_type    | BTREE
   Comment       | 
   Index_comment | 
   
   1 row in set
   Time: 0.012s
   

7. 刪除資料表 dcount 的主鍵，因每個資料表只能有一個主鍵，所以不用指定欄位。

   mariadb root@localhost:dyw> alter table dcount drop primary key;                
   You're about to run a destructive command.
   Do you want to proceed? (y/n): y
   Your call!
   Query OK, 6 rows affected
   Time: 0.043s
   

8. 查詢資料表 dcount 沒有索引。

   mariadb root@localhost:dyw> show index from dcount\G                            
   
   mariadb root@localhost:dyw>