13.5 經常使用sql語句

經常使用sql語句目錄概要

• select count(*) from mysql.user;
• select * from mysql.db;
• select db from mysql.db;
• select db,user from mysql.db;
• select * from mysql.db where host like '192.168.%';
• insert into db1.t1 values (1, 'abc');
• update db1.t1 set name='aaa' where id=1;
• truncate table db1.t1;
• drop table db1.t1;
• drop database db1;

經常使用sql語句

• 增刪改查，就是mysql和其餘關係型數據庫經常使用的select語句操做命令

查詢語句

1. 首先登陸root下的mysql

[root@hf-01 ~]#  mysql -uroot -p'hanfeng'
Warning: Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 18
Server version: 5.6.35 MySQL Community Server (GPL)

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

2. 使用db1庫

mysql> use db1;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql>

3. 查看當前庫的全部表show tables;

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1            |
+---------------+
1 row in set (0.00 sec)

mysql>

4. 查看錶的行數 select count(*) from mysql.user;

• 庫和表中間有個分割符，就是用點 . 分割

mysql> select count(*) from mysql.user;
+----------+
| count(*) |
+----------+
|       12 |
+----------+
1 row in set (0.01 sec)

mysql>

• 就是說user表有12行內容

5. 查看全部的內容 select * from mysql.db;（這樣看起來會很亂） ——>能夠在後面加上\G，如select * from mysql.db\G;

• 這裏的 * 表示查看全部內容

mysql> select * from mysql.db/G;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '/G' at line 1
mysql> select * from mysql.db\G;
*************************** 1. row ***************************
                 Host: %
                   Db: test
                 User: 
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: Y
          Create_priv: Y
            Drop_priv: Y
           Grant_priv: N
      References_priv: Y
           Index_priv: Y
           Alter_priv: Y
Create_tmp_table_priv: Y
     Lock_tables_priv: Y
     Create_view_priv: Y
       Show_view_priv: Y
  Create_routine_priv: Y
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: Y
         Trigger_priv: Y
*************************** 2. row ***************************
 等等等，只截取了一部分

6. 查看db庫的全部內容 select db from mysql.db; 第一個db是字段

mysql> select db from mysql.db;
+---------+
| db      |
+---------+
| test    |
| test\_% |
| db1     |
| db1     |
+---------+
4 rows in set (0.01 sec)

mysql>

7. 查db字段和user字段 select db,user from mysql.db;

mysql> select db,user from mysql.db;
+---------+-------+
| db      | user  |
+---------+-------+
| test    |       |
| test\_% |       |
| db1     | user2 |
| db1     | user2 |
+---------+-------+
4 rows in set (0.00 sec)

mysql>

8. 模糊查詢 select * from mysql.db where host like '192.168.%';

• like 就是模糊匹配

mysql> select * from mysql.db where host like '192.168.%'\G;
*************************** 1. row ***************************
                 Host: 192.168.133.1
                   Db: db1
                 User: user2
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: N
          Create_priv: N
            Drop_priv: N
           Grant_priv: N
      References_priv: N
           Index_priv: N
           Alter_priv: N
Create_tmp_table_priv: N
     Lock_tables_priv: N
     Create_view_priv: N
       Show_view_priv: N
  Create_routine_priv: N
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: N
         Trigger_priv: N
*************************** 2. row ***************************
                 Host: 192.168.133.2
                   Db: db1
                 User: user2
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: N
          Create_priv: N
            Drop_priv: N
           Grant_priv: N
      References_priv: N
           Index_priv: N
           Alter_priv: N
Create_tmp_table_priv: N
     Lock_tables_priv: N
     Create_view_priv: N
       Show_view_priv: N
  Create_routine_priv: N
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: N
         Trigger_priv: N
2 rows in set (0.00 sec)

ERROR: 
No query specified

mysql>

插入語句

1. 查看建立的表

mysql> desc db1.t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(4)   | YES  |     | NULL    |       |
| name  | char(40) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql>

2. 查看db1.t1表的內容，會發現爲空

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql>

3. 插入數據到 insert into db1.t1 values (1, 'abc');

• 插入1, 'abc'到db1.t1表

mysql> insert into db1.t1 values (1, 'abc');
Query OK, 1 row affected (0.02 sec)

mysql>

4. 再來查詢db1.t1

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | abc  |
+------+------+
1 row in set (0.00 sec)

mysql>

4. 這樣就成功了插入了一條數據，在插入的時候 name 這個字段應該是是一個字符串，字符串須要加上一個單引號 ' ' ，數字能夠不加單引號

mysql> insert into db1.t1 values (1, 234);
Query OK, 1 row affected (0.01 sec)

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | abc  |
|    1 | 234  |
+------+------+
2 rows in set (0.00 sec)

mysql>

5.這裏沒有作限制，這裏id和name均可以是相同的，同一個字段裏有相同的數字，相同的值 6. 也能夠作一些限制，在插入相同的id的時候，就會衝突

update操做

• 更改db1.t1表 的字符串爲name 的數據 和 字符串爲id 的數據
• update db1.t1 set name='aaa' where id=1;

mysql> update db1.t1 set name='aaa' where id=1;
Query OK, 2 rows affected (0.01 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | aaa  |
|    1 | aaa  |
+------+------+
2 rows in set (0.00 sec)

mysql>

delete操做

• 刪除db1.t1表 的數據 和 字符串爲id 的數據
• delete from db1.t1 where id=1;

mysql> delete from db1.t1 where id=1;
Query OK, 2 rows affected (0.01 sec)

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql>

truncate清空一個表

• 清空表數據 truncate table db1.t1;
  • 即便表的數據清空了，但表的字段依舊存在的

mysql> insert into db1.t1 values (1, 234);
Query OK, 1 row affected (0.00 sec)

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | 234  |
+------+------+
1 row in set (0.00 sec)

mysql> truncate table db1.t1;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql> desc db1.t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(4)   | YES  |     | NULL    |       |
| name  | char(40) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql>

• truncate 只是清空的內容，而drop 會清空表的數據並清除表的框架
• drop 會把表的框架也丟掉 drop table db1.t1;

mysql> drop table db1.t1;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from db1.t1;    //由於表的架構已經不存在了
ERROR 1146 (42S02): Table 'db1.t1' doesn't exist
mysql>

• 丟掉表 drop database db1;

總結

• 在使用mysql的時候，少用 * 這樣的操做，由於如果一個表裏面的內容不少，select count(*)這樣操做就會很耗時，浪費資源
• 數據庫中經常使用引擎是myisam和innodb，默認mysql庫裏面都是使用的myisam引擎
  • 特色：myisam引擎，能自動去統計有多少行
    • 在select count(*)查看錶的時候會很快
    • use mysql;
    • show create table user\G;
  • 特色：innodb引擎，不會自動統計行數，每次去查詢，每次去統計行數，就會很耗時
    • use db1
    • show create table t1;
  • 因此select count(*)這種操做盡可能減小，會耗費太多資源