28 tips you need to know to learn MySQL

With the continuous development of information technology and the rapid growth of the Internet industry, MySQL, as an open source database, has been widely used and developed. At present, MySQL has become a very important member in the field of relational database.

Whether it is operation and maintenance, development, testing, or architect, database technology is a must-have salary increase artifact. So, it has always been said that learning databases, learning MySQL, what exactly do you want to learn about it?

How to quickly master MySQL?

Develop interest

Interest He is the best teacher. No matter what knowledge he is learning, his interest can greatly improve the learning efficiency. No matter whether you are learning MySQL5.7 or MySQL8.0, there is no exception!

Strengthen the basics of SQL

Technology in the computer field places great emphasis on basics. You may not realize this when you first start learning. With the deepening of technology application, only those with solid basic skills can go faster and farther on the road of technology. For learning MySQL, SQL statement is the most basic part, and many operations are implemented through SQL statements. Therefore, in the process of learning, readers should write more SQL statements and use different implementation statements to complete the same function, so as to deeply understand the differences.

Learn new knowledge in a timely manner

Using the search engine correctly and effectively, you can search for a lot of related knowledge about MySQL. At the same time, you can refer to other people's ideas for solving problems, learn from other people's experiences, and obtain the latest technical information in a timely manner.

Multiple practical operations

The database system is extremely operable and requires a lot of hands-on computer operations. Only in the process of actual operation can we discover problems and think about methods and ideas to solve them. Only in this way can we improve our actual operation ability.

Let’s share 28 must-know tips for learning MySQL!

#1. How to use special characters in MySQL?

Symbols such as single quotes ', double quotes ", backslashes \, these Symbols cannot be directly input and used in MySQL, otherwise unexpected results will occur.

Example:

Assume that a row of records needs to be stored in the Lucifer table, with a value of lucifer's dog, the single quotation mark ', if not escaped, cannot be executed successfully:

mysql> create table lucifer (id int,name char(100));
Query OK, 0 rows affected (0.02 sec)

mysql> insert into lucifer values (1,'lucifer's dog');
    '> 
    '> mysql> 

^C
mysql>

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In MySQL, these special characters are called escape characters. They need to start with the backslash symbol \ when inputting, so when using single quotes and double quotes, should be entered respectively. \' or \". When entering a backslash, you should enter \\. Other special characters include carriage return \r and line feed. \n, tab character \tab, backspace character \b, etc.

mysql> create table lucifer (id int,name char(100));
Query OK, 0 rows affected (0.03 sec)

mysql> insert into lucifer values (1,'lucifer\'s dog');
Query OK, 1 row affected (0.00 sec)

mysql> select * from lucifer;
+------+---------------+
| id   | name          |
+------+---------------+
|    1 | lucifer's dog |
+------+---------------+
1 row in set (0.00 sec)
mysql>

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? Note: is in When inserting these special characters into the database, they must be escaped.

2. Can files be stored in MySQL?

The answer is of course yes!

The BLOB and TEXT field types in MySQL can store large amounts of data Files, you can use these data types to store images, sounds or large-capacity text content, such as web pages or documents.

mysql> create table view(id int unsigned NOT NULL AUTO_INCREMENT, catid int,title varchar(256),picture MEDIUMBLOB, content TEXT,PRIMARY KEY (id));
Query OK, 0 rows affected (0.03 sec)

mysql> show fields from view;
+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| id      | int unsigned | NO   | PRI | NULL    | auto_increment |
| catid   | int          | YES  |     | NULL    |                |
| title   | varchar(256) | YES  |     | NULL    |                |
| picture | mediumblob   | YES  |     | NULL    |                |
| content | text         | YES  |     | NULL    |                |
+---------+--------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql>

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Although you can use BLOB or TEXT to store large-capacity data, the processing of these fields will Reduce database performance.

? 注意： 如果并非必要，可以选择只储存文件的路径。

3、MySQL 中如何执行区分大小写的字符串比较？

MySQL 是 不区分大小写 的，因此字符串比较函数也不区分大小写。

mysql> select 'TRUE' from dual where 'DOG' = 'dog';
+------+
| TRUE |
+------+
| TRUE |
+------+
1 row in set (0.00 sec)

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如果想执行区分大小写的比较，可以在字符串前面添加 BINARY 关键字。

mysql> select 'TRUE' from dual where BINARY'DOG' = 'dog';
Empty set (0.00 sec)

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例如默认情况下，’DOG‘=’dog‘ 返回结果为 TRUE，如果使用 BINARY 关键字，BINARY’DOG’=‘dog’ 结果为 FALSE，在区分大小写的情况下，’DOG’ 与 ’dog’ 并不相同。

4、如何从日期时间值中获取年、月、日等部分日期或时间值？

MySQL 中，日期时间值以字符串形式存储在数据表中，因此可以使用字符串函数分别截取日期时间值的不同部分。

mysql> create table lucifer(date date);
Query OK, 0 rows affected (0.04 sec)

mysql> show fields from lucifer;
+-------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------+------+-----+---------+-------+
| date  | date | YES  |     | NULL    |       |
+-------+------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> insert into lucifer values (now());
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from lucifer;
+------------+
| date       |
+------------+
| 2021-11-25 |
+------------+
1 row in set (0.00 sec)

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例如某个名称为 date 的字段有值 2021-11-25，如果只需要获得年值，可以输入 LEFT(date, 4)，这样就获得了字符串左边开始长度为 4 的子字符串，即 YEAR 部分的值；

mysql> select LEFT(date, 4) from lucifer;
+---------------+
| LEFT(date, 4) |
+---------------+
| 2021          |
+---------------+
1 row in set (0.00 sec)

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如果要获取月份值，可以输入 MID(date,6,2)，字符串第 6 个字符开始，长度为 2 的子字符串正好为 date 中的月份值。同理，读者可以根据其他日期和时间的位置，计算并获取相应的值。

mysql> select MID(date,6,2) from lucifer;
+---------------+
| MID(date,6,2) |
+---------------+
| 11            |
+---------------+
1 row in set (0.00 sec)

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5、如何改变默认的字符集？

CONVERT() 函数改变指定字符串的默认字符集！

MySQL 的安装和配置过程中，其中的一个步骤是可以选择 MySQL 的默认字符集。但是，如果只改变字符集，没有必要把配置过程重新执行一遍，在这里，一个简单的方式是 修改配置文件。

读者可以在修改字符集时使用 SHOW VARIABLES LIKE 'character_set_%'; 或者 status 命令查看当前字符集，以进行对比。

mysql> SHOW VARIABLES LIKE 'character_set_%';
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | latin1                     |
| character_set_connection | latin1                     |
| character_set_database   | utf8mb3                    |
| character_set_filesystem | binary                     |
| character_set_results    | latin1                     |
| character_set_server     | utf8mb4                    |
| character_set_system     | utf8mb3                    |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

mysql> status
--------------
mysql  Ver 8.0.26-0ubuntu0.21.04.3 for Linux on aarch64 ((Ubuntu))

Connection id:          10
Current database:
Current user:           root@localhost
SSL:                    Not in use
Current pager:          stdout
Using outfile:          ''
Using delimiter:        ;
Server version:         8.0.26-0ubuntu0.21.04.3 (Ubuntu)
Protocol version:       10
Connection:             Localhost via UNIX socket
Server characterset:    utf8mb4
Db     characterset:    utf8mb4
Client characterset:    latin1
Conn.  characterset:    latin1
UNIX socket:            /var/run/mysqld/mysqld.sock
Binary data as:         Hexadecimal
Uptime:                 36 min 55 sec

Threads: 2  Questions: 325  Slow queries: 0  Opens: 181  Flush tables: 3  Open tables: 69  Queries per second avg: 0.146
--------------

mysql>

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MySQL 配置文件名称为 my.cnf，该文件在 MySQL 的安装目录下面。修改配置文件中的 default-character-set 和 character-set-server 参数值，将其改为想要的字符集名称，如 gbk、gb2312、latinl 等，修改完之后重新启动 MySQL 服务，即可生效。

## 找到 my.cnf 位置
root@modb:~# find /etc -iname my.cnf -print
/etc/alternatives/my.cnf
/etc/mysql/my.cnf

## 修改字符集
在[client ]下面加入
default-character-set=utf8
在[ mysqld ] 下面加
character_set_server=utf8

## 重启 mysql 生效
service mysql restart

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此时，登录 MySQL 后使用 SHOW VARIABLES LIKE 'character_set_%'; 或者 status 命令查看修改结果！

6、DISTINCT 可以应用于所有的列吗？

查询结果中，如果需要对列进行降序排序，可以使用 DESC，这个关键字只能对其前面的列 进行降序排列。

mysql> select * from lucifer;
+------+----------+
| id   | name     |
+------+----------+
|    1 | lucifer  |
|    2 | lucifer1 |
|    3 | lucifer2 |
+------+----------+
3 rows in set (0.00 sec)

mysql> select * from lucifer order by id desc;
+------+----------+
| id   | name     |
+------+----------+
|    3 | lucifer2 |
|    2 | lucifer1 |
|    1 | lucifer  |
+------+----------+
3 rows in set (0.00 sec)

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例如，要对多列都进行降序排序，必须要在每一列的列名后面加 DESC 关键字。

mysql> select * from lucifer order by id desc,name desc;
+------+----------+
| id   | name     |
+------+----------+
|    3 | lucifer2 |
|    2 | lucifer1 |
|    1 | lucifer  |
+------+----------+
3 rows in set (0.00 sec)

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而 DISTINCT 不同，DISTINCT 不能部分使用。换句话说，DISTINCT 关键字应用于所有列而不仅是它后面的第一个指定列。

例如，查询 2 个字段 sex，age，如果不同记录的这 2 个字段的组合值都不同，则所有记录都会被查询出来。

mysql> select * from lucifer;
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   20 |
|    1 | xiaoliu   | female |   21 |
|    1 | xiaozhang | female |   21 |
|    1 | xiaowu    | female |   21 |
+------+-----------+--------+------+
4 rows in set (0.00 sec)

mysql> select distinct sex,age from lucifer;
+--------+------+
| sex    | age  |
+--------+------+
| male   |   20 |
| female |   21 |
+--------+------+
2 rows in set (0.00 sec)

mysql>

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7、ORDER BY 可以和 LIMIT 混合使用吗？

在使用 ORDER BY 子句时，应保证其位于 FROM 子句之后，如果使用 LIMIT，则必须位于 ORDER BY 之后，如果子句顺序不正确，MySQL 将产生错误消息。

✅ 正确用法：

mysql> select * from lucifer order by age desc limit 2,4;
+------+--------+--------+------+
| id   | name   | sex    | age  |
+------+--------+--------+------+
|    1 | xiaowu | female |   21 |
|    1 | xiaoli | male   |   20 |
+------+--------+--------+------+
2 rows in set (0.00 sec)

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❎ 错误用法：

mysql> select * from lucifer limit 2,4 order by age desc;
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 'order by age desc' at line 1
mysql>

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8、什么时候使用引号？

在查询的时候，会看到在 WHERE 子句中使用条件，有的值加上了单引号，而有的值未加。

mysql> select * from lucifer where sex = 'female';
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoliu   | female |   21 |
|    1 | xiaozhang | female |   21 |
|    1 | xiaowu    | female |   21 |
+------+-----------+--------+------+
3 rows in set (0.00 sec)

mysql>

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单引号用来限定字符串，如果将值与字符串类型列进行比较，则需要限定引号；而用来与数值进行比较则不需要用引号。

mysql> select * from lucifer where age = 20;
+------+--------+------+------+
| id   | name   | sex  | age  |
+------+--------+------+------+
|    1 | xiaoli | male |   20 |
+------+--------+------+------+
1 row in set (0.00 sec)

mysql>

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9、在 WHERE子句中 AND 和 OR 必须使用圆括号吗？

任何时候使用具有 AND 和 OR 操作符的 WHERE 子句，都应该使用圆括号明确操作顺序。

mysql> select * from lucifer where (age = 20 or sex = 'female') and name != 'xiaowu';
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   20 |
|    1 | xiaoliu   | female |   21 |
|    1 | xiaozhang | female |   21 |
+------+-----------+--------+------+
mysql> 3 rows in set (0.00 sec)

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如果条件较多，即使能确定计算次序，默认的计算次序也可能会使 SQL 语句不易理解，因此使 用括号明确操作符的次序，是一个好的习惯。

10、更新或者删除表时必须指定 WHERE子 句吗？

个人建议所有的 UPDATE 和 DELETE 语句全都在 WHERE 子句中指定条件。

mysql> update lucifer set age = 22 where name = 'xiaoliu';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from lucifer where name = 'xiaoliu';
+------+---------+--------+------+
| id   | name    | sex    | age  |
+------+---------+--------+------+
|    1 | xiaoliu | female |   22 |
+------+---------+--------+------+
1 row in set (0.00 sec)

mysql>

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如果省略 WHERE 子句，则 UPDATE 或 DELETE 将被应用到表中所有的行。

mysql> update lucifer set age = 22;
Query OK, 3 rows affected (0.01 sec)
Rows matched: 4  Changed: 3  Warnings: 0

mysql> select * from lucifer;
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   22 |
|    1 | xiaoliu   | female |   22 |
|    1 | xiaozhang | female |   22 |
|    1 | xiaowu    | female |   22 |
+------+-----------+--------+------+
4 rows in set (0.00 sec)

mysql>

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因此，除非确实打算更新或者删除所有记录，否则要注意使用不带 WHERE 子句的 UPDATE 或 DELETE 语句。

? Note: It is recommended to use the SELECT statement to confirm the records that need to be deleted before updating and deleting the table to avoid irreversible results.

#11. Indexes are so important to database performance, how should they be used?

Advantages of indexes:

• By creating a unique index, you can ensure the uniqueness of each row of data in the database table Uniqueness.
• You can set indexes for all MySQL column types.
• can greatly speed up data query, which is the main reason for using indexes.
• It can speed up the connection between tables in terms of achieving referential integrity of data.
• When using grouping and sorting clauses for data query, the time of grouping and sorting in the query can also be significantly reduced

Disadvantages:

• #Creating and maintaining index groups takes time, and the time spent increases as the amount of data increases.
• Indexes need to occupy disk space. In addition to the data space occupied by the data table, each index also occupies a certain amount of physical space. If you have a large number of indexes, the index files may reach their maximum file size faster than the data files.
• When the data in the table is added, deleted, and modified, the index must also be dynamically maintained, which reduces the data maintenance speed.

When using indexes, you need to consider the advantages and disadvantages of indexes.

Choosing the right index for a database is a complex task. An index with fewer columns requires less disk space and less maintenance overhead. If multiple combined indexes are created on a large table, the index file will also expand quickly.

On the other hand, more indexes can cover more queries. You may need to experiment with several different designs to find the most effective index. Indexes can be added, modified, and dropped without affecting the database schema or application design.

因此，应尝试多个不同的索引从而建立最优的索引。

12、尽量使用短索引（前缀索引）

对字符串类型的字段进行索引，如果可能应该指定一个前缀长度。

例如，如果有一个 CHAR(255) 的列，如果在前 10 个或 30 个字符内，多数值是惟一的，则不需要对整个列进行索引。

mysql> select * from lucifer;
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   22 |
|    1 | xiaoliu   | female |   22 |
|    1 | xiaozhang | female |   22 |
|    1 | xiaowu    | female |   22 |
+------+-----------+--------+------+
4 rows in set (0.00 sec)

mysql> create index idx_lucifer_name on lucifer (name(4));
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from lucifer;
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name         | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| lucifer |          1 | idx_lucifer_name |            1 | name        | A         |           1 |        4 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
1 row in set (0.01 sec)

mysql>

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短索引不仅可以提高查询速度而且可以节省磁盘空间、减少 I/O 操作。

13、MySQL 存储过程和函数有什么区别？

在本质上它们都是存储程序。

函数：

• 只能通过 return 语句返回单个值或者表对象；
• 限制比较多，不能用临时表，只能用表变量，还有一些函数都不可用等等；
• 可以嵌入在 SQL 语句中使用，可以在 SELECT 语句中作为查询语句的一个部分调用；

存储过程：

• Return is not allowed, but multiple values ​​can be returned through the out parameter;
• The restrictions are relatively small;
• Generally executed as an independent part;

14. Can the content in the stored procedure be changed?

Can't!

Currently, MySQL does not provide modification of existing stored procedure code. If you must modify the stored procedure, you must use the DROP statement to delete it and then rewrite the code or create a new storage process.

I have to say that Oracle does better in this regard.

#15. Can other stored procedures be called within a stored procedure?

Can!

The stored procedure contains a user-defined set of SQL statements. You can use the CALL statement to call the stored procedure. Of course, you can also use the CALL statement to call other stored procedures in the stored procedure, but you cannot use the DROP statement to delete other stored procedures. process.

#16. The parameters of the stored procedure should not be the same as the field names in the data table.

When defining the stored procedure parameter list, attention should be paid to distinguishing the parameter names from the field names in the database table, otherwise unexpected results will occur.

17. Can the parameters of the stored procedure be in Chinese?

Under normal circumstances, there may be situations where Chinese parameters are passed in to a stored procedure. For example, a stored procedure searches for the user's information based on the user's name. Parameter values ​​may be in Chinese. At this time, you need to add character set gbk at the end when defining the stored procedure, otherwise an error will occur when calling the stored procedure using Chinese parameters. For example, defining the userInfo stored procedure, the code is as follows:

CREATE PROCEDURE useInfo(IN u_name VARCHAR (50) character set gbk, OUT u_age INT)

#18. What is the difference and connection between views and tables in MySQL?

The difference between the two:

• Views are compiled SQL statements and are based on SQL The result set of the statement is visualized in a table, while the table is not;
• The view does not have the actual physical records, while the base table does;
• The table is the content, and the view is the window;
• The table occupies physical space but the view does not occupy physical space. The view is just a logical concept. The table can be modified in time, but the view can only be used Create a statement to modify;
• View is a way to view the data table. You can query the data composed of certain fields in the data table. It is just a collection of some SQL statements. From a security perspective, views can prevent users from accessing data tables, so users do not know the table structure; the
• table belongs to the table in the global schema and is a real table; the view belongs to the local schema The table is a virtual table;
• The creation and deletion of the view only affects the view itself, not the corresponding basic table;

# #The connection between the two:

The view (view) is a table built on the basic table. Its structure (i.e., the defined columns) and content (i.e., all records) come from the basic table , which exists based on the existence of the basic table.

一个视图可以对应一个基本表，也可以对应多个基本表。

视图是基本表的抽象和在逻辑意义上建立的新关系。

19、使用触发器时须特别注意！

在使用触发器的时候需要注意，对于相同的表，相同的事件只能创建一个触发器。

mysql> create trigger lucifer_tri before insert on lucifer for each row set NEW.id=NEW.id+1;
Query OK, 0 rows affected (0.01 sec)

mysql> 
mysql> 
mysql> select * from lucifer;
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   22 |
|    1 | xiaoliu   | female |   22 |
|    1 | xiaozhang | female |   22 |
|    1 | xiaowu    | female |   22 |
|    1 | lucifer   | male   |   20 |
|    1 | lucifer   | male   |   20 |
+------+-----------+--------+------+
6 rows in set (0.00 sec)

mysql> insert into lucifer values(1,'lucifer','male',20);
Query OK, 1 row affected (0.00 sec)

mysql> select * from lucifer;
+------+-----------+--------+------+
| id   | name      | sex    | age  |
+------+-----------+--------+------+
|    1 | xiaoli    | male   |   22 |
|    1 | xiaoliu   | female |   22 |
|    1 | xiaozhang | female |   22 |
|    1 | xiaowu    | female |   22 |
|    1 | lucifer   | male   |   20 |
|    1 | lucifer   | male   |   20 |
|    2 | lucifer   | male   |   20 |
+------+-----------+--------+------+
7 rows in set (0.00 sec)

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比如对表 lucifer 创建了一个 BEFORE INSERT 触发器，那么如果对表 lucifer 再次创建一个 BEFORE INSERT 触发器，MySQL 将会报错，此时，只可以在表 lucifer 上创建 AFTER INSERT 或者 BEFORE UPDATE 类型的触发器。

mysql> create trigger lucifer_tri before insert on lucifer for each row set NEW.id=NEW.id+1;
ERROR 1359 (HY000): Trigger already exists
mysql>

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灵活的运用触发器将为操作省去很多麻烦。

20、及时删除不再需要的触发器

触发器定义之后，每次执行触发事件，都会激活触发器并执行触发器中的语句。

如果需求发生变化，而触发器没有进行相应的改变或者删除，则触发器仍然会执行旧的语句，从而会影响新的数据的完整性。

mysql> drop trigger lucifer_tri;
Query OK, 0 rows affected (0.03 sec)

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因此，要将不再使用的触发器及时删除。

21、应该使用哪种方法创建用户？（3种方式）

创建用户有 3 种方法：

• 使用 CREATE USER 语句创建用户
• 在 mysql.user 表中添加用户
• 使用 GRANT 语句创建用户（仅限 MySQL 8 版本以下使用）

一般情况， 最好使用 GRANT 或者 CREATE USER 语句，而不要直接将用户信息插入 user 表，因为 user 表中存储了全局级别的权限以及其他的账户信息，如果意外破坏了 user 表中的记录，则可能会对 MySQL 服务器造成很大影响。

-- 使用 CREATE USER 语句创建用户
mysql> create user 'lucifer'@'localhost' identified by 'lucifer';
Query OK, 0 rows affected (0.01 sec)

mysql> 

-- 在 mysql.user 表中添加用户
mysql> select MD5('lucifer');
+----------------------------------+
| MD5('lucifer')                   |
+----------------------------------+
| cae33a0264ead2ddfbc3ea113da66790 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> INSERT INTO mysql.user(Host, User, authentication_string, ssl_cipher, ssuex509_i09_sr, x5ubject) VALUES ('lohoscalt',uci 'lfer MD5('1',lucifer'), '', '',; '')
Query OK, 1 row affected (0.01 sec)

mysql> 

-- 使用 GRANT 语句创建用户
mysql> GRANT SELECT ON*.* TO 'lucifer2'@localhost IDENTIFIED BY 'lucifer';
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 'IDENTIFIED BY 'lucifer'' at line 1
mysql>

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? 注意： 由于测试使用的是 MySQL 8 版本，已经不支持 GRANT 直接创建用户，5.7 版本依然是支持的。

22、mysqldump 备份的文件只能在 MySQL 中使用吗？

逻辑备份工具，适用于所有的存储引擎，支持温备、完全备份、部分备份、对于 InnoDB 存储引擎支持热备。

mysqldump 备份的文本文件实际是数据库的一个副本，使用该文件不仅可以在 MySQL 中恢复数据库，而且通过对该文件的简单修改，可以使用该文件在 SQL Server 或者 Sybase 等其他数据库中恢复数据库。

root@modb:~# mysqldump -uroot -p hr > /root/hr.db
Enter password: 
root@modb:~# 
root@modb:~# ll hr.db 
-rw-r--r-- 1 root root 25327 Nov 26 08:52 hr.db

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这在某种程度上实现了数据库之间的迁移。

23. How to choose a backup tool?

Based on the backup method (whether the database needs to be offline), backup can be divided into:

• Hot Backup
• Cold Backup
• Warm Backup

Performed in MySQL Different backup methods also need to consider whether the storage engine supports it. For example, MyISAM does not support hot backup, but supports warm backup and cold backup. InnoDB supports hot standby, warm standby and cold standby.

Generally, the data we need to back up is divided into the following categories:

• Table data
• Binary log, InnoDB transaction log
• Code (stored procedure, stored function, trigger, event scheduler)
• Server configuration file

The following are several commonly used backup tools:

• mysqldump：逻辑备份工具，适用于所有的存储引擎，支持温备、完全备份、部分备份、对于 InnoDB 存储引擎支持热备。
• cp、tar 等归档复制工具：物理备份工具，适用于所有的存储引擎、冷备、完全备份、部分备份。
• lvm2 snapshot：借助文件系统管理工具进行备份。
• mysqlhotcopy：名不副实的一个工具，仅支持 MyISAM 存储引擎。
• xtrabackup：一款由 percona 提供的非常强大的 InnoDB/XtraDB 热备工具，支持完全备份、增量备份。

直接复制数据文件是最为直接、快速的备份方法，但缺点是基本上不能实现增量备份。备份时必须确保没有使用这些表。如果在复制一个表的同时服务器正在修改它，则复制无效。备份 文件时，最好关闭服务器，然后重新启动服务器。

24、平时应该打开哪些日志？

日志既会影响 MySQL 的性能，又会占用大量磁盘空间。因此，如果不必要，应尽可能少地 开启日志。

根据不同的使用环境，可以考虑开启不同的日志。

例如，在开发环境中优化查询效率低的语句，可以开启慢查询日志；

开启慢查询日志： 可以让MySQL记录下查询超过指定时间的语句，通过定位分析性能的瓶颈，才能更好的优化数据库系统的性能。

-- 检查是否开启慢查询
mysql> show variables like 'slow_query%';
+---------------------+------------------------------+
| Variable_name       | Value                        |
+---------------------+------------------------------+
| slow_query_log      | OFF                          |
| slow_query_log_file | /var/lib/mysql/modb-slow.log |
+---------------------+------------------------------+
2 rows in set (0.00 sec)

mysql> show variables like 'long_query_time';
+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.01 sec)

-- 开启慢查询日志
mysql> set global slow_query_log='ON'; 
Query OK, 0 rows affected (0.00 sec)

-- 设置查询超过10秒就记录
mysql> set global long_query_time=10;
Query OK, 0 rows affected (0.00 sec)

-- 再次检查是否开启
mysql> show variables like 'slow_query%';
mysql> +---------------------+------------------------------+
| Variable_name       | Value                        |
+---------------------+------------------------------+
| slow_query_log      | ON                           |
| slow_query_log_file | /var/lib/mysql/modb-slow.log |
+---------------------+------------------------------+
2 rows in set (0.00 sec)

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如果需要记录用户的所有查询操作，可以开启通用查询日志；

mysql> show variables like 'general_log%';
+------------------+-------------------------+
| Variable_name    | Value                   |
+------------------+-------------------------+
| general_log      | OFF                     |
| general_log_file | /var/lib/mysql/modb.log |
+------------------+-------------------------+
2 rows in set (0.00 sec)

-- 开启通用查询日志
mysql> SET GLOBAL general_log=1; 
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'general_log%';
+------------------+-------------------------+
| Variable_name    | Value                   |
+------------------+-------------------------+
| general_log      | ON                      |
| general_log_file | /var/lib/mysql/modb.log |
+------------------+-------------------------+
2 rows in set (0.00 sec)

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如果需要记录数据的变更，可以开启二进制日志；错误日志是默认开启的。

mysql> show variables like 'log_bin%';
+---------------------------------+-----------------------------+
| Variable_name                   | Value                       |
+---------------------------------+-----------------------------+
| log_bin                         | ON                          |
| log_bin_basename                | /var/lib/mysql/binlog       |
| log_bin_index                   | /var/lib/mysql/binlog.index |
| log_bin_trust_function_creators | OFF                         |
| log_bin_use_v1_row_events       | OFF                         |
+---------------------------------+-----------------------------+
5 rows in set (0.00 sec)

mysql>

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25、如何使用二进制日志？

二进制日志主要用来记录数据变更。

如果需要记录数据库的变化，可以开启二进制日志。基于二进制日志的特性，不仅可以用来进行数据恢复，还可用于数据复制。

root@modb:/var/lib/mysql# ls binlog*
binlog.000001  binlog.000002  binlog.index
root@modb:/var/lib/mysql# mysqlbinlog binlog.000001 | mysql -u root -p                                            
Enter password: 
root@modb:/var/lib/mysql#

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在数据库定期备份的 情况下，如果出现数据丢失，可以先用备份恢复大部分数据，然后使用二进制日志恢复最近备份后变更的数据。在双机热备情况下，可以使用 MySQL 的二进制日志记录数据的变更，然后将变更部分复制到备份服务器上。

26、如何使用慢查询日志？

慢查询日志主要用来记录查询时间较长的日志。

在开发环境下，可以开启慢查询日志来记录查询时间较长的查询语句，然后对这些语句进行优化。

root@modb:/var/lib/mysql# cat /var/lib/mysql/modb-slow.log
/usr/sbin/mysqld, Version: 8.0.26-0ubuntu0.21.04.3 ((Ubuntu)). started with:
Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
Time                 Id Command    Argument
root@modb:/var/lib/mysql#

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通过配 long_query_time 的值，可以灵活地掌握不同程度的慢查询语句。

27、是不是索引建立得越多越好？

合理的索引可以提高查询的速度，但不是索引越多越好。

在执行插入语句的时候，MySQL 要为新插入的记录建立索引。所以过多的索引会导致插入操作变慢。原则上是只有查询用的字段才建立索引。

使用索引时，需要综合考虑索引的优点和缺点。

28、如何使用查询缓冲区？

查询缓冲区可以提高查询的速度，但是这种方式只适合查询语句比较多、更新语句比较少 的情况。

默认情况下查询缓冲区的大小为 0，也就是不可用。可以修改 queiy_cache_size 以调整查询缓冲区大小；修改 query_cache_type 以调整查询缓冲区的类型。

在 my.cnf 中修改 query_cache_size 和 query_cache_type 的值如下所示：

[mysqld]
query_cache_size= 512M 
query_cache_type= 1
query_cache_type=1

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表示开启查询缓冲区。

只有在查询语句中包含 SQL_NO_CACHE 关键字时，才不会使用查询缓冲区。可以使用 FLUSH QUERY CACHE 语句来刷新缓冲区，清理查询缓冲区中的碎片。

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