【MySQL】二级MySQL考试 救场帮助表

周六去考二级，应用第一题就是添加外键约束

草，写了半天老说语法不对，然后急中生智，觉得默认的库里应该有文档说明表

以下是SQL查询过程：

-- 猜测是在mysql库里面
mysql> USE mysql;
Database changed

-- 查看这个库下的表
mysql> SHOW TABLES;
+------------------------------------------------------+
| Tables_in_mysql                                      |
+------------------------------------------------------+
| columns_priv                                         |
| component                                            |
| db                                                   |
| default_roles                                        |
| engine_cost                                          |
| func                                                 |
| general_log                                          |
| global_grants                                        |
| gtid_executed                                        |
| help_category                                        |
| help_keyword                                         |
| help_relation                                        |
| help_topic                                           |
| innodb_index_stats                                   |
| innodb_table_stats                                   |
| password_history                                     |
| plugin                                               |
| procs_priv                                           |
| proxies_priv                                         |
| replication_asynchronous_connection_failover         |
| replication_asynchronous_connection_failover_managed |
| role_edges                                           |
| server_cost                                          |
| servers                                              |
| slave_master_info                                    |
| slave_relay_log_info                                 |
| slave_worker_info                                    |
| slow_log                                             |
| tables_priv                                          |
| time_zone                                            |
| time_zone_leap_second                                |
| time_zone_name                                       |
| time_zone_transition                                 |
| time_zone_transition_type                            |
| user                                                 |
+------------------------------------------------------+
35 rows in set (0.46 sec)

　　

然后发现这四张Help表，考试的时候我没关注过分类表

展开查看发现，里面已经说明了MySQL的内容：

mysql> SELECT * FROM help_category;
+------------------+---------------------------------------+--------------------+-----+
| help_category_id | name                                  | parent_category_id | url |
+------------------+---------------------------------------+--------------------+-----+
|                0 | Contents                              |                  0 |     |
|                1 | Help Metadata                         |                  0 |     |
|                2 | Data Types                            |                  0 |     |
|                3 | Administration                        |                  0 |     |
|                4 | Functions                             |                  0 |     |
|                5 | Enterprise Encryption Functions       |                  4 |     |
|                6 | Language Structure                    |                  0 |     |
|                7 | Geographic Features                   |                  0 |     |
|                8 | MBR                                   |                  7 |     |
|                9 | WKT                                   |                  7 |     |
|               10 | Comparison Operators                  |                  4 |     |
|               11 | Logical Operators                     |                  4 |     |
|               12 | Flow Control Functions                |                  4 |     |
|               13 | Numeric Functions                     |                  4 |     |
|               14 | Date and Time Functions               |                  4 |     |
|               15 | String Functions                      |                  4 |     |
|               16 | Cast Functions and Operators          |                  4 |     |
|               17 | XML                                   |                  4 |     |
|               18 | Bit Functions                         |                  4 |     |
|               19 | Encryption Functions                  |                  4 |     |
|               20 | Locking Functions                     |                  4 |     |
|               21 | Information Functions                 |                  4 |     |
|               22 | Spatial Functions                     |                  4 |     |
|               23 | WKT Functions                         |                 22 |     |
|               24 | WKB Functions                         |                 22 |     |
|               25 | Geometry Constructors                 |                 22 |     |
|               26 | Geometry Property Functions           |                 22 |     |
|               27 | Point Property Functions              |                 22 |     |
|               28 | LineString Property Functions         |                 22 |     |
|               29 | Polygon Property Functions            |                 22 |     |
|               30 | GeometryCollection Property Functions |                 22 |     |
|               31 | Geometry Relation Functions           |                 22 |     |
|               32 | MBR Functions                         |                 22 |     |
|               33 | GTID                                  |                  4 |     |
|               34 | Aggregate Functions and Modifiers     |                  4 |     |
|               35 | GROUP BY Functions and Modifiers      |                  4 |     |
|               36 | Window Functions                      |                  4 |     |
|               37 | Performance Schema Functions          |                  4 |     |
|               38 | Internal Functions                    |                  4 |     |
|               39 | Miscellaneous Functions               |                  4 |     |
|               40 | Data Definition                       |                  0 |     |
|               41 | Data Manipulation                     |                  0 |     |
|               42 | Transactions                          |                  0 |     |
|               43 | Compound Statements                   |                  0 |     |
|               44 | Account Management                    |                  0 |     |
|               45 | Table Maintenance                     |                  0 |     |
|               46 | User-Defined Functions                |                  0 |     |
|               47 | Components                            |                  0 |     |
|               48 | Plugins                               |                  0 |     |
|               49 | Utility                               |                  0 |     |
|               50 | Storage Engines                       |                  0 |     |
+------------------+---------------------------------------+--------------------+-----+
51 rows in set (0.05 sec)

　　

因为是关键字或者其他问题，尝试翻查这个关键字帮助表

当时写外键语句是这样：

ALTER TABLE xxx
ADD CONSTRAINT FOREIGN KEY xxx(xxx) REFERENCE xx(xx);

　　

报错提醒发生在 reference这里

所以我当时就搜素两个关键字，外键和引用这两个

mysql> SELECT * FROM help_keyword WHERE `name` LIKE '%reference%';
+-----------------+------------+
| help_keyword_id | name       |
+-----------------+------------+
|             653 | REFERENCE  |
|             673 | REFERENCES |
+-----------------+------------+
2 rows in set (0.03 sec)

mysql> SELECT * FROM help_keyword WHERE `name` LIKE '%FOREIGN%';
+-----------------+---------+
| help_keyword_id | name    |
+-----------------+---------+
|             599 | FOREIGN |
+-----------------+---------+
1 row in set (0.02 sec)

　　

然后官方给这个文档帮助表设定的说明关系是，关键字和主题是一个多对多的关系

用一个关系表维护，这里要先看【帮助主题】绑定的【关键字ID】

mysql> SELECT * FROM help_relation WHERE help_keyword_id = 599;
+---------------+-----------------+
| help_topic_id | help_keyword_id |
+---------------+-----------------+
|           508 |             599 |
|           518 |             599 |
|           520 |             599 |
|           521 |             599 |
+---------------+-----------------+
4 rows in set (0.12 sec)

我们还可以再看看之前reference关键字关联的

mysql> SELECT * FROM help_relation WHERE help_keyword_id = 653;
+---------------+-----------------+
| help_topic_id | help_keyword_id |
+---------------+-----------------+
|           519 |             653 |
|           532 |             653 |
+---------------+-----------------+
2 rows in set (0.02 sec)

mysql> SELECT * FROM help_relation WHERE help_keyword_id = 673;
+---------------+-----------------+
| help_topic_id | help_keyword_id |
+---------------+-----------------+
|           520 |             673 |
|           521 |             673 |
|           610 |             673 |
+---------------+-----------------+
3 rows in set (0.02 sec)

　　

可以发现599ID 和673ID都有共同两个帮助主题

所以确定是References关键字，然后看看520ID的主题是什么：

由于主题表输出的内容太多，格式完全混乱了，当时考试那个命令终端就是一直刷，还没办法停下

然后先查看主题表的字段：

mysql> DESC help_topic;
+------------------+-------------------+------+-----+---------+-------+
| Field            | Type              | Null | Key | Default | Extra |
+------------------+-------------------+------+-----+---------+-------+
| help_topic_id    | int unsigned      | NO   | PRI | NULL    |       |
| name             | char(64)          | NO   | UNI | NULL    |       |
| help_category_id | smallint unsigned | NO   |     | NULL    |       |
| description      | text              | NO   |     | NULL    |       |
| example          | text              | NO   |     | NULL    |       |
| url              | text              | NO   |     | NULL    |       |
+------------------+-------------------+------+-----+---------+-------+
6 rows in set (0.08 sec)

　　

最主要的是example样例和decription描述,但是example查询为空，只能看描述了

mysql> SELECT example FROM help_topic WHERE help_topic_id = 520;
+---------+
| example |
+---------+
|         |
+---------+

　　

520ID没找到，就找521，结果一看正好就是这个：

就是少写了S导致的

MySQL supports foreign keys, which permit cross-referencing related
data across tables, and foreign key constraints, which help keep the
related data consistent.

A foreign key relationship involves a parent table that holds the
initial column values, and a child table with column values that
reference the parent column values. A foreign key constraint is defined
on the child table.

The essential syntax for a defining a foreign key constraint in a
CREATE TABLE or ALTER TABLE statement includes the following:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (col_name, ...)
    REFERENCES tbl_name (col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

URL: https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html