MySQL has a convenient HELP command that allows you to quickly find explanations on one aspect of the SQL language without having to resort to documentation or other manuals. Let’s see how to use it.
Fill in the help tables of mySQL:
Often the help contents are not installed by default, so you need to load them manually. They can be downloaded from the documentation page on the official MySQL website. What we are going to download is a compressed package containing a text file, which in turn contains the SQL queries that will populate some system tables. Once downloaded we will unpack it in a convenient location, assign it a short name (like help.sql) and launch the following command from the Terminal:
mysql --user=myuser --password=mypassword mysql help.sql
Of course you need to replace myuser and mypassword with a real existing username and password. If they are correct, MySQL should quickly enter the help contents into the system tables.
The syntax of the HELP command is as follows:
HELP _topic_.
You can also use it from the command line:
help _argument_
In both cases the argument is a string (so in SQL you have to enclose it between quotes!) and the use of upper or lower case characters is indifferent.
Topic can be any aspect of the SQL language on which we need information. For example, if we want information about HELP, we write in SQL:
HELP ‘help
If the search returns more topics:
We often get what we’re looking for right away. In some cases, however, as in the case of HELP, the search will produce two or more results.
If this happens, the command will return a recordset of two fields:
- name: is the name of the argument found;
- is_en_category: is ‘Y’ if it is an argument category, ‘N’ if it is a single argument;
- source_category_name: this field is not always present if it indicates the category in which the argument (or subcategory) in question is located.
In this case we will have ‘HELP COMMAND’ and ‘HELP STATEMENT’, which are arguments and not categories. We can view them with:
HELP 'help command
or:
HELP 'help statement
The first refers to help as a command to be entered in the command line, the second to HELP as an SQL statement.
Navigate between categories:
If we look for the name of a category, we can explore its content. We will then see the topics it contains (if any), the father category (if any) and the daughter categories (if any).
HELP 'functions
Since among the results we will have ‘String Functions’, we can decide to explore its content:
HELP 'String Functions'.
The root category, which contains all the others, is ‘Contents’.
Display a topic
Let’s try it:
HELP 'ascii
HELP 'mediumint
HELP 'like'
We’re gonna get a three-field record:
- name: the name of the topic;
- description: the syntax and other explanations;
- example: an example, if available.
The system tables:
There are four system tables used by HELP and they are contained in the system database ‘mysql’. Their structure is quite simple, let’s examine it.
The categories are contained in the help_category table, which has the following fields:
help_category_id: primary key;
name: category name, on this field there is a UNIQUE index;
parent_category_id: is the id of the parent category, 0 if there is no parent category;
url: this field is always empty, it should probably contain the url corresponding to the category in the official documentation on the MySQL site.
The arguments are contained in the help_topic table, which has the following fields:
- help_topic_id: primary key;
- name: name of the argument, on this field there is a UNIQUE index;
- help_category_id: the id of the category to which the argument belongs;
- description: the explanation of the argument;
- example: an example;
- url: the url of the official documentation page that corresponds to the argument.
Since the HELP instruction never returns the url field, if you need more information, you can use the following query to quickly find out which url to consult:
SELECT url FROM help_topic WHERE name='argument
The last two tables probably do not interest us, but we analyse them anyway for completeness.
Some keywords are associated with topics (not categories). They are used internally for research, in a completely transparent way, and are contained in the help_keyword table, which has the following fields:
- help_keyword_id: primary key;
- name: the keyword, on this field there is a UNIQUE index.
The relationship between keywords and arguments, being a many to many relationship, is represented in the help_relation table, which has the following fields:
- help_keyword_id: the id of a keyword;
help_topic_id: the id of an argument.
Both fields together make up the primary key.
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