help_topic

• 该表提供查询帮助主题的详细内容（详细帮助信息）
• 表结构定义

CREATE TABLE `help_topic` (
  `help_topic_id` int(10) unsigned NOT NULL,
  `name` char(64) NOT NULL,
  `help_category_id` smallint(5) unsigned NOT NULL,
  `description` text NOT NULL,
  `example` text NOT NULL,
  `url` text NOT NULL,
  PRIMARY KEY (`help_topic_id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 STATS_PERSISTENT=0 COMMENT='help topics';

• 表字段含义

• help_topic_id：帮助主题详细信息在表记录中对应的ID
• name：帮助主题名称，与help_keyword表中的name字段值相等
• help_category_id：帮助类别ID，与help_category表中的help_category_id字段值相等
• description：帮助主题的详细信息（这里就是我们通常查询帮助信息真正想看的内容，例如：告诉我们某某语句如何使用的语法与注意事项等）
• example：帮助主题的示例信息（这里告诉我们某某语句如何使用的示例）
• url：该帮助主题对应在MySQL官方在线手册中的URL链接地址

• 表记录内容示例

root@localhost : mysql 01:13:31> select * from help_topic limit 1\G;
*************************** 1. row ***************************
   help_topic_id: 0
            name: JOIN
help_category_id: 28
     description: MySQL supports the following JOIN syntaxes for the table_references
part of SELECT statements and multiple-table DELETE and UPDATE
statements:

table_references:
    escaped_table_reference [, escaped_table_reference] ...

escaped_table_reference:
    table_reference
  | { OJ table_reference }

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [PARTITION (partition_names)]
        [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

index_hint_list:
    index_hint [, index_hint] ...

index_hint:
    USE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
  | IGNORE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
  | FORCE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
    index_name [, index_name] ...

A table reference is also known as a join expression.

A table reference (when it refers to a partitioned table) may contain a
PARTITION option, including a comma-separated list of partitions,
subpartitions, or both. This option follows the name of the table and
precedes any alias declaration. The effect of this option is that rows
are selected only from the listed partitions or subpartitions---in
other words, any partitions or subpartitions not named in the list are
ignored For more information, see
http://dev.mysql.com/doc/refman/5.7/en/partitioning-selection.html.

The syntax of table_factor is extended in comparison with the SQL
Standard. The latter accepts only table_reference, not a list of them
inside a pair of parentheses.

This is a conservative extension if we consider each comma in a list of
table_reference items as equivalent to an inner join. For example:

SELECT * FROM t1 LEFT JOIN (t2, t3, t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

is equivalent to:

SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

In MySQL, JOIN, CROSS JOIN, and INNER JOIN are syntactic equivalents
(they can replace each other). In standard SQL, they are not
equivalent. INNER JOIN is used with an ON clause, CROSS JOIN is used
otherwise.

In general, parentheses can be ignored in join expressions containing
only inner join operations. MySQL also supports nested joins (see
http://dev.mysql.com/doc/refman/5.7/en/nested-join-optimization.html).

Index hints can be specified to affect how the MySQL optimizer makes
use of indexes. For more information, see
http://dev.mysql.com/doc/refman/5.7/en/index-hints.html.

URL: http://dev.mysql.com/doc/refman/5.7/en/join.html


         example: SELECT left_tbl.*
  FROM left_tbl LEFT JOIN right_tbl ON left_tbl.id = right_tbl.id
  WHERE right_tbl.id IS NULL;

             url: http://dev.mysql.com/doc/refman/5.7/en/join.html
1 row in set (0.00 sec)

---

上一篇：help_relation表 |下一篇：Server端帮助文档