MySQL Basics Tutorial 10 - Function Full-text Search Function

Grammar:

• MATCH (col1,col2,...) AGAINST (expr [IN BOOLEAN MODE | WITH QUERY EXPANSION])

MySQL supports full-text indexing and search functions. The full-text index type FULLTEXT index in MySQL. FULLTEXT indexes are only available on MyISAM tables; they can be created from CHAR, VARCHAR, or TEXT columns as part of a CREATE TABLE statement, or added later using ALTER TABLE or CREATE INDEX. For larger data sets, entering your data into a table that does not have a FULLTEXT index and then creating the index is faster than entering the data into an existing FULLTEXT index.

Full text search is performed with the MATCH() function.

mysql> CREATE TABLE articles (    
->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,    
->   title VARCHAR(200),    
->   body TEXT,    
->   FULLTEXT (title,body)    
-> );Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO articles (title,body) VALUES    
-> ('MySQL Tutorial','DBMS stands for DataBase ...'),    
-> ('How To Use MySQL Well','After you went through a ...'),    
-> ('Optimizing MySQL','In this tutorial we will show ...'),    
-> ('1001 MySQL Tricks','1. Never run mysqld as root. 2. ...'),    
-> ('MySQL vs. YourSQL','In the following database comparison ...'),    
-> ('MySQL Security','When configured properly, MySQL ...');Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM articles    
-> WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+------------------------------------------+
| id | title             | body          |
+----+-------------------+------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...            |
+----+-------------------+------------------------------------------+
2 rows in set (0.00 sec)

The MATCH() function performs a natural language search within the database for a string. A database is a set of 1 or 2 columns contained in FULLTEXT. The search string is given as a parameter to AGAINST(). For each row in the table, MATCH() returns a correlation value, that is, a similarity measure between the search string and the text in that row in the specified column in the MATCH() table.

By default, the search is performed in a case-insensitive manner. However, you can perform a case-sensitive full-text search by using a binary sort on the indexed columns. For example, you can give a latin1_bin sorting method to a column that uses the latin1 character set, making full-text searches case-sensitive.

As in the above example, when MATCH() is used in a WHERE statement, the relevant value is a non-negative floating point number. Zero correlation means no similarity. The correlation is calculated based on the number of words in the line, the number of uniques in the line, the total number of words in the database, and the number of files (lines) that contain the unique word.

For natural language full-text search, it is required that the columns named in the MATCH() function are the same as the columns contained in some FULLTEXT indexes in your table. For the above query, please note that the columns named in the MATCH() function (title and full text) are the same as the columns in the FULLTEXT index of the article table. If you want to search the title and full text separately, you should create a FULLTEXT index on each column.

Alternatively you can run a Boolean search or search using query expansion.

The above example basically shows how to use the MATCH() function that returns rows in a decreasing order of correlation. The following example shows how to retrieve the relevant value explicitly. The order of the returned rows is uncertain because the SELECT statement does not contain a WHERE or ORDER BY clause:

mysql> SELECT id, MATCH (title,body) AGAINST ('Tutorial')    
-> FROM articles;
+----+-----------------------------------------+
| id | MATCH (title,body) AGAINST ('Tutorial') |
+----+-----------------------------------------+
|  1 |                        0.65545833110809 |
|  2 |                                       0 |
|  3 |                        0.66266459226608 |
|  4 |                                       0 |
|  5 |                                       0 |
|  6 |                                       0 |
+----+-----------------------------------------+
6 rows in set (0.00 sec)

The following example is more complicated. The query returns the relevant values, sorting the rows in order of decreasing relevance. To achieve this result, you should specify MATCH() twice: once in the SELECT list and once in the WHERE clause. This causes no additional housekeeping because the MySQL optimizer notices that the two MATCH() calls are identical and activates the full-text search code only once.

mysql> SELECT id, body, MATCH (title,body) AGAINST    
-> ('Security implications of running MySQL as root') AS score    
-> FROM articles WHERE MATCH (title,body) AGAINST    
-> ('Security implications of running MySQL as root');
+----+-------------------------------------+-----------------+
| id | body                                | score           |
+----+-------------------------------------+-----------------+
|  4 | 1. Never run mysqld as root. 2. ... | 1.5219271183014 |
|  6 | When configured properly, MySQL ... | 1.3114095926285 |
+----+-------------------------------------+-----------------+
2 rows in set (0.00 sec)

There are 2 rows in the table (0.00 seconds)

MySQL FULLTEXT execution treats any sequence of single-word character prototypes (letters, numbers, and underscore parts) as a word. This sequence may also contain single quotes ('), but there will be no more than one on a line. This means aaa'bbb will be treated as one word, while aaa''bbb will be treated as 2 words. Single quotes before or after a word will be removed by the FULLTEXT parser; 'aaa'bbb' will become aaa'bbb.

The FULLTEXT parser determines where a word begins and ends by looking for certain delimiters, such as ' ' (space mark), , (comma), and . (period). If words are not separated by delimiters (such as in Chinese), the FULLTEXT parser cannot determine the start and end positions of a word. In order to be able to add words or other indexed terms to a FULLTEXT index in such a language, you must preprocess them so that they are separated by some arbitrary delimiter such as ".

Some words will be ignored in full-text searches:

• ##Any words that are too short will be ignored. The default minimum length of words found by full-text searches is 4 characters.

Words in stop words are ignored. A stop word is a word like "the" or "some" that is too common to be considered semantic, but there is a built-in stop word. It can be overridden through user-defined lists

• ## Each correct word in the vocabulary and query is measured according to its importance in the vocabulary and query. In this way, a word that appears in many documents has a lower importance (and even many words have zero importance) because of its lower semantic value in this particular vocabulary. On the contrary, if the word is rare, Then it gets a higher importance. The importance of the words is then combined and used to calculate the relevance of the row.

  这项技术最适合同大型词库一起使用 (事实上, 此时它经过仔细的调整 )。对于很小的表，单词分布并不能充分反映它们的语义价值， 而这个模式有时可能会产生奇特的结果。例如, 虽然单词 “MySQL” 出现在文章表中的每一行，但对这个词的搜索可能得不到任何结果：

  mysql> SELECT * FROM articles

  -> WHERE MATCH (title,body) AGAINST ('MySQL');

  找不到搜索的词(0.00 秒)

  这个搜索的结果为空，原因是单词 “MySQL” 出现在至少全文的50%的行中。 因此, 它被列入停止字。对于大型数据集，使用这个操作最合适不过了----一个自然语言问询不会从一个1GB 的表每隔一行返回一次。对于小型数据集，它的用处可能比较小。

  一个符合表中所有行的内容的一半的单词查找相关文档的可能性较小。事实上, 它更容易找到很多不相关的内容。我们都知道，当我们在因特网上试图使用搜索引擎寻找资料的时候，这种情况发生的频率颇高。可以推论，包含该单词的行因其所在特别数据集 而被赋予较低的语义价值。 一个给定的词有可能在一个数据集中拥有超过其50%的域值，而在另一个数据集却不然。

  当你第一次尝试使用全文搜索以了解其工作过程时，这个50% 的域值提供重要的蕴涵操作：若你创建了一个表，并且只将文章的1、2行插入其中， 而文中的每个单词在所有行中出现的机率至少为  50% 。那么结果是你什么也不会搜索到。一定要插入至少3行，并且多多益善。需要绕过该50% 限制的用户可使用布尔搜索代码。

  1. 布尔全文搜索

  利用IN BOOLEAN MODE修改程序， MySQL 也可以执行布尔全文搜索：

  mysql> SELECT * FROM articles WHERE MATCH (title,body)    
  -> AGAINST ('+MySQL -YourSQL' IN BOOLEAN MODE);
  +----+-----------------------+-------------------------------------+
  | id | title                 | body                                |
  +----+-----------------------+-------------------------------------+
  |  1 | MySQL Tutorial        | DBMS stands for DataBase ...        |
  |  2 | How To Use MySQL Well | After you went through a ...        |
  |  3 | Optimizing MySQL      | In this tutorial we will show ...   |
  |  4 | 1001 MySQL Tricks     | 1. Never run mysqld as root. 2. ... |
  |  6 | MySQL Security        | When configured properly, MySQL ... |
  +----+-----------------------+-------------------------------------+

  这个问询检索所有包含单词“MySQL”的行，但不检索包含单词“YourSQL”的行。

  布尔全文搜索具有以下特点：

  • 它们不使用 50% 域值。.

  • 它们不会按照相关性渐弱的顺序将行进行分类。你可以从上述问询结果中看到这一点：相关性最高的行是一个包含两个“MySQL” 的行，但它被列在最后的位置，而不是开头位置。

  • 即使没有FULLTEXT，它们仍然可以工作，尽管这种方式的搜索执行的速度非常之慢。

  • 最小单词长度全文参数和最大单词长度全文参数均适用。

  • 停止字适用。

  布尔全文搜索的性能支持以下操作符：

  • +

  一个前导的加号表示该单词必须 出现在返回的每一行的开头位置。

  • -

  一个前导的减号表示该单词一定不能出现在任何返回的行中。

  • (无操作符)

  在默认状态下(当没有指定 + 或–的情况下)，该单词可有可无，但含有该单词的行等级较高。这和MATCH() ... AGAINST()不使用IN BOOLEAN MODE修改程序时的运作很类似。

  • > fb0fed43b83fb0652d5f29eaa4ee0614 操作符增强其影响，而 c338afddb723986043c602cf4e852541turnover 86987e0d0a2efc04ebede17b4748f1be

    REPAIR TABLE

    tbl_name QUICK;Note that if you Using

    myisamchk

    to perform an operation that modifies the table index (such as repair or analysis), the FULLTEXT index is rebuilt using the default full-text parameter values ​​for minimum and maximum word length and stop words, unless you specify otherwise . This will cause the query to fail.

    发生这个问题的原因是只有服务器认识这些参数。它们的存储位置不在  MyISAM 索引文件中。若你已经修改了最小单词长度或最大单词长度或服务器中的停止字，为避免这个问题，为你对mysqld所使用的myisamchk 指定同样的ft_min_word_len、 ft_max_word_len和ft_stopword_file值。例如，假如你已经将最小单词长度设置为 3, 则你可以这样修改一个带有myisamchk的表：

    shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

    为保证 myisamchk 及服务器对全文参数使用相同的值， 可将每一项都放在供选文件中的 [mysqld]和 [myisamchk] 部分：

    [mysqld]
    ft_min_word_len=3
    [myisamchk]
    ft_min_word_len=3

    使用 REPAIR TABLE、 ANALYZE TABLE、OPTIMIZE TABLE或ALTER TABLE来代替使用 myisamchk 。这些语句通过服务器来执行，服务器知道使用哪个全文参数值更加合适。

     以上就是MySQL基础教程10 —— 函数之全文搜索功能的内容，更多相关内容请关注PHP中文网（www.php.cn）！