左边加入Where子句

Question

我需要从设置表中检索所有默认设置,但如果存在x字符,还需要获取字符设置.

但是此查询仅检索字符= 1的那些设置,而不是用户未设置任何人的默认设置.

SELECT `settings`.*, `character_settings`.`value`
FROM (`settings`)
LEFT JOIN `character_settings` 
ON `character_settings`.`setting_id` = `settings`.`id`
WHERE `character_settings`.`character_id` = '1'  

所以我应该需要这样的东西:

array(
    '0' => array('somekey' => 'keyname', 'value' => 'thevalue'),
    '1' => array('somekey2' => 'keyname2'),
    '2' => array('somekey3' => 'keyname3')
)

其中,当键0包含具有字符值的默认值时,键1和键2是默认值.

Answer 1

该where子句过滤掉left join不成功的行.将其移至联接:

SELECT  `settings`.*, `character_settings`.`value`
FROM    `settings`
LEFT JOIN 
       `character_settings` 
ON     `character_settings`.`setting_id` = `settings`.`id`
        AND `character_settings`.`character_id` = '1'  

Answer 2

在进行OUTER JOIN(ANSI-89或ANSI-92)时,过滤位置很重要,因为在JOIN生成之前ON应用了子句中指定的条件.在JOIN之后应用该子句中提供的OUTER JOINed表的标准.这可以产生非常不同的结果集.相比较而言,这并不重要,如果在提供了标准内连接或从句-结果将是相同的.WHEREONWHERE

  SELECT  s.*, 
          cs.`value`
     FROM SETTINGS s
LEFT JOIN CHARACTER_SETTINGS cs ON cs.setting_id = s.id
                               AND cs.character_id = 1

Answer 3

如果我正确理解了您的问题,那么如果他们没有与character_settings表相关联的连接,或者如果该连接记录的character_id = 1,则需要设置数据库中的记录.

你应该这样做

SELECT `settings`.*, `character_settings`.`value`
FROM (`settings`)
LEFT OUTER JOIN `character_settings` 
ON `character_settings`.`setting_id` = `settings`.`id`
WHERE `character_settings`.`character_id` = '1' OR
`character_settings`.character_id is NULL