在sql语句中使where子句可选

Question

这是我当前的查询：

SELECT `performers`.`hash`,
        `performers`.`alias`,
        `performers`.`date_updated`,
        `performers`.`status`, 
        IF(`performers`.`status` = 'active', 'deleted','active') AS `statususe`,
        `images`.`image_hash_file` 
FROM `performers` 
LEFT JOIN `images` ON `images`.`asset_id` = `performers`.`id` 
WHERE (`images`.`asset_type` = 'performer') 
ORDER BY `alias` ASC LIMIT 12`

其中有一个where子句

WHERE (`images`.`asset_type` = 'performer')

我希望它是可选的，这样如果where子句不合适，它仍会显示表演者表中没有连接到完成图像记录的记录。

Answer 1

尝试这样的事情：

WHERE (`images`.`asset_type` = 'performer' OR `images`.`asset_type` IS NULL) 

Answer 2

您可以在LEFT JOIN中添加WHERE子句，因此不匹配的行仍在结果集中。

SELECT `performers`.`hash`, `performers`.`alias`, 
  `performers`.`date_updated`, `performers`.`status`,
  IF(`performers`.`status` = 'active', 'deleted','active') AS `statususe`,
  `images`.`image_hash_file`
FROM `performers`
LEFT JOIN `images` ON `images`.`asset_id` = `performers`.`id` AND `images`.`asset_type` = 'performer'
ORDER BY `alias` ASC
LIMIT 12

WHERE和ON子句之间的主要区别在于ON子句在JOIN期间工作，当WHERE子句在所有连接之后工作时，不会从结果集中删除匹配的行。

Answer 3

您应该将where条件移至on子句：

SELECT p.`hash`, p.`alias`, p.`date_updated`, p.`status`, 
       IF(p.`status` = 'active',  'deleted', 'active') AS statususe,
       i.`image_hash_file` 
FROM `performers`  p LEFT JOIN
     `images`i
     ON i.`asset_id` = p.`id` and
        i.`asset_type` = 'performer' 
ORDER BY `alias` ASC
LIMIT 12

我还修改了查询以使用表别名。它们使查询更容易编写和阅读。