Question

我有以下查询

SELECT 
  `c`.`id` AS `confession_id`,
  `c`.`account_id` AS `confession_author_id`,
  `c`.`confession_text`,
  `c`.`anonymous`,
  `c`.`total_likes`,
  `c`.`total_comments`,
  `c`.`creation_date`,
  `a`.`username`,
  `col`.`name` AS `college`,
  `col`.`avatar_url`,
  `cll`.`deleted` AS `tacc_exists` 
FROM
  `confessions` AS `c` 
  LEFT JOIN `accounts` AS `a` 
    ON `a`.`id` = `c`.`account_id` 
  LEFT JOIN `colleges` AS `col` 
    ON `col`.`id` = `c`.`college_id` 
  LEFT JOIN `confession_likes_log` AS `cll` 
    ON `cll`.`confession_id` = `c`.`id` 
    AND `cll`.`account_id` = :taccd_user 
    AND `c`.`college_id` IN ('9') 
ORDER BY `c`.`id` DESC 
LIMIT 10

假设根据提供的大学ID选择指定大学的条目

AND `c`.`college_id` in ('9')

虽然由于某种原因，即使college_id不是指定的那个，它也在选择条目。我在查找如何使用左连接执行和子句时遇到了麻烦，所以我可能没有正确地格式化查询。

Answer 1

这是您的where条款：

              FROM `confessions` AS `c` LEFT JOIN
                   `colleges` AS `col`
                    ON `col`.`id` = `c`.`college_id` LEFT JOIN
                   `confession_likes_log` AS `cll`
                    ON `cll`.`confession_id` = `c`.`id` AND
                       `cll`.`account_id` = :taccd_user AND
                        `c`.`college_id` in ('9')

left join的工作方式是：在on子句中运行测试。如果第一个表中的给定行不匹配，则包括右表中具有NULL值的行。你猜怎么着。这甚至适用于第一个表中的条件。

所以，过滤器c.college_id in ('9') isn't doing what you think. You need to move it to a where where子句：

              FROM `confessions` AS `c` LEFT JOIN
                   `colleges` AS `col`
                    ON `col`.`id` = `c`.`college_id` LEFT JOIN
                   `confession_likes_log` AS `cll`
                    ON `cll`.`confession_id` = `c`.`id` AND
                       `cll`.`account_id` = :taccd_user
              WHERE `c`.`college_id` in ('9')

mysql查询问题

1 个答案: