Question

我从mySQL数据库中选择一些数据。我想只选择project_id不为NULL或为空的数据：

$result = $db->query('SELECT * 
         FROM data 
         LEFT JOIN projects ON data.project=projects.project_id
         GROUP BY data.id
         WHERE projects.project_id IS NOT NULL
         ORDER BY data.id DESC
         ')->fetchAll(PDO::FETCH_ASSOC);

但是我收到一条错误消息：

未捕获的PDOException：SQLSTATE [42000]：语法错误或访问违规：1064您的SQL语法有错误;检查手册对应于您的MySQL服务器版本，以获得正确的语法在第21行'WHERE projects.project_id IS NOT NOT ORDER BY d'附近使用

Answer 1

where子句出现在group by

之前

我相信您已经拥有主键，因此您无需明确添加null支票。如果您只需要匹配的数据集，请使用inner join代替left join。

Answer 2

这应该有效，您只是没有使用where和order by

来遵循正确的语法

$result = $db->query('SELECT * 
          FROM data 
          LEFT JOIN projects ON data.project=projects.project_id
          WHERE projects.project_id IS NOT NULL
          GROUP BY data.id
          ORDER BY data.id DESC
          ')->fetchAll(PDO::FETCH_ASSOC);

Answer 3

查询应该有一个正确的语法，因为group by将在where之前。查询应该是

          SELECT * FROM data 
          LEFT JOIN projects ON data.project=projects.project_id
          WHERE projects.project_id IS NOT NULL
          GROUP BY data.id
          ORDER BY data.id DESC

如何检查表格单元格是空还是NULL？

3 个答案: