Question

这是我的疑问：

SELECT messages.id AS m_id, messages.user_id AS m_uid, messages.project_id AS m_pid, messages.date_created AS m_dc, messages.type AS m_type, messages.file_url AS m_fu, messages.message_text AS m_text, messages.deleted AS m_del,
               projects.id AS p_id, projects.name AS p_name, projects.company_id AS p_cid,
               users.id AS u_id, users.name AS u_name
        FROM messages, projects, users
        HAVING `m_pid` = '$project_id' AND m_uid = u_id
        ORDER BY `m_dc` DESC

我一直在使用HAVING而不是WHERE，因为WHERE似乎没有使用我正在使用的AS关键字。结果是我的查询返回的结果超出了我的需要。我想我应该在这里使用JOIN，但我无法真正掌握它们。请帮忙！

:)谢谢

Answer 1

这是真的，you should use JOIN instead of the FROM table1, table2 syntax。

更直接地回答您的问题：WHERE子句对表中的列进行操作 - HAVING子句对结果集（包括您的别名）进行操作。

您的表格如何相互关联？这是你需要首先回答的问题 - 一旦得到答案，你就把它直接放在JOIN的ON子句中。

SELECT messages.id AS m_id, messages.user_id AS m_uid, messages.project_id AS m_pid, messages.date_created AS m_dc, messages.type AS m_type, messages.file_url AS m_fu, messages.message_text AS m_text, messages.deleted AS m_del, projects.id AS p_id, projects.name AS p_name, projects.company_id AS p_cid,
users.id AS u_id, users.name AS u_name
FROM messages
JOIN users ON messages.user_id = user.id
JOIN projects ON projects.user_id = user.id # this is just a guess
WHERE messages.id = $project_id
ORDER BY `m_dc` DESC

为了更好地了解JOIN如何将表关联在一起，您可能希望查看这个方便的page showing JOINs as Venn diagrams。

Answer 2

试试这个？

SELECT m.id AS m_id, 
    m.user_id AS m_uid, 
    m.project_id AS m_pid, 
    m.date_created AS m_dc, 
    m.type AS m_type, 
    m.file_url AS m_fu, 
    m.message_text AS m_text, 
    m.deleted AS m_del,               
    p.id AS p_id, 
    p.name AS p_name, 
    p.company_id AS p_cid,               
    u.id AS u_id, 
    u.name AS u_name
FROM messages as m
INNER JOIN projects as p on p.id = m.project_id
INNER JOIN users as u ON u.id = m.user_id              
WHERE m.project_id = '$project_id' 
ORDER BY ....

Answer 3

SELECT 
       messages.id AS m_id, messages.user_id AS m_uid, messages.project_id AS m_pid, 
       messages.date_created AS m_dc, messages.type AS m_type, messages.file_url AS m_fu, 
       messages.message_text AS m_text, messages.deleted AS m_del,
       projects.id AS p_id, projects.name AS p_name, projects.company_id AS p_cid,
       users.id AS u_id, users.name AS u_name
FROM 
  messages 
JOIN 
  users ON messages.user_id = users.id
JOIN 
  projects ON projects.id=messages.project_id
WHERE 
  `m_pid` = '$project_id'
ORDER BY 
  `m_dc` DESC

PHP / MySQL：我想我需要在这里使用JOIN，但我不知道怎么做

3 个答案: