Question

SELECT n.type, n.type_id, n.data,
( CASE WHEN (n.type='users') THEN users.u_name ELSE NULL END ) AS user_name,
( CASE WHEN (n.type='blogs') THEN blogs.b_name ELSE NULL END ) AS blog_name
FROM notifications AS n
LEFT JOIN users ON ( CASE
                     WHEN (n.type='users') AND n.type_id=users.id THEN 1
                     ELSE 0
                     END = 1 )
LEFT JOIN blogs ON ( CASE 
                     WHEN (n.type='blogs') AND n.type_id=blogs.id THEN 1
                     ELSE 0
                     END = 1 )
WHERE n.receiver_id = 1
ORDER BY n.id DESC LIMIT 20

它有效，但用途：使用临时;使用filesort，使用连接缓冲区（flat，BNL join），使用连接缓冲区（incremental，BNL join）。这看起来不太好。

此外，LEFT Joins应该是INNER，但是在内部它不显示任何结果......（和相同的temporarys，filesorts，缓冲区等）

我尝试了不同的语法（可能是错的），结果相同：

 ( CASE WHEN (n.type='blogs') THEN n.type_id=blogs.id ELSE NULL END )

我可以做两个查询，但我试图在一个查询中执行：）

Answer 1

SELECT n.type, n.type_id, n.data,
       users.u_name as user_name,
       blogs.b_name AS blog_name
FROM notifications AS n
LEFT JOIN users ON n.type='users' AND n.type_id=users.id 
LEFT JOIN blogs ON n.type='blogs' AND n.type_id=blogs.id
WHERE n.receiver_id = 1
ORDER BY n.id DESC 
LIMIT 20

你不需要这些案件。 left join是正确的方法，因为通知来自用户或博客，而不是两者。

Answer 2

尝试以下查询：

这使用内连接和直接连接条件。请注意，SELECT子句中有一个额外的列n.id.

SELECT  n.id, n.type, n.type_id, n.data, u.u_name AS user_name, NULL AS blog_name
FROM    notification n INNER JOIN users u ON n.receiver_id = 1 AND n.type = 'users' AND n.type_id = u.id
UNION ALL
SELECT  n.id, n.type, n.type_id, n.data, NULL AS user_name, b.b_name AS blog_name
FROM    notification n INNER JOIN blogs b ON n.receiver_id = 1 AND n.type = 'blogs' AND n.type_id = b.id
ORDER BY 1 DESC LIMIT 20;

Answer 3

试试这个：

SELECT n.type, n.type_id, n.data, 
  (select u.u_name from users as u where u.id = n.type_id and n.type = 'users'), 
  (select b.b_name from blogs as b where b.id = n.type_id and n.type = 'blogs')
FROM notifications AS n
WHERE n.receiver_id = 1
ORDER BY n.id DESC LIMIT 20

MySQL：使用条件连接改进查询

3 个答案: