Question

我有三个表：pages，blocks和pages_blocks。

我的数据库架构如下所示：

CREATE TABLE `blocks` (
  `id` smallint(6) NOT NULL AUTO_INCREMENT,
  `name` char(40) NOT NULL,
  `content` text NOT NULL
  PRIMARY KEY (`id`)
);

CREATE TABLE `pages` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` char(100) NOT NULL,
  `content` mediumtext NOT NULL
  PRIMARY KEY (`id`)
);

CREATE TABLE `pages_blocks` (
  `page_id` int(11) DEFAULT NULL,
  `block_id` smallint(6) DEFAULT NULL,
  `location` enum('left','right') DEFAULT NULL,
  `display_order` smallint(6) DEFAULT NULL
);

获取特定页面的“左”和“右”块的完美SQL代码是什么？

非常感谢您的帮助。

Answer 1

一种方法是左右两个连接搜索：

select  *
from    pages p
left join
        (
        select  *
        from    pages_blocks pb
        join    blocks b
        on      pb.block_id = b.id
        where   pb.location = 'left'
        ) left_block
on      left_block.page_id = p.id
left join
        (
        select  *
        from    pages_blocks pb
        join    blocks b
        on      pb.block_id = b.id
        where   pb.location = 'right'
        ) right_block
on      right_block.page_id = p.id
where   p.id = 42

这样做的好处是只返回一行而不使用group by。

编辑：如果可以有多个“右”和“左”块，您可以查询所有块，如：

select  pb.location
,       pb.display_order
,       b.name
,       b.content
from    pages p
join    pages_blocks pb
on      pb.page_id = p.id
join    blocks b
on      pb.block_id = b.id
where   p.id = 42
order by
        pb.location
,       pb.display_order

Answer 2

您可以使用UNION：

select a.content 
from blocks a, pages_blocks b 
where a.id=b.block_id and b.page_id=123 and b.location='left' 
union 
select a.content 
from blocks a, pages_blocks b 
where a.id=b.block_id and b.page_id=123 and b.location='right'

获取页面块

2 个答案: