Question

I have three tables: pages, blocks and pages_blocks.

My database schema looks as follow:

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
);

What would be the perfect SQL code to grab the “left” and “right” blocks for a specific page?

Your help is much appreciated.

Answer 1

One way would be two joins searching left and right:

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

This has the advantage of returning only one row without using group by.

EDIT: if there can be multiple “right” and “left” blocks, you could query all blocks like:

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