MySQL 与 group_concat 左连接答案

【问题标题】：MySQL left join with group_concatMySQL 与 group_concat 左连接
【发布时间】：2013-08-25 15:46:19
【问题描述】：

我在创建表和查询时遇到了问题：http://www.sqlfiddle.com/#!9/3404e/1

这是表创建脚本：

CREATE TABLE IF NOT EXISTS `collection` (
  `id` bigint(20) unsigned NOT NULL,
  `name` varchar(50) NOT NULL,
  `label` varchar(120) NOT NULL,
  `label_plural` varchar(120) NOT NULL);

INSERT INTO `collection` (`id`, `name`, `label`, `label_plural`) VALUES
    (1, 'account', 'Account', 'Accounts');

CREATE TABLE IF NOT EXISTS `field` (
  `id` bigint(20) unsigned NOT NULL,
  `name` varchar(50) NOT NULL,
  `label` varchar(120) NOT NULL,
  `collection_id` bigint(20) unsigned NOT NULL);

INSERT INTO `field` (`id`, `name`, `label`, `collection_id`) VALUES
    (1, 'name', 'Name', 1),
    (2, 'state', 'State', 1);

CREATE TABLE IF NOT EXISTS `option` (
  `id` bigint(20) unsigned NOT NULL,
  `record_type_id` bigint(20) unsigned DEFAULT NULL,
  `field_id` bigint(20) unsigned NOT NULL,
  `value` varchar(120) NOT NULL);

INSERT INTO `option` (`id`, `record_type_id`, `field_id`, `value`) VALUES
    (1, NULL, 2, 'CO'),
    (2, NULL, 2, 'NE'),
    (3, NULL, 2, 'BC'),
    (4, NULL, 2, 'MB'),
    (5, 1, 2, 'CO'),
    (6, 1, 2, 'NE'),
    (7, 2, 2, 'BC'),
    (8, 2, 2, 'MB');

CREATE TABLE IF NOT EXISTS `record_type` (
  `id` bigint(20) unsigned NOT NULL,
  `name` varchar(120) NOT NULL,
  `collection_id` bigint(20) unsigned NOT NULL);

INSERT INTO `record_type` (`id`, `name`, `collection_id`) VALUES
    (1, 'US', 1),
    (2, 'Canada', 1);

这是我要运行的查询：

select
    `field`.`name`,
    `field`.`label`,
    ifnull(group_concat(`option`.`value` separator ';'), '') as `options`
from
    `field`
join
    `collection` on
        `collection`.`id` = `field`.`collection_id`
join
    `record_type` on
        `record_type`.`collection_id` = `collection`.`id`
left join
    `option` on
        `option`.`record_type_id` = `record_type`.`id` and
        `option`.`field_id` = `field`.`id`
where
    `record_type`.`name` = 'US' and
    `collection`.`name` = 'account';

我期待的是如下两行：

+-------+-------+---------+
| name  | label | options |
+-------+-------+---------+
| name  | Name  | NULL    |
| state | State | CO;NE   |
+-------+-------+---------+

但我只收到状态行。如果我删除 group_concat 行，我会得到如下三行，所以我知道所有内容都会返回：

+-------+-------+--------+
| name  | label | option |
+-------+-------+--------+
| name  | Name  | NULL   |
| state | State | CO     |
| state | State | NE     |
+-------+-------+--------+

【问题讨论】：

标签： mysql left-join group-concat

【解决方案1】：

您基本上缺少GROUP BY、GROUP_CONCAT() 连接组中的所有值，而没有GROUP BY 只有一个组。

试试这个；

select
    `field`.`name`, `field`.`label`,
    group_concat(`option`.`value` separator ';') as `options`
from `field`
join `collection` 
  on `collection`.`id` = `field`.`collection_id`
join `record_type` 
  on `record_type`.`collection_id` = `collection`.`id`
left join `option` 
  on `option`.`record_type_id` = `record_type`.`id` 
 and `option`.`field_id` = `field`.`id`
where `record_type`.`name` = 'US' 
  and `collection`.`name` = 'account'
GROUP BY `field`.`name`,`field`.`label`

An SQLfiddle with the correction.

【讨论】：

【解决方案2】：

您的查询的主要问题是您没有group by 子句。

此外，您的where 条件正在“撤消”left joins，将它们变成inner joins。以下实现了您想要的：

select
    `field`.`name`,
    `field`.`label`,
    ifnull(group_concat(`option`.`value` separator ';'), '') as `options`
from
    `field`
join
    `collection` on
        `collection`.`id` = `field`.`collection_id` and
        `collection`.`name` = 'account'
join
    `record_type` on
        `record_type`.`collection_id` = `collection`.`id` and
        `record_type`.`name` = 'US' 
left join
    `option` on
        `option`.`record_type_id` = `record_type`.`id` and
        `option`.`field_id` = `field`.`id`
group by field.name, field.label;

【讨论】：