我有一个JSON字段来保存帖子的标签。
id:1, content:'...', tags: ["tag_1", "tag_2"]
id:2, content:'...', tags: ["tag_3", "tag_2"]
id:3, content:'...', tags: ["tag_1", "tag_2"]
我只想列出所有标签的流行程度(甚至没有它们):
tag_2:3,
tag_1:2,
tag_3:1
答案 0 :(得分:2)
以下是设置:
create table t ( id serial primary key, content json);
insert into t set content = '{"tags": ["tag_1", "tag_2"]}';
insert into t set content = '{"tags": ["tag_3", "tag_2"]}';
insert into t set content = '{"tags": ["tag_1", "tag_2"]}';
如果您知道任何标记数组中的最大标记数,则可以使用UNION提取所有标记:
select id, json_extract(content, '$.tags[0]') AS tag from t
union
select id, json_extract(content, '$.tags[1]') from t;
+----+---------+
| id | tag |
+----+---------+
| 1 | "tag_1" |
| 2 | "tag_3" |
| 3 | "tag_1" |
| 1 | "tag_2" |
| 2 | "tag_2" |
| 3 | "tag_2" |
+----+---------+
您需要与最长数组中的标记数一样多的联合子查询。
然后,您可以将其放在派生表中并对其执行聚合:
select tag, count(*) as count
from (
select id, json_extract(content, '$.tags[0]') as tag from t
union
select id, json_extract(content, '$.tags[1]') from t
) as t2
group by tag
order by count desc;
+---------+-------+
| tag | count |
+---------+-------+
| "tag_2" | 3 |
| "tag_1" | 2 |
| "tag_3" | 1 |
+---------+-------+
如果您将标记存储在第二个表而不是JSON数组中,这将更容易:
create table tags ( id bigint unsigned, tag varchar(20) not null, primary key (id, tag));
insert into tags set id = 1, tag = 'tag_1';
insert into tags set id = 1, tag = 'tag_2';
insert into tags set id = 2, tag = 'tag_3';
insert into tags set id = 2, tag = 'tag_2';
insert into tags set id = 3, tag = 'tag_1';
insert into tags set id = 3, tag = 'tag_2';
select tag, count(*) as count
from tags
group by tag
order by count desc;
+-------+-------+
| tag | count |
+-------+-------+
| tag_2 | 3 |
| tag_1 | 2 |
| tag_3 | 1 |
+-------+-------+
无论您拥有多少个标签,此解决方案都可以使用。您不需要知道每个ID的标签列表的最大长度。
当您需要存储'文件时,JSON很不错。半结构化数据,但仅当您将文档视为一个不可减少的数据值时。只要您需要访问文档的元素并对它们应用关系操作,面向文档的方法就会显示出它的弱点。