MySQL从多值字段获取唯一值

Question

我有一个具有以下结构的MySQL表

id       categories
1        ["Pizza","Subs","Sandwiches","Seafood","Italian"]
2        ["Pizza","Salad","Sandwiches","Subs"]
3        ["Fast Food","Burgers","Salad","American","Cafe"]
4        ["Coffee","Cafe"]

我需要通过SQL查询获取所有唯一类别名称的列表

Answer 1

假设您最多有10个类别。 您可以使用substring_index()获取列表。

select distinct category
from (select substring_index(substring_index(categories, ',', n.n), ',', -1) as category
      from (select replace(replace(replace(categories, '"', ''), '[', ''), ']', '') as categories
            from t
          ) t cross join
          (select 1 as n union all select 2 union all select 3 union all select 4 union all select 5 union all
           select 6 union all select 7 union all select 8 union all select 9 union all select 10
          ) n
      ) c
where category is not null;

如果列表较长，只需将更多值添加到n的子查询中。 复杂的replace()假设您实际上并不想要双引号和方括号，并且实际上并不需要区分它们。

如评论中所述，这是一个非常糟糕的数据结构。 您应该有一个用于item_categories的表， item_categories有一个项目行和一个类别。