MySQL從多值字段獲取唯一值

Question

我有一個具有以下結構的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"]

我需要通過SQL查詢獲取所有唯一類別名稱的列表

Answer 1

假設您最多有10個類別。 您可以使用substring_index()獲取列表。

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;

如果列表較長，只需將更多值添加到n的子查詢中。 復雜的replace()假設您實際上並不想要雙引號和方括號，並且實際上並不需要區分它們。

如評論中所述，這是一個非常糟糕的數據結構。 您應該有一個用於item_categories的表， item_categories有一個項目行和一個類別。