mysql的獲取從多值字段中唯一值

Question

我有一個MySQL表結構如下

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查詢來獲取所有獨特的類別名稱的列表

Answer 1

讓我們假設你有一個最大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; 

如果你有一個較長的列表，只需添加更多的價值的子查詢n。複雜的replace()假設你實際上不需要雙引號和方括號，並且他們不是真的需要區分這些類別。

正如評論中指出的那樣，這是一個非常糟糕的數據結構。對於item_categories，您應該有一個表格，其中一行用於一個項目和一個類別。