選擇所有多對多的關系

Question

給出以下表格：

author
--------------
id int(11)
name varchar

author_type
---------------
id int(11)
type varchar

author_has_type
---------------
author_id int(11)
author_type_id int(11)

我正在嘗試編寫一個執行以下操作的查詢;

給定一個作者類型，它將顯示以下內容

author.name           types
-The authors name-    -All types of the author-

現在，我正在嘗試使用的查詢是

Select author.name, GROUP_CONCAT(author_type.type) as types
from author
left join author_has_type on author.id = author_has_type.author_id
left join author_type on author_has_type.author_type_id = author_type.id
where author_type.type = "EXAMPLE TYPE"

但是，當我這樣做時，它只返回group_concat中的給定類型。 我理解為什么會發生這種情況，但是有沒有辦法讓每一種類型的作者？

Answer 1

你需要一個group by和刪除where子句：

Select a.name, GROUP_CONCAT(t.type) as types
from author a left join
     author_has_type aht
     on a.id = aht.author_id left join
     author_type t
     on aht.author_type_id = t.id
group by a.name;

編輯：

我想你想要的是：

Select a.name, GROUP_CONCAT(t.type) as types
from author a left join
     author_has_type aht
     on a.id = aht.author_id left join
     author_type t
     on aht.author_type_id = t.id
group by a.name
having max(author_type.type = 'EXAMPLE TYPE') > 0;

Answer 2

您需要將分組添加到查詢中。 並且要限制要輸出哪些行，請使用帶有子查詢的HAVING。

SELECT author.name, GROUP_CONCAT(author_type.type) as types
FROM author
LEFT JOIN author_has_type on author.id = author_has_type.author_id
LEFT JOIN author_type on author_has_type.author_type_id = author_type.id
GROUP BY (author.name)
HAVING author.id IN (
    SELECT id FROM author 
    JOIN author_has_type ON author.id = author_has_type.author_id
    JOIN author_type ON author_has_type.author_type_id = author_type.id
    WHERE author_type.type = "EXAMPLE TYPE"
)

Answer 3

SELECT author.name, 
       GROUP_CONCAT(author_type.type) as types
  FROM author
  LEFT JOIN author_has_type on author.id = author_has_type.author_id
  LEFT JOIN author_type on author_has_type.author_type_id = author_type.id
       (SELECT author.ID 
         FROM author
         LEFT JOIN author_has_type on author.id = author_has_type.author_id
         LEFT JOIN author_type on author_has_type.author_type_id = author_type.id
        WHERE author_type.type = "EXAMPLE TYPE") temp
    ON author.id=temp.id

Answer 4

SELECT 
    author.name, 
    result.types 
FROM (
    SELECT author.id, GROUP_CONCAT(types.type) AS types
    FROM 
        author
    INNER JOIN author_has_type
        ON author.id = author_has_type.author_id
    INNER JOIN author_type 
        ON author_has_type.author_type_id = author_type.id 
        AND author_type.type = "funny"
    INNER JOIN author_has_type AS has_types
        ON author.id = has_types.author_id
    INNER JOIN author_type AS types
        ON has_types.author_type_id = types.id 
    GROUP BY author.id
) AS result 
INNER JOIN author 
    USING(id)

添加兩個內部聯接以過濾到要查找的類型。 然后使用接下來的兩個連接來獲取作者的所有類型。

將查詢分組並將其放入子查詢中。 在外部查詢中使用結果將authordata加入到結果中。

例