SQL：将MySQL中的行转换为列（SELECT语句）

Question

我得到了 table orders 和 order_comments。每个订单可以有 0 到 n 条评论。我想按特定顺序获取所有订单及其评论的列表。

Table orders:

order_id | order_nr 
1 | 5252
4 | 6783
5 | 6785

Table order_comments

id_order_comments | order_fk | created_at | email | content
1 | 4 | 2015-01-12 | jack | some text here
2 | 5 | 2015-01-13 | marta | some text here
3 | 5 | 2015-01-14 | beata | some text here
4 | 4 | 2015-01-16 | julia | some text here

因此，我想为每个订单获取 1 行。评论应显示在单独的列中，从最早的评论开始。所以在这种情况下所需的输出是：

order_id | 1_comment_created_at | 1_comment_author | 1_comment_content | 2_comment_created_at | 2_comment_author | 2_comment_content 
1 | NULL | NULL | NULL | NULL | NULL | NULL
4 | 2015-01-12 | jack | some text here |  2015-01-16 | Julia | some text here
5 | 2015-01-13 | marta | some text here | 2015-01-14 | beata | some text here

我发现了这个：MySQL - Rows to Columns - 但我不能使用 'create view'。
我发现了这个：http://dev.mysql.com/doc/refman/5.5/en/while.html - 但我无法在此数据库中创建过程。

我得到了什么:

SELECT @c := (SELECT count(*) FROM order_comments GROUP BY order_fk ORDER BY count(*) DESC LIMIT 1);

SET @rank=0;
SET @test=0;

SELECT
  CASE WHEN @test < @c AND temp.comment_id = @test THEN temp.created_at END AS created,
  CASE WHEN @test < @c AND temp.comment_id = @test THEN temp.author END AS author,
  CASE WHEN @test < @c AND temp.comment_id = @test THEN temp.content END AS content
  /*But I cannot set @test as +1. And I cannot name column with variable - like CONCAT(@test, '_created')*/

FROM (
  SELECT @rank := @rank +1 AS comment_id, created_at, author, content
  FROM order_comments 
  WHERE order_fk = 4
  ORDER BY created_at
) AS temp

问题: 我想搜索超过 1 个订单。我也应该得到没有评论的订单。我能做什么？

Answer 1

你可以为这种类型的数据透视表使用变量，但查询有点复杂，因为你需要枚举每个订单的值：

SELECT o.order_id,
       MAX(case when rank = 1 then created_at end) as created_at_1,
       MAX(case when rank = 1 then email end) as email_1,
       MAX(case when rank = 1 then content end) as content_1,
       MAX(case when rank = 2 then created_at end) as created_at_2,
       MAX(case when rank = 2 then email end) as email_2,
       MAX(case when rank = 2 then content end) as content_2,
FROM orders o LEFT JOIN
     (SELECT oc.*,
             (@rn := if(@o = order_fk, @rn + 1,
                        if(@o := order_fk, 1, 1)
                       )
             ) as rank
      FROM order_comments oc CROSS JOIN
           (SELECT @rn := 0, @o := 0) vars
      ORDER BY order_fk, created_at
     ) oc
     ON o.order_id = oc.order_fk
GROUP BY o.order_id;

SQL：将MySQL中的行转换为列（SELECT语句）

SQL: transform rows into columns in MySQL (SELECT statement)

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