Question

我这里有一点问题。有三个表：

表1

category_id        parent_id
     1                 0
     2                 1
     3                 0
     4                 0
     5                 10

表2

category_id         Name
     1              Cars
     2              Toys
     3              Gifts
     4              For women
     5              For men

表3

product_id       category_id
    1                 2
    1                 1
    2                 3
    2                 1
    3                 2
    4                 3
    4                 5

product_id列中的数字可以重复多次。 我想加入表格并将每个product_id排成一行，如下所示：

product_id       category1      category2
    1              Toys           Cars
    2              Gifts          Cars
    3              Toys           Null
    4              Gifts          For men

parent_id 列包含指定类别类型和级别的数字，可以是主要类别也可以是子类别。这是我的代码，但我在

中得到0

    SELECT product_to_category.product_id, sum(CASE(category.parent_id) WHEN 1 
    THEN category_description.name ELSE 0 END) as category1,
    sum(CASE(category.parent_id) WHEN 2
    THEN category_description.name ELSE 0 END) as category2
from category LEFT JOIN category_description ON 
category_description.category_id = category.category_id 
LEFT JOIN product_to_category ON category.category_id = product_to_category.category_id
group by category.category_id

当我执行它时，我在category_main列

上得到0值

product_id          category1        category2
    1                   0                0
    2                   0                0
    3                   0                0
    4                   0                0

Answer 1

更改并检查

SELECT tab.product_id, 
       Sum(category_id1) AS Category_id1, 
       Sum(category_id2) AS Category_id2 
INTO   #temptable 
FROM  (SELECT a.product_id, 
          0 AS Category_id1, 
          Min(a.category_id) AS Category_id2 
   FROM   #table3 a 
          INNER JOIN (SELECT product_id, 
                             Count(category_id) AS Category_id 
                      FROM   #table3 
                      GROUP  BY product_id 
                      HAVING Count(category_id) > 1) b 
                  ON a.product_id = b.product_id 
   GROUP  BY a.product_id 
   UNION ALL 
   SELECT a.product_id, 
          Max(a.category_id) AS Category_id1, 
          0                  AS Category_id2 
   FROM   #table3 a 
          INNER JOIN (SELECT product_id, 
                             Count(category_id) AS Category_id 
                      FROM   #table3 
                      GROUP  BY product_id 
                      HAVING Count(category_id) > 1) b 
                  ON a.product_id = b.product_id 
   GROUP  BY a.product_id 
   UNION ALL 
   SELECT a.product_id, 
          a.category_id AS Category_id1, 
          0             AS Category_id2 
   FROM   #table3 a 
          INNER JOIN (SELECT product_id, 
                             Count(category_id) AS Category_id 
                      FROM   #table3 
                      GROUP  BY product_id 
                      HAVING Count(category_id) <= 1) b 
                  ON a.product_id = b.product_id) tab 
GROUP  BY tab.product_id 

SELECT #temptable.product_id, 
       CAT1.name AS Category_id1, 
       CAT2.name AS Category_id2 
FROM   #temptable 
       LEFT OUTER JOIN #table2 CAT1 
                    ON #temptable.category_id1 = CAT1.category_id 
       LEFT OUTER JOIN #table2 CAT2 
                ON #temptable.category_id2 = CAT2.category_id

Answer 2

为什么不使用COUNT（）和WHERE？你想要计算category.parent_id等于1的数量，对吧？所以：

SELECT category.category_id, COUNT(category_description.name)
category_main  FROM category 
LEFT JOIN category_description 
    ON category_description.category_id = category.category_id 
WHERE category.parent_id = 1 
GROUP BY category.category_id;

Answer 3

SELECT 
category.category_id, 
category_main = CASE
                   WHEN category.parent_id = 1  
                   THEN category_description.name 
                   ELSE '0' 
                END
FROM category 
LEFT JOIN category_description ON 
category_description.category_id = category.category_id

Answer 4

您是否需要不同列中的每个类别？

如果您不需要，请使用group_concat

http://www.w3resource.com/mysql/aggregate-functions-and-grouping/aggregate-functions-and-grouping-group_concat.php

就是这样：

SELECT t3.product_id, GROUP_CONCAT(t2.name) FROM table3 t3
LEFT JOIN table2 t2 on t2.category_id = t3.category_id
LEFT JOIN table1 t1 on t1.parent_id = t2.category_id
GROUP BY t3.product_id

mysql求和文本值的大小写错误

4 个答案: