Question

我正在尝试合并三个表。第一个表有id和data_name，第二个表data_id，option_id和property，第三个表user_id，data_id。

例如：

first table
+----+-----------+
| id | data_name |
+----+-----------+
| 1  + veri1     |
| 2  + veri2     |
| 3  + veri3     |
+---++-----------+

second table 
+----------+----------+-----------+
data_id    | property | option_id |
+----------+----------+-----------+
| 1        | blue     | 1         |
| 1        | cold     | 2         |
| 2        | gray     | 1         |
| 2        | hot      | 2         |
| 3        | green    | 1         |
| 3        | cold     | 1         |
+----------+----------+-----------+

third table
+----------+-----------+
| user_id  | data_id   |
+----------+-----------+
| 1        | 2         |
| 2        | 3         |
| 3        | 1         |
+----------+-----------+

我想得到：

user: 1
data: veri2
properties: gray - hot

如何为此编写SQL？

Answer 1

此查询应该这样做。但是，property将以逗号分隔，而不是-。

Select user_id,data_name, GROUP_CONCAT(property)
From Table1 INNER JOIN Table2 ON Table1.id = Table2.data_id
INNER JOIN Table3 ON Table1.id = Table3.data_id
GROUP BY Table1.id

Answer 2

首先从FROM子句开始。您想要存储的数据在哪里？

FROM user_first_table
JOIN property_second_table
JOIN user_data_third_table

然后考虑你应该加入的条件（我发现重新排序表更容易）

FROM user_data_third_table t 
JOIN user_first_table f ON f.id = t.user_id
JOIN property_second_table s ON s.data_id = t.data_id

然后使用WHERE子句缩小范围，例如：只想要veri1的数据

FROM user_data_third_table t 
JOIN user_first_table f ON f.id = t.user_id
JOIN property_second_table s ON s.data_id = t.data_id
WHERE f.data_name = 'veri1'

然后标记您想要的数据字段

SELECT f.id as 'user', f.data_name as 'data', s.property as 'properties'
FROM user_data_third_table t 
JOIN user_first_table f ON f.id = t.user_id
JOIN property_second_table s ON s.data_id = t.data_id
WHERE f.data_name = 'veri1'

如何编写查询以将多个字符串聚合到一个字段中？

2 个答案: