从SQL Server检索数据,并基于分组在行上连接结果
我已经研究一个问题好几天了,终于找到了一个适合我的解决方案。如果这个解决方案对其他人有用,我会问一个问题,然后自己回答 我对包含超过100万条记录的大型SQL Server数据库具有只读访问权限。数据库中的一些表通过查找表以多对多关系进行链接。为了简化问题,可以如下所示对表格进行说明:从SQL Server检索数据,并基于分组在行上连接结果,sql,sql-server,group-by,many-to-many,concatenation,Sql,Sql Server,Group By,Many To Many,Concatenation,我已经研究一个问题好几天了,终于找到了一个适合我的解决方案。如果这个解决方案对其他人有用,我会问一个问题,然后自己回答 我对包含超过100万条记录的大型SQL Server数据库具有只读访问权限。数据库中的一些表通过查找表以多对多关系进行链接。为了简化问题,可以如下所示对表格进行说明: table names |-----------| | id | name | |----|------| | 1 | dave | | 2 | phil | | 3 | john | tabl
table names
|-----------|
| id | name |
|----|------|
| 1 | dave |
| 2 | phil |
| 3 | john | table foods_relationship table clothes_relationship
| 4 | pete | |--------------------------| |----------------------------|
|-----------| | id | names_id | foods_id | | id | names_id | clothes_id |
|----|----------|----------| |----|----------|------------|
table foods | 1 | 1 | 1 | | 1 | 1 | 1 |
|---------------| | 2 | 1 | 3 | | 2 | 1 | 3 |
| id | food | | 3 | 1 | 4 | | 3 | 1 | 4 |
|----|----------| | 4 | 2 | 2 | | 4 | 2 | 2 |
| 1 | beef | | 5 | 2 | 3 | | 5 | 2 | 3 |
| 2 | tomatoes | | 6 | 2 | 4 | | 6 | 2 | 4 |
| 3 | bacon | | 7 | 2 | 5 | | 7 | 3 | 1 |
| 4 | cheese | | 8 | 3 | 3 | | 8 | 3 | 3 |
| 5 | apples | | 9 | 3 | 5 | | 9 | 3 | 5 |
|---------------| | 10 | 4 | 1 | | 10 | 4 | 2 |
| 11 | 4 | 2 | | 11 | 4 | 4 |
table clothes | 12 | 4 | 3 | | 12 | 4 | 5 |
|---------------| | 13 | 4 | 5 | |----------------------------|
| id | clothes | |--------------------------|
|----|----------|
| 1 | trousers |
| 2 | shorts |
| 3 | shirt |
| 4 | socks |
| 5 | jumper |
| 6 | jacket |
|---------------|
可以使用以下SQL重新创建表(改编自MySQL数据库,因此可能需要稍加调整才能在SQL Server中工作):
我想查询数据库并以某种方式获得以下输出:
|-------------------------------------------------------------|
| name | food | clothes |
|------|------------------------------|-----------------------|
| dave | beef,cheese,bacon | trousers,socks,shirt |
| john | apples,bacon | jumper,shirt,trousers |
| pete | beef,apples,bacon,tomatoes | shorts,jumper,socks |
| phil | bacon,tomatoes,apples,cheese | shirt,shorts,socks |
|-------------------------------------------------------------|
但是,运行一个SELECT查询,将“名称”表与一个或两个其他表(通过各自的查找表)联接,会导致每个名称有多行。例如:
SELECT
names.name,
foods.food
FROM
names
LEFT JOIN food_relationships ON names.id = food_relationships.names_id
LEFT JOIN foods ON food_relationships.foods_id = foods.id;
…生成以下一组结果:
|-----------------|
| name | food |
|------|----------|
| dave | beef |
| dave | bacon |
| dave | cheese |
| phil | tomatoes |
| phil | bacon |
| phil | cheese |
| phil | apples |
| john | bacon |
| john | apples |
| pete | beef |
| pete | tomatoes |
| pete | bacon |
| pete | apples |
|-----------------|
如果SELECT查询从两个表返回数据,则问题会更加复杂:
SELECT
names.name,
foods.food,
clothes.clothes
FROM
names
LEFT JOIN food_relationships ON names.id = food_relationships.names_id
LEFT JOIN foods ON food_relationships.foods_id = foods.id
LEFT JOIN clothes_relationships ON names.id = clothes_relationships.names_id
LEFT JOIN clothes ON clothes_relationships.clothes_id = clothes.id;
|-----------------------------|
| name | food | clothes |
|------|----------|-----------|
| dave | beef | trousers |
| dave | beef | shirt |
| dave | beef | socks |
| dave | bacon | trousers |
| dave | bacon | shirt |
| dave | bacon | socks |
| dave | cheese | trousers |
| dave | cheese | shirt |
| dave | cheese | socks |
| phil | tomatoes | shorts |
| phil | tomatoes | shirt |
| phil | tomatoes | socks |
| phil | bacon | shorts |
| phil | bacon | shirt |
| phil | bacon | socks |
| phil | cheese | shorts |
| phil | cheese | shirt |
| phil | cheese | socks |
| phil | apples | shorts |
| phil | apples | shirt |
| phil | apples | socks |
| ...
| etc.
问题是,如何查询SQL Server数据库以检索所有数据,但将其处理为每人只有一行?如果数据库是MySQL,解决方案将相对简单,因为MySQL有一个连接行的GROUP_CONCAT函数。因此,对于其中一个表,我可以使用:
SELECT
names.name,
GROUP_CONCAT(foods.food)
FROM
names
LEFT JOIN food_relationships ON names.id = food_relationships.names_id
LEFT JOIN foods ON food_relationships.foods_id = foods.id
GROUP BY (names.name);
…给予:
name food
dave beef,cheese,bacon
john apples,bacon
pete beef,apples,bacon,tomatoes
phil bacon,tomatoes,apples,cheese
要从“名称”和“衣服”表中获得等效数据,我可以使用以下方法:
SELECT
temp_foods_table.name AS 'name',
temp_foods_table.food AS 'food',
temp_clothes_table.clothes AS 'clothes'
FROM
(
SELECT
names.name,
GROUP_CONCAT(foods.food) AS 'food'
FROM
names
LEFT JOIN food_relationships ON names.id = food_relationships.names_id
LEFT JOIN foods ON food_relationships.foods_id = foods.id
GROUP BY (names.name)
) AS temp_foods_table
LEFT JOIN
(
SELECT
names.name,
GROUP_CONCAT(clothes.clothes) AS 'clothes'
FROM
names
LEFT JOIN clothes_relationships ON names.id = clothes_relationships.names_id
LEFT JOIN clothes ON clothes_relationships.clothes_id = clothes.id
GROUP BY (names.name)
) AS temp_clothes_table
ON temp_foods_table.name = temp_clothes_table.name;
…给出以下结果:
name food clothes
dave beef,cheese,bacon trousers,socks,shirt
john apples,bacon jumper,shirt,trousers
pete beef,apples,bacon,tomatoes shorts,jumper,socks
phil bacon,tomatoes,apples,cheese shirt,shorts,socks
然而,SQLServer中的情况似乎不那么直截了当。对于单个表,有一些建议的在线解决方案,包括使用公共表表达式或XML路径。然而,所有的解决方案似乎都有缺点,并给人一种明显的印象,即它们是变通解决方案,而不是专门设计的功能。每个建议的解决方案都有一些缺点(例如,for XML路径解决方案假定文本是XML,因此,文本中包含的特殊字符可能会导致问题)。此外,一些评论者表示担心,这些解决方案是基于未记录或不推荐的功能,因此从长远来看可能不可靠
因此,我决定不必拘泥于SQL,而是使用Python和Pandas处理检索后的数据。我总是将数据传输到Pandas数据框以进行绘图和分析,所以这并不是一个很大的不便。为了在多个列上连接数据,我使用了groupby()。但是,由于有两个多对多表,因此每列中都存在重复,因此,最终连接的字符串包含所有这些重复。为了只有唯一的值,我使用了Python集(根据定义,Python集只能包含唯一的值)。这种方法唯一的潜在缺点是没有维护字符串的顺序,但在我的情况下,这不是一个问题。最终的Python解决方案如下所示:
导入必要的库:
>>> import pandas as pd
>>> import pymssql
>>> import getpass
输入连接到数据库所需的详细信息:
>>> myServer = input("Enter server address: ")
>>> myUser = input("Enter username: ")
>>> myPwd = getpass.getpass("Enter password: ")
创建连接:
>>> myConnection = pymssql.connect(server=myServer, user=myUser, password=myPwd, port='1433')
>>> myLatestData = pd.io.sql.read_sql(myQuery, con=myConnection)
>>> myConnection.close()
定义查询以检索必要的数据:
>>> myQuery = """SELECT
names.name,
foods.food,
clothes.clothes
FROM
names
LEFT JOIN food_relationships ON names.id = food_relationships.names_id
LEFT JOIN foods ON food_relationships.foods_id = foods.id
LEFT JOIN clothes_relationships ON names.id = clothes_relationships.names_id
LEFT JOIN clothes ON clothes_relationships.clothes_id = clothes.id """
运行查询,将结果放入dataframe并关闭连接:
>>> myConnection = pymssql.connect(server=myServer, user=myUser, password=myPwd, port='1433')
>>> myLatestData = pd.io.sql.read_sql(myQuery, con=myConnection)
>>> myConnection.close()
连接多行中的字符串并删除重复项:
>>> tempDF = tempDF.groupby('name').agg(lambda col: ','.join(set(col)))
打印最终数据帧:
>>> print(tempDF)
name food clothes
dave beef,bacon,cheese socks,trousers,shirt
john bacon,apples jumper,trousers,shirt
pete tomatoes,beef,bacon,apples socks,jumper,shorts
phil tomatoes,bacon,cheese,apples socks,shorts,shirt
对我来说,这个解决方案比尝试在SQL查询中进行所有数据处理更直观。希望这对其他人有所帮助。如果是MS Sql Server 您可以使用STUFF函数。例如 声明@Heroes表( [HeroName]VARCHAR(20) ) 插入@Heroes([HeroName]) 价值观(‘超人’、‘蝙蝠侠’、‘铁人’、‘金刚狼’) 选择内容((选择“,”+[HeroName] 来自@英雄 按[姓名]订购 对于XML路径(“”),1,1,“”)作为[Output] 输出 蝙蝠侠,铁人,超人,狼獾 我认为这应该回答你的问题
谢谢其他帖子的一些评论者建议,如果字符串包含可以解释为XML的字符,那么使用FOR XML路径解决方法将失败。