如何用python查询数据库

Question

我有下面的PostgreSQL表:

cust     prod  day  month  year state  quant
0    Bloom    Pepsi    2     12  2011    NY   4232
1    Bloom    Bread   23      5  2015    PA   4167
2    Bloom    Pepsi   22      1  2016    CT   4404
3    Bloom   Fruits   11      1  2010    NJ   4369
4    Bloom     Milk    7     11  2016    CT    210

我必须找到并显示每个州Bloom的平均销售额，并像这样显示:

CUST   AVG_NY  AVG_CT AVG_NJ
Bloom  28923   3241   1873

我将数据转换成以下形式:

[('Bloom', 'Pepsi', 2, 12, 2011, 'NY', 4232), ('Bloom', 'Eggs', 30, 11, 2010, 'NJ', 559), ('Bloom', 'Yogurt', 25, 7, 2014, 'PA', 17), ('Bloom', 'Yogurt', 3, 4, 2011, 'NJ', 1203), ('Bloom', 'Coke', 7, 2, 2010, 'NY', 1229), ('Bloom', 'Coke', 6, 10, 2018, 'PA', 2867), ('Bloom', 'Soap', 6, 1, 2015, 'CT', 4623), ('Bloom', 'Milk', 8, 9, 2010, 'NJ', 1106), ('Bloom', 'Milk', 19, 4, 2013, 'NY', 3516), ('Bloom', 'Soap', 7, 6, 
2015, 'PA', 3404)]

下面是我的代码，这可能是最糟糕的方式:

connection = psycopg2.connect(user="postgres",
password="ss",
host="127.0.0.1",
port="8800",
database="postgres")
cursor = connection.cursor()
postgreSQL_select_Query = "select * from sales"
cursor.execute(postgreSQL_select_Query)
mobile_records = cursor.fetchall()
def takeSecond(elem):
return elem[0][0]

mobile_records.sort(key=takeSecond)
Bloom1 = []
for i in mobile_records:
if i[5] == 'NY' and i[0] == 'Bloom':
Bloom1.append(i)
s1 = 0
for j in Bloom1:
s1 += j[6]
avg1 = s1/len(Bloom1)

Bloom2 = []
for i in mobile_records:
if i[5] == 'CT' and i[0] == 'Bloom':
Bloom2.append(i)
s2 = 0
for j in Bloom2:
s2 += j[6]
avg2 = s2/len(Bloom2)

Bloom3 = []
for i in mobile_records:
if i[5] == 'NJ' and i[0] == 'Bloom':
Bloom3.append(i)
s3 = 0
for j in Bloom3:
s3 += j[6]
avg3 = s3/len(Bloom3) 

我如何开始实现这一点?

Answer 1

您应该更深入地了解SQL。完全没有必要那样做。请使用group by语句。

statement = "SELECT state, AVG(quant) FROM sales WHERE cust = Bloom GROUP BY state"

执行后，您可以简单地遍历返回的列表并检查每个状态。

data = cursor.fetchall()
for dataset in data:
if dataset[0] == 'NJ':
# do something with dataset[1]

注意:dataset[0]存储状态，dataset[1]存储平均值。