How to query database with python

Question

I have the below PostgreSQL table:

     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

I have to find and display the average sale of Bloom per state and display like this:

CUST   AVG_NY  AVG_CT AVG_NJ
Bloom  28923   3241   1873

I converted the data to the below form:

[('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)]

Below is my code and is probably the worsts way to do so:

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)

How do I even start to achieve this?

I would say check the @MΛIK's answer. I would assume you mean the states by precondition. So, it is just a simple WHERE clause used along with GROUP BY. — aykcandem
– aykcandem, Commented Oct 15, 2021 at 6:42

Maik Hasler · Accepted Answer · 2021-10-15 06:38:13Z

2

You should take a deeper look into SQL. It's totally not necessary to do it like that. Just use a group by statement.

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

After executing it you can simply loop through the returned list and check each state.

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

Note: dataset[0] stores the state and dataset[1] the average.

answered Oct 15, 2021 at 6:38

Maik Hasler

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4 Comments

Swayam Shah Over a year ago

So, I have to get this data only using Python and select * from sales is the only query I can use as part of the assignment. That's why I can't use group by or any other SQL statement

Maik Hasler Over a year ago

@swombhai Ahh okay, now I understand your problem! You should have said that before! Is it allowed to select specific columns or only everything?

Swayam Shah Over a year ago

Everything, also I think I should use Pandas for this.

Maik Hasler Over a year ago

@swombhai Pandas would make it easier but it's also approachable with plain python.

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