How to execute multiple queries in pandas?

Question

How to execute the following queries with sqlalchemy ?

import pandas as pd
import urllib
from sqlalchemy import create_engine
from sqlalchemy.types import NVARCHAR
params = urllib.parse.quote_plus(r'DRIVER={SQL Server};SERVER=localhost\SQLEXPRESS;Trusted_Connection=yes;DATABASE=my_db;autocommit=true;MultipleActiveResultSets=True')
conn_str = 'mssql+pyodbc:///?odbc_connect={}'.format(params)
engine = create_engine(conn_str, encoding = 'utf-8-sig')

with engine.connect() as con:
    con.execute('Declare @latest_date nvarchar(8);')
    con.execute('SELECT @latest_date = max(date) FROM my_table')
    df = pd.read_sql_query('SELECT * from my_db where date = @latest_date', conn_str)

However, an error occured:

sqlalchemy.exc.ProgrammingError: (pyodbc.ProgrammingError) ('42000', '[42000] [Microsoft][ODBC SQL Server Driver][SQL Server]Must declare the scalar variable "@latest_date". (137) (SQLExecDirectW)')

How to solve this problem?

Thanks.

Answer 1

You don't need to declare a variable and use so many queries, you can do it just with one query:

SELECT * 
FROM my_db 
WHERE date = (SELECT max(date) 
              FROM my_db)

And then you can use, i use backticks because date is a reserved word:

with engine.connect() as con:
    query="SELECT * FROM my_db WHERE `date` = (SELECT max(`date`) FROM my_db)"
    df = pd.read_sql(query, con=con)