Python: Setting data types when using to_sql

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Issue: column encoding is altered when saving a DataFrame from Python to Oracle

When dealing with databases, it's important to pay attention to data types to ensure that the table structure is preserved when saving a DataFrame from Python. The following is a basic code snippet to save a DataFrame to an Oracle database using SQLAlchemy and pandas:

import pandas as pd
from sqlalchemy import create_engine

# Define the table structure
data = {
    'id': [1, 2, 3],
    'name': ['Alice', 'Bob', 'Charlie'],
    'hire_date': ['2020-01-15', '2019-05-20', '2021-02-10'],
    'insert_datetime': ['2021-09-15 10:00:00', '2021-09-16 11:30:00', '2021-09-17 09:45:00']
}

df = pd.DataFrame(data)

# Display the DataFrame and Data Types
print(df)
print(df.dtypes)

# Create SQLAlchemy engine
engine = create_engine('oracle://username:password@hostname:port/service_name')

# Use pd.to_sql() to replace/update the table in Oracle
df.to_sql('employee', con=engine, if_exists='replace', index=False)

id     name   hire_date      insert_datetime
0   1    Alice  2020-01-15  2021-09-15 10:00:00
1   2      Bob  2019-05-20  2021-09-16 11:30:00
2   3  Charlie  2021-02-10  2021-09-17 09:45:00

id                  int64
name               object
hire_date          object
insert_datetime    object
dtype: object

However, running this code may reveal a bug where the column encoding is altered in the Oracle database table. Upon inspection, it becomes apparent that only the first column id retains its integer data format, while the remaining columns name, hire_date, and insert_datetime are changed to CLOB (Character Large Object) encoding in Oracle.

Solution: using the `dtype` parameter when invoking `to_sql()`

To ensure that the correct data types are preserved when writing a DataFrame to an Oracle database, one approach is to explicitly define the data types for each column when using to_sql(). This can be achieved by creating an SQLAlchemy Table object with specified data types and then using the dtype parameter when invoking to_sql().

import pandas as pd
from sqlalchemy import create_engine, types

# Define the table structure
data = {
    'id': [1, 2, 3],
    'name': ['Alice', 'Bob', 'Charlie'],
    'hire_date': ['2020-01-15', '2019-05-20', '2021-02-10'],
    'insert_datetime': ['2021-09-15 10:00:00', '2021-09-16 11:30:00', '2021-09-17 09:45:00']
}

df = pd.DataFrame(data)

# convert data type to datetime
df['hire_date'] = pd.to_datetime(df['hire_date'], format='%Y-%m-%d')
df['insert_datetime'] = pd.to_datetime(df['insert_datetime'], format='%Y-%m-%d %H:%M:%S')

# Create SQLAlchemy engine
engine = create_engine('oracle://username:password@hostname:port/service_name')

# Set data types
dtype_dic = {'id': types.INTEGER(), 'name': types.NVARCHAR(length=50),'hire_date': types.DATE(),'insert_datetime': types.DateTime()}

# Use pd.to_sql() to replace/update the table in Oracle
df.to_sql('employee', con=engine, schema='TMP01', if_exists='replace', index=False, dtype=dtype_dic)