I need some help with designing a database. My aim is to persistently store a number of pandas DataFrames in a searchable way, and from what I've read SQLite is perfect for this task.

Each DataFrame contains about a million rows of particle movement data like this:

              z            y            x  frame  particle
0     49.724138    45.642857   813.035714      0         0
3789  14.345679  2820.537500  4245.162500      0         1
3788  10.692308  2819.210526  1646.842105      0         2
3787  34.100000  2817.700000  1375.300000      0         3
3786   8.244898  2819.729167  1047.375000      0         4

Using sqlalchemy I can already store each DataFrame as a table in a new DataBase:

from sqlalchemy import create_engine
import pandas as pd

engine = create_engine("sqlite:////mnt/storage/test.db")

exp1.to_sql("exp1", engine, if_exists="replace")
exp2.to_sql("exp2", engine, if_exists="replace")
exp3.to_sql("exp3", engine, if_exists="replace")

But this is too basic. How can I store each DataFrame/experiment with a couple of metadata fields like Name, Date in such a way that later on it's possible to return all experiments conducted by a certain person, or on a specific date?

I will add more columns over time. Assuming each DataFrame/experiment has a column velocity, how could I retrieve all experiments where the mean temperature value is below or above an arbitrary threshold?


1 Answer 1


You've created 3 separate tables (well 2, pending the apparent typo?). If you want to unify the data, you probably shouldn't be forcibly overwriting target tables with if_exists="replace"

From the .to_sql() documentation

  • replace: Drop the table before inserting new values.

  • append: Insert new values to the existing table.

Assuming your similarly named files have the same schema, you can edit the last 3 lines as follows.

exp1.to_sql("exp", engine, if_exists="append")
exp2.to_sql("exp", engine, if_exists="append")
exp3.to_sql("exp", engine, if_exists="append")

This will insert all three datasets to a single table named exp instead of 3 separate tables.

If each csv isn't uniquely identified from the others within itself - for example if exp1.csv looks like this...


...then you can append the experiment identifier to each dataset as needed in the dataframe. For example by...

>>> exp1['ExpName'] = 'exp1'
>>> exp1
    Name        Date  Temperature ExpName
0  Peter  2020-01-01           50    exp1
1   Paul  2020-01-01           55    exp1
2   Mary  2020-01-01           53    exp1
3   Jane  2020-01-01           49    exp1

...which will allow you to group by experiment in any follow-on SQL you may run against your database.

...how could I retrieve all experiments where the mean temperature value is below or above an arbitrary threshold?

...well given an arbitrary additional two datasets of...

➜  /tmp cat exp2.csv
➜  /tmp cat exp3.csv
➜  /tmp

...that you likewise appended the expN identifier to in the dataframe, then you would run the following SQL to retrieve experiments where the average temp was below 53

FROM exp
HAVING AVG(Temperature) < 53;

Which I'll leave to you to plug into SQLAlchemy as you like :)

  • Thanks! So there is no good way to store "single field" values? Filling a whole column with the same entry a million times seems sort of unclean. Apart from that this is much easier to work with than what I had before.
    – smcs
    Mar 3, 2020 at 17:42
  • The "append experiment" line is based on the initial stated premise of multiple csv files and was included in the event that your dataframes do not already uniquely isolate experiments within themselves. Appending values conditionally to a dataframe is likely another question (for SO, not dba.se), but it's also probably an answered question Mar 3, 2020 at 17:45
  • The DataFrames do not have a unique identifier at this time. With pandas, as far as I can see, the only way to add such an identifier is to add a whole column with the same value, like you did. I thought maybe since I'm moving to SQLite now that there might be a better solution, e.g. that it's possible to add "date" or "name" as a single field, as metadata basically.
    – smcs
    Mar 3, 2020 at 17:57
  • According to this, there is no one perfect way to do it and the answer there actually supports your way of adding a whole column.
    – smcs
    Mar 3, 2020 at 18:50

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