This is to demonstrate the similarity between pandas and SQL. The same post as Python code is available here

Create

SQL use CREATE TABLE and INSERT INTO to build a data table. Pandas can use read_csv() or read_excel() to load data from readily-available sources, for example (URL can be replaced with file path):

url = "https://research.stlouisfed.org/fred2/series/UNRATE/downloaddata/UNRATE.csv"
df_csv = pd.read_csv(url, parse_dates=True)

url = "http://api.worldbank.org/v2/en/indicator/gc.dod.totl.gd.zs?downloadformat=excel"
df_xl = pd.read_excel(url, sheet_name='Data', skiprows=3)

or we can create the data manually. For example, and empty DataFrame is defined by providing only the column names:

customer_cols = ["CustomerID", "CustomerName", "ContactName", "Address", "City", "PostalCode", "Country"]
df_empty = pd.DataFrame(columns=customer_cols)

and we can provide rows of data directly (matching the columns) to create a populated DataFrame:

customer_data = [
    [1, "Alfreds Futterkiste", "Maria Anders", "Obere Str. 57", "Berlin", "12209", "Germany"],
    [2, "Ana Trujillo Emparedados y helados", "Ana Trujillo", "Avda. de la Constitución 2222", "México D.F.", "05021", "Mexico"],
    [3, "Antonio Moreno Taquería", "Antonio Moreno", "Mataderos 2312", "México D.F.", "05023", "Mexico"],
    [4, "Around the Horn", "Thomas Hardy", "120 Hanover Sq.", "London", "WA1 1DP", "UK"],
    [5, "Berglunds snabbköp", "Christina Berglund", "Berguvsvägen 8", "Luleå", "S-958 22", "Sweden"],
    [89, "White Clover Markets", "Karl Jablonski", "305 - 14th Ave. S. Suite 3B", "Seattle", "98128", "USA"],
    [90, "Wilman Kala", "Matti Karttunen", "Keskuskatu 45", "Helsinki", "21240", "Finland"],
    [91, "Wolski", "Zbyszek", "ul. Filtrowa 68", "Walla", "01-012", "Poland"],
    [92, "Cardinal", "Tom B. Erichsen", "Skagen 21", "Stavanger", "4006", "Norway"],
]
df = pd.DataFrame(customer_data, columns=customer_cols)

We can also provide data in Python dict, either as list of dict, each dict correspond to one row, or dict of list, each correspond to one column. The DataFrame will have the columns in unspecified order, which we need to use filter() to enforce an order afterwards:

lst_of_dict = [dict(zip(customer_cols, row)) for row in customer_data]
df_lod = pd.DataFrame(lst_of_dict).filter(customer_cols)

dict_of_lst = {col: [row[i] for row in customer_data] for i, col in enumerate(customer_cols)}
df_dol = pd.DataFrame(dict_of_lst).filter(customer_cols)

We can also use a list of named tuples rather than list of dict to present data to pandas:

import collections
record = collections.namedtuple("record", customer_cols)
lst_of_ntup = [record(*row) for row in customer_data]
df_ntup = pd.DataFrame(lst_of_ntup)

sometimes we use random data to test out features. Below is an example to use numpy to generate standard normal distributed numbers of 100 rows and 4 columns:

df_random = pd.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))

Select

SQL use SELECT * FROM table to show all data from a table. Printing pandas DataFrame is similar to that. Besides, we can convert data from a DataFrame to other formats. For example, convert to list of dicts, either using to_dict("records") or constructing each row of data in a loop:

lst_of_dict = df.to_dict("records")

lst_of_dict = [row.to_dict() for _, row in df.iterrows()]

lst_of_dict = [dict(zip(df.columns, row)) for row in df.values]

We can also extract the underlying numpy array of a DataFrame:

df.values

The SQL statement SELECT column FROM table (i.e., cover all rows but some subset of columns) is equivalent to the following in pandas. The copy() is a safety precaution that the extracted data will become independent of the original one so that it can be manipulated:

df2 = df[["CustomerName", "ContactName"]].copy()

Instead of using a selector, we can also use filter() function for the same purpose. Only difference is that filter() will silently ignore those columns that do not exist:

df2 = df.filter(["CustomerName", "ContactName", "foo"]).copy()

Count and distinct

SQL can remove duplicated rows of data by SELECT DISTINCT column1, column2 FROM table_name. Pandas can do similar column selection and then apply drop_duplicates():

df2 = df.filter(["CustomerID", "CustomerName"]).drop_duplicates()

Similarly, a count of distinct can also be done. SQL will be SELECT COUNT(DISTINCT Country) FROM Customers while pandas can use len() function:

count = len(df.filter(["Country"]).drop_duplicates())

The full dimension of a DataFrame is provided by .shape, which counts the total number of rows and columns in a tuple:

assert df.shape == (len(df), len(df.columns)))

But if we count only the non-null cells, we use count(). For example, this counts the number of rows with non-null country:

df["Country"].count()

and this counts the number of cells in the whole DataFrame that are not null:

df.count()

Row slicing

SQL has the LIMIT clause to control the amount of output. Pandas counterpart is the iloc[] and loc[] slicers. For example, SQL statement SELECT * FROM Customers LIMIT 3 becomes the following, for slicing rows without referencing to the row indices:

df_subset = df.iloc[:3]

or using loc[] to slice by row indices (row ids in some DBMS):

df_subset = df.loc[[1,3,5]]

or just use these shorthands to investigate the first or last few rows of a DataFrame:

df_subset = df.head(4)
df_subset = df.tail(4)

The iloc[] and loc[] can indeed filter by column as well, similar to SQL statement SELECT column1, column2 FROM Customers LIMIT 3 OFFSET 1. iloc[] requires both row and column are specified as offset ranges while loc[] requires they are specified as indices/labels. Such as the following:

df_subset = df.iloc[2:5, 0:2]
df_subset = df.loc[[2,3,4], ["CustomerID", "CustomerName"]]
df_subset = df.loc[2:4, "CustomerID":"CustomerName"]

Row indices and column labels of a DataFrame are provided respectively by

df.index
df.columns

Both iloc[] and loc[] accept slicing by enumerated list or a range. But in iloc[] the range is interpreted as if a Python range which the upperbound is not included in the output. However, loc[] will include both ends of the range.

SQL has WHERE clause to select rows based on some boolean criteria. In pandas, we select using boolean indexing, i.e., by providing a boolean vector of size as same number of rows of the DataFrame. Expressions that we can use in WHERE clause may contain operators such as =, <>, <, >, >=, <=, BETWEEN, LIKE, IN, AND, OR, NOT, IS NULL, and IS NOT NULL. Similar operation also exists in pandas:

boolindex = (df["Country"] == "Mexico") | df["City"].str.startswith("S") | (df["CustomerId"] < 50)
df_subset = df[boolindex]

SELECT * FROM Customers WHERE Country='Germany' AND (City='Berlin' OR # City='München') becomes

df_subset = df[df["Country"].eq("Germany") & df["City"].isin(["Berlin", "München"])]

Using IS NULL: SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL

df_subset = df[df["Address"].isna()].filter(["CustomerName", "ContactName", "Address"])

Pandas provide boolean operators & (and), | (or), ~ (not) and functions .gt(x), .ge(x), .lt(x), .le(x), .eq(x), .ne(x), .notna(), .isna(), isin(x). String functions are available as .str.*. Simple boolean is further supported by query():

df_subset = df.query("CustomerID % 2 == 0")

Nested query can also be used intuitively, e.g. SELECT * FROM Customers WHERE Country IN (SELECT Country FROM Suppliers) becomes:

df_subset = df[df["Country"].isin(df_suppliers["Country"])]

Expression on output

SQL can provide expression as SELECT result column. Pandas need to create such result column as additional columns. For example SELECT CustomerID, CustomerID^2 AS "ID2" FROM Customer is equivalent to:

df_result = df[["CustomerID"]]
df_result["ID2"] = df["CustomerID"] ** 2

or we can combine it as one line:

df_result = df[["CustomerID"]].assign(ID2=df["CustomerID"]**2)

Such simple expression also supported by eval()

df["ID2"] = df.eval("CustomerID ** 2")

Alternatively, we can call eval() in-place instead of using assignment:

df.eval("ID2 = CustomerID ** 2", inplace=True)

More complicated expression have to be defined as a function or lambda that return a row and apply() on the DataFrame on row-by-row basis.

def names(row):
    tokens = row["CustomerName"].split()
    row["FirstName"] = tokens[0]
    row["LastName"] = tokens[-1]
    return row
df_result = df.apply(names, axis=1).filter(["FirstName", "LastName"])

The result of DataFrame.apply() is a DataFrame if the function returns a Series, and it is a Series if the function returns a scalar. If the function modifies the row, as above, the change is in-place to the DataFrame.

We also have functions to modify a column (Series) directly, e.g.

df["Customer"] = df["CustomerName"].str.upper().apply(lambda n: n.split()[-1]) + \
                 " " + df["CustomerName"].apply(lambda n: n.split()[0])

Similar to the case of evaluting new result column, we can assign a new column by providing a series directly, as long as the list has same length as the number of rows in the DataFrame:

df["foo"] = [1, 2, 3, 4]

Case statements

Besides using a function to apply(), we can mimick case statements by a series of mask() or where(). For example:

SELECT OrderID, Quantity,
CASE
    WHEN Quantity > 30 THEN "The quantity is greater than 30"
    WHEN Quantity = 30 THEN "The quantity is 30"
    ELSE "The quantity is under 30"
END AS QuantityText
FROM OrderDetails

becomes

df_order["QuantityText"] = "The quantity is under 30"
df_order["QuantityText"] = df_order["QuantityText"] \
                           .mask(df_order["Quantity"].eq(30), "The quantity is 30") \
                           .mask(df_order["Quantity"].gt(30), "The quantity is greater than 30")
df_result = df_order[["OrderID", "Quantity", "QuantityText"]]

mask() and where() has the same syntax that it takes a boolean mask and a data to replace to. Only that mask() replace the original value whenever the boolean is True while where() replace whenever the boolean is False. The condition should be specified in reverse order as CASE statement because value is taken as last-match-wins while the CASE statement is first-match-wins.

Aggregate functions

Aggregate functions allows us to find the sum, mean, min, max, and so on. SQL aggregate functions are provided as DataFrame or Series methods in pandas, e.g. SELECT MIN(Price) AS SmallestPrice FROM Products becomes

smallest_price = df["Products"]["Price"].min()

SELECT SUM(Quantity) FROM OrderDetail WHERE OrderID=10248 becomes:

df = pd.DataFrame(orderdetail_data, columns=orderdetail_cols)
total = df[df["OrderID"] == 10248]["Quantity"].sum()

And we can also use sum() as count function when applied to boolean index, for example, SELECT COUNT() FROM OrderDetail WHERE Quantity > 10 becomes:

count = df["Quantity"].gt(10).sum()

Ordering

Pandas DataFrame has sort_values() to correspond to the ORDER clause in SQL. Example:

df_sorted = df.sort_values("Country", ascending=True)

Another example, SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC becomes

df_sorted = df.sort_values(["Country", "CustomerName"], ascending=[True, False])

If we have to sort by a function, we can use a temporary column

df.assign(f = df['one']**2 + df['two']**2).sort_values('f').drop('f', axis=1)

Besides sort by value, we can also sort by index. This is useful if we have a row index (axis=0), or want to sort the column by string order (axis=1). For example:

df.sort_index(axis=0, ascending=False)

Append and insert

SQL has UNION to combine tables. Pandas does that with concat():

df_combined = pd.concat([df1, df2])

or using append()

df_new = df.append(df2, sort=False)

another example of append(): in SQL SELECT City FROM Customers UNION SELECT City FROM Suppliers ORDER BY City becomes

cities = df_customers["Country"].append(df_suppliers["Country"]).sort_values()

Besides, we can also append a row or DataFrame to an existing DataFrame:

df_new = df.append({}, ignore_index=True)

Or, add a row in-place by providing a new index,

df.loc[101] = [99, np.nan, n.nan, np.nan, np.nan, 98765, "USA"]

If we insert new row to the DataFrame but not every columns are provided, we can use the .loc[] slicing. For example the SQL:

INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway')

becomes

df.loc[101, ["CustomerName", "ContactName", "Address", "City", "PostalCode", "Country"]] = \
    ['Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway']

Casting

We can change the data type of a column by CAST() in SQL and by astype() in pandas DataFrame:

df["CustomerID"] = df["CustomerID"].astype("int")

In case we do not need conversion like str to int, we can ask pandas to infer the type of each column automatically:

df = df.infer_objects()

Special case is the datatime object: Pandas do not use Python datetime object but carries its own version so that “not a time” can be defined. To convert timestamp, we use:

dt64.to_pydatetime()
dt64.timestamp()

Update data

Update a particular cell: We specify the row index and column, such as UPDATE Customer SET City='Seattle' WHERE rowid=4

df.loc[4].City = 'Seattle'

We can also using a condition to select the rows, e.g. in SQL UPDATE Customers SET ContactName='Alfred Schmidt', City='Frankfurt' WHERE CustomerID=1 becomes

df.loc[df["CustomerID"]==1, ["City", "ContactName"]] = ["Frankfurt", "Alfred Schmidt"]

Similar construct can be used to update columns by an expression, e.g. UPDATE df SET B=B*C WHERE A>10 becomes

selector = df["A"] > 10
df.loc[selector, "B"] *= df[selector]["C"]

Above, we cannot write as df.loc[selector]["B"] *= df[selector]["C"] because the result of row slicing is a new DataFrame and the update is not propagated back.

Pandas has a simpler interface to replace all cells by a mapping table. For example, replace all empty string "" into None:

df = df.replace({"":None})

or if the cell the replace are all the “nan” cells, we have fillna() (but not to replace to None):

df = df.fillna("foo")

Update columns and delete data

To rename a column, SQL has ALTER TABLE command. In pandas, we provide a dict as name mappings and using rename() function:

df.rename(columns={"CustomerID": "ID"})

To delete columns from a DataFrame, we use drop():

df_trimed = df.drop("Country", axis=1)
df_trimed = df.drop(["Country", "PostalCode"], axis=1)

Deleting rows is logically same as SELECT those rows to retain. For example, select no rows but retain the column structure:

df_empty = df.iloc[0:0]
df_empty = df.loc[0:-1]

We can also remove rows by row index (default axis argument to be 0):

df_trimed = df.drop([0, 1])

If delete is conditioned on certain rows with nan (default to all rows), we have a simpler function:

df_trimed = df.dropna(subset=["Country", "City"])

Table joins

SQL table join is merge() in pandas DataFrame. For example:

SELECT o.OrderID, o.OrderDate, c.CustomerName
FROM Customers AS c, Orders AS o
WHERE c.CustomerName="Around the Horn" AND c.CustomerID=o.CustomerID

becomes

df_result = df_customer \
            .merge(df_orders, how="inner", on="CustomerID") \
            .pipe(lambda df: df["CustomerName"]=="Around the Horn") \
            .filter(["OrderID", "OrderDate", "CustomerName"])

which the merge() can specify how to be any of "inner", "left", "right", or "outer".

If instead of join by column we want to join by row index, the function is join()

df_result = df1.join(df2)

Group by

SQL GROUP BY is retained in pandas. For example, reading the first row of each group (SELECT DISTINCT ON in PostgreSQL):

df.groupby("id", as_index=False).first()

and the SQL SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country ORDER BY COUNT(CustomerID) DESC becomes

df_result =  df.filter(["Country", "CustomerID"]) \
               .groupby("Country", as_index=False) \
               .count() \
               .sort_values("CustomerID", ascending=False)

Mostly we use groupby() for more complicated calculations. For example, the weighted average, by way of numpy:

df_wac = df_orders.groupby(["Deal"]) \
         .apply(lambda grp: np.average(grp["Coupon"], weights=grp["CurrBal"])) \
         .rename("WAC") \
         .reset_index()

Take the most common (i.e., mode) term can similarly using mode() of a row group:

df_mode = df[[col1, col2, col3, col4]].groupby(col1).agg(lambda x: x.mode().iloc[0])

The HAVING clause in SQL is simply additional row slicing after groupby(). For example,

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders
FROM Orders
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID
WHERE LastName = 'Davolio' OR LastName = 'Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25

becomes

df_result = df_orders.merge(df_empl, how="inner", on="EmployeeID") \
            .pipe(lambda df: df[df["LastName"].isin(["Davolio", "Fuller"])]) \
            .groupby("LastName") \
            .rename("NumberOfOrders") \
            .reset_index() \
            .pipe(lambda df: df[df["NumberOfOrders"] > 25])

By default, groupby() will make the group value as row index unless we specify as_index=False. We do reset_index() at the end to make index back to an ordinary column (hence convert a Series into a two-column DataFrame).

The GroupBy object has a different set of member functions. For example, we use size() to produce a equiv for SELECT Country, COUNT(*) AS Counts FROM Customers GROUP BY Country

df_count = df.groupby('Country').size().reset_index(name='Counts')

The above reset_index() is called with a name argument, which assigns the column name to the result of size() in addition to making the group-by index as a column. This is useful in case of different aggregate functions to be called in the same group-by, e.g., in SQL SELECT Country, SUM(Quantity) AS TotalQty, COUNT(*) AS Count FROM Product # GROUP BY Country becomes

groups = df_product.groupby("Country", as_index=False)
df_result = groups \
            .sum()[["Country", "Quantity"]].rename(columns={"Quantity":"TotalQty"}) \
            .merge(groups.size().reset_index(name="Count"), on="Country")

Pivot table

Pivot table is something cannot be done in SQL but easily achieved in spreadsheet. For example, we start with a table of data

data = [ln.split() for ln in """
1  1    0.10    0.22     0.55     0.77
2  1    0.20    0.12     0.99     0.125
1  2    0.13    0.15     0.15     0.245
2  2    0.33    0.1      0.888    0.64
""".strip().splitlines()]
cols = "A  B  01-2016  02-2017  03-2017  04-2020".split()
df = pd.DataFrame(data, columns=cols).astype("float64")
df[["A","B"]] = df[["A","B"]].astype("int")

now the DataFrame looks like

>>> df
   A  B 01-2016 02-2017 03-2017 04-2020
0  1  1    0.10    0.22    0.55    0.77
1  2  1    0.20    0.12    0.99   0.125
2  1  2    0.13    0.15    0.15   0.245
3  2  2    0.33     0.1   0.888    0.64

and we melt() the DataFrame to combine all dates columns into one:

df_melted = df.melt(id_vars=["A", "B"], var_name="date")
df_melted["value"] = df_melted["value"].astype("float64")

which the DataFrame produced is like

>>> df.melted(id_vars=["A","B"], var_name="date")
    A  B     date  value
0   1  1  01-2016   0.10
1   2  1  01-2016   0.20
2   1  2  01-2016   0.13
3   2  2  01-2016   0.33
4   1  1  02-2017   0.22
5   2  1  02-2017   0.12
6   1  2  02-2017   0.15
7   2  2  02-2017    0.1
8   1  1  03-2017   0.55
9   2  1  03-2017   0.99
10  1  2  03-2017   0.15
11  2  2  03-2017  0.888
12  1  1  04-2020   0.77
13  2  1  04-2020  0.125
14  1  2  04-2020  0.245
15  2  2  04-2020   0.64

and to reverse, we use pivot_table():

df_pivot = df_melted \
           .pivot_table(values="value", index="date", columns=["A", "B"]).T \
           .reset_index()

and this will become

>>> df_pivot
date  A  B  01-2016  02-2017  03-2017  04-2020
0     1  1     0.10     0.22    0.550    0.770
1     1  2     0.13     0.15    0.150    0.245
2     2  1     0.20     0.12    0.990    0.125
3     2  2     0.33     0.10    0.888    0.640

There is a pivot() function besides pivot_table() but the former only allow a single column to be the index or columns argument.

Other algorithmic usages

Convert a DataFrame into iterable of rows with row index using iterrows():

for i, row in df_random.iterrows():
    row["A"] = row["A"] * 2

export data into external format: To Excel and to CSV, or even to string using tabulate module in Python:

with pd.ExcelWriter(filename, engine="openpyxl") as writer:
    df.to_excel(writer, sheetname, index=False)
df.to_csv(filename)
print(tabulate.tabulate(df, headers="keys", show_index=False))

Export to HTML can be more complicated:

html = df.to_html(na_rep='N/A', index=False, justify='center', float_format=lambda f: "{0:,.2f}".format(f))

numcols = df.select_dtypes(include=[np.number]).columns
html = df.style.set_properties(subset=numcols, **{'text-align':'right'}) \
         .set_uuid("order") \
         .format(lambda v: v if v==v else '') \
         .format({col: lambda v:"{0:,.2f}".format(v) if v==v else ''
                  for col in ret.select_dtypes(include=[np.float]).columns}) \
         .render()

To investigate the column types of a DataFrame:

df.dtype

Pandas has squeeze() function to lower the dimension: It will produce a series from DataFrame if there is only one row, or retain as DataFrame otherwise.

df_or_series = df[df["location"] == "c"].squeeze()

If the DataFrame is large and we want to parallelize it, we can build a few rows at a time in separate processes, using a process pool:

pool = multiprocessing.Pool(4)
df = pd.concat(pool.map(func, list_of_scalar))
df = pd.concat(pool.starmap(func, list_of_tuples))

but the above will need the function to be defined at module level and pickle-able in order to send to difference processes.

If we want a mapping to a column’s value, we can prepare a dict of the mapping, and apply the dict’s get() function to the column Series. Pandas provides the map() function for the same:

mapping_series = df_customer.filter(["CustomerID", "CustomerName"]).set_index("CustomerID")["CustomerName"]
df_order["CustomerName"] = df_order["CustomerID"].map(mapping_series)

Series has a different set of member functions. For example, combining two series:

s1 = pd.Series({'foo':1, 'bar':2})
s2 = s1.combine_first(pd.Series({'fubar':3}))

or converting a series into list:

assert s1.tolist() == list(s1)

Terminology

Panel (3D), DataFrame (3D), Series (1D)

  • Series index: row id of each item in vector
  • DataFrame index: row id (numeric from 0 onward by default)
  • DataFrame column: column id
  • Concat series into dataframe:
    • by axis 0 = output in vector
    • by axis 1 = concat column vectors horizontally into a table
  • Concat dataframes into another dataframe:
    • by axis 0 = stacking vertically
    • by axis 1 = stacking horizontally

Data Sources

Following are some handy source of tabular data that can be used as test data sets to try out pandas functions: