In DataFusion an expression is an abstraction that represents a computation. Expressions are used as the primary inputs and outputs for most functions within DataFusion. As such, expressions can be combined to create expression trees, a concept shared across most compilers and databases.
The first expression most new users will interact with is the Column, which is created by calling :py:func:`~datafusion.col`. This expression represents a column within a DataFrame. The function :py:func:`~datafusion.col` takes as in input a string and returns an expression as it's output.
Literal expressions represent a single value. These are helpful in a wide range of operations where a specific, known value is of interest. You can create a literal expression using the function :py:func:`~datafusion.lit`. The type of the object passed to the :py:func:`~datafusion.lit` function will be used to convert it to a known data type.
In the following example we create expressions for the column named color and the literal scalar string red. The resultant variable red_units is itself also an expression.
.. ipython:: python
red_units = col("color") == lit("red")
When combining expressions that evaluate to a boolean value, you can combine these expressions using boolean operators. It is important to note that in order to combine these expressions, you must use bitwise operators. See the following examples for the and, or, and not operations.
.. ipython:: python
red_or_green_units = (col("color") == lit("red")) | (col("color") == lit("green"))
heavy_red_units = (col("color") == lit("red")) & (col("weight") > lit(42))
not_red_units = ~(col("color") == lit("red"))
For columns that contain arrays of values, you can access individual elements of the array by index
using bracket indexing. This is similar to calling the function
:py:func:`datafusion.functions.array_element`, except that array indexing using brackets is 0 based,
similar to Python arrays and array_element is 1 based indexing to be compatible with other SQL
approaches.
.. ipython:: python
from datafusion import SessionContext, col
ctx = SessionContext()
df = ctx.from_pydict({"a": [[1, 2, 3], [4, 5, 6]]})
df.select(col("a")[0].alias("a0"))
Warning
Indexing an element of an array via [] starts at index 0 whereas
:py:func:`~datafusion.functions.array_element` starts at index 1.
To check if an array is empty, you can use the function :py:func:`datafusion.functions.array_empty` or datafusion.functions.empty. This function returns a boolean indicating whether the array is empty.
.. ipython:: python
from datafusion import SessionContext, col
from datafusion.functions import array_empty
ctx = SessionContext()
df = ctx.from_pydict({"a": [[], [1, 2, 3]]})
df.select(array_empty(col("a")).alias("is_empty"))
In this example, the is_empty column will contain True for the first row and False for the second row.
To get the total number of elements in an array, you can use the function :py:func:`datafusion.functions.cardinality`. This function returns an integer indicating the total number of elements in the array.
.. ipython:: python
from datafusion import SessionContext, col
from datafusion.functions import cardinality
ctx = SessionContext()
df = ctx.from_pydict({"a": [[1, 2, 3], [4, 5, 6]]})
df.select(cardinality(col("a")).alias("num_elements"))
In this example, the num_elements column will contain 3 for both rows.
To concatenate two arrays, you can use the function :py:func:`datafusion.functions.array_cat` or :py:func:`datafusion.functions.array_concat`. These functions return a new array that is the concatenation of the input arrays.
.. ipython:: python
from datafusion import SessionContext, col
from datafusion.functions import array_cat, array_concat
ctx = SessionContext()
df = ctx.from_pydict({"a": [[1, 2, 3]], "b": [[4, 5, 6]]})
df.select(array_cat(col("a"), col("b")).alias("concatenated_array"))
In this example, the concatenated_array column will contain [1, 2, 3, 4, 5, 6].
To repeat the elements of an array a specified number of times, you can use the function :py:func:`datafusion.functions.array_repeat`. This function returns a new array with the elements repeated.
.. ipython:: python
from datafusion import SessionContext, col, literal
from datafusion.functions import array_repeat
ctx = SessionContext()
df = ctx.from_pydict({"a": [[1, 2, 3]]})
df.select(array_repeat(col("a"), literal(2)).alias("repeated_array"))
In this example, the repeated_array column will contain [[1, 2, 3], [1, 2, 3]].
Columns that contain struct elements can be accessed using the bracket notation as if they were Python dictionary style objects. This expects a string key as the parameter passed.
.. ipython:: python
ctx = SessionContext()
data = {"a": [{"size": 15, "color": "green"}, {"size": 10, "color": "blue"}]}
df = ctx.from_pydict(data)
df.select(col("a")["size"].alias("a_size"))
As mentioned before, most functions in DataFusion return an expression at their output. This allows us to create a wide variety of expressions built up from other expressions. For example, :py:func:`~datafusion.expr.Expr.alias` is a function that takes as it input a single expression and returns an expression in which the name of the expression has changed.
The following example shows a series of expressions that are built up from functions operating on expressions.
.. ipython:: python
from datafusion import SessionContext
from datafusion import column, lit
from datafusion import functions as f
import random
ctx = SessionContext()
df = ctx.from_pydict(
{
"name": ["Albert", "Becca", "Carlos", "Dante"],
"age": [42, 67, 27, 71],
"years_in_position": [13, 21, 10, 54],
},
name="employees"
)
age_col = col("age")
renamed_age = age_col.alias("age_in_years")
start_age = age_col - col("years_in_position")
started_young = start_age < lit(18)
can_retire = age_col > lit(65)
long_timer = started_young & can_retire
df.filter(long_timer).select(col("name"), renamed_age, col("years_in_position"))