adding UDF example

[edublancas]

closes #49

Registering SQLite UDFs is possible since version 0.5.1 when we added support for registering existing SQLAlchemy engines; this PR adds an example.

[idomic]

.

[idomic]

This is a good example. Can't we allow the functionality without passing existing engine? It adds complexity and friction to the users.

[edublancas]

we cannot. the engine has to be prepared beforehand to register the function. I'm unsure what you're proposing

[idomic]

Can't you do something like that?

from sqlalchemy import create_engine, func

# Define a function that takes in a value and returns its square
def my_square_function(x):
    return x * x

# Create an engine and connect to a database
engine = create_engine('postgresql://username:password@localhost:5432/mydatabase')
connection = engine.connect()

# Use the func module to define the UDF as a SQL expression
square_udf = func.my_square_function(column)

# Use the UDF in a SELECT statement
query = (
    'SELECT *, {} AS squared_value FROM mytable'.format(square_udf)
)
result = connection.execute(query)

# Print the result set
for row in result:
    print(row)

# Close the connection
connection.close()
In this example, we define a function my_square_function that takes in a value and returns its square. We then use the func module to create a SQL expression for the function, and use it in a SELECT statement.

*Keep in mind that this example uses PostgreSQL as the database backend, and you may need to modify the example slightly to work with a different database.

[edublancas]

can you provide an example of how you think the API should be when using %%sql? The snippet you shared is not using it

[idomic]

# Load the extension and connect to the database
%load_ext sql
%sql postgresql://username:password@localhost:5432/mydatabase

# Define a function that takes in a value and returns its square
def my_square_function(x):
    return x * x

# Use the func module to define the UDF as a SQL expression
from sqlalchemy import func
square_udf = func.my_square_function(column)

# Use the UDF in a SELECT statement
result = %sql SELECT *, :square_udf AS squared_value FROM mytable

# Print the result set
print(result)

We might want to even create a tighter integration to the func part, maybe something like:

# Define the UDF as a SQL expression
%sql --udf my_square_function

# And then consume it
%sql SELECT *, :square_udf AS squared_value FROM mytable

[edublancas]

Ok, I see. I think we can open an issue and discuss the API, I agree it's simpler. For now, I'd say let's get this example in the docs so people know they can do it, then we simplify it.

btw, I don't think sqlalchemy.func is what we want, based on this, it looks like it's module to construct SQL from Python, not really for UDFs.

[idomic]

Alright, I'll keep the original issue open.

I think it's func with another thing like select(), the developer should dive into it.