Develop Reference

Is it necessary to specify the database name in the code if the connection to the database is already established?

Let's say I make a request like:
query_select = "SELECT `client_id` FROM `my_databse_name`.`orders` ORDER BY `client_id` DESC"
await cursor.execute(query_select)
In this example, I append the database name to the table name (my_databse_name.orders) instead of just writing orders. Is this addition redundant?
Moreover, I've already installed the connection, and when I installed it, I specified the database name - my_databse_name.
I even checked if I remove that name and pass it like this:
query_select = "SELECT `client_id` FROM `orders` ORDER BY `client_id` DESC"
Then my call works correctly anyway. But in a lot of examples, I often see references to tables where the database name is added. Therefore, I'm not sure what should I do and also add. Although I would prefer to remove the database name from my queries.
The question is - does such adding a name affect the reliability of work, etc.?
I am using MySQL.

Selecting a column with a slash in SQL Alchemy

I need to select some columns from a table with SQL Alchemy. Everything works fine except selecting the one column with '/' in the name. My query looks like:
query = select([func.sum(Table.c.ColumnName),
func.sum(Table.c.Column/Name),
])
Obviously the issue comes from the second line with the column 'Column/Name'. Is there a way in SQL Alchemy to overcome special characters in a column name?
edit:
I've it all inside some class but simplified version of a process looks like this. I create an engine (all necessary db data is inside create_new_engine() function) and map all tables in db into metadata.
def map(self):
from sqlalchemy.engine.base import Engine
# check if engine exist
if not isinstance(self.engine, Engine):
self.create_new_engine()
self.metadata = MetaData({'schema': 'dbo'})
self.metadata.reflect(bind=self.engine)
Then I map a single table with:
def map_table(self, table_name):
table = "{schema}.{table_name}".format(schema=self.metadata.schema, table_name=table_name)
table = self.metadata.tables[table]
return table
In the end I use pandas read_sql_query to run above query with connection and engine established earlier.
I'm connecting to SQL Server.

Since Table.c points to a plain python obect. Try in pure Python
query = select([func.sum(Table.c.ColumnName),
func.sum(getattr(Table.c, 'Column/Name')),
])
So in your case (from comments above) :
func.sum(getattr(Table.c, 'cur/fees'))

How do I create a database with database name as variable using Python?

I am learning how to work with Python and MySQL. I wanted to know how can I create a database with it's name as variable using python. I have tried this but it does not work at all. I have used this method in insert query so thought it might work in this too.
db_query=('''CREATE DATABASE VALUES(?)''')
VALUES=email2
my_cursor.execute(db_query,VALUES)
mydb.commit()

The database name is not an expression that can be computed in MySQL, it has to be literal, so you can't use a placeholder or function call there.
If the database name is in a Python variable, use string formatting to substitute it.
email2 = 'desired_db_name`
db_query = f'CREATE DATABASE `{email2}`'
my_cursor.execute(db_query)
mydb.commit()
If the DB name comes from user input, make sure you validate it carefully first.

Pushing dataframe to postgres using sqlalchemy and psycogp2

I am trying to write dataframes to postgres . For that DBAPI used is psycogp2.
localconn='postgresql+psycopg2://postgres/PSWD#localhost:5432/localPG'
localEng=engine.create_engine(localconn)
df.to_sql("DUMMY", localEng)
But its throwing error (psycopg2.OperationalError) could not translate host name postgres to address: Name or service not known
localPG is the database name.
Where I am doing wrong?

The format you have written is wrong, use the following:
localEng = create_engine('postgresql+psycopg2://[user]:[pass]#[host]:[port]/[schema]', echo=False)
and of course, you should replace every parameter between the bracket with the equivalent database credentials.

Binding variable to table name with cx_Oracle

I'm using cx_Oracle in Python and can't get a variable be used as table name, like in this simple example:
query = "select * from some.:usertable.userinfo"
bindvars = {'usertable':usertable}
cursor.execute(query, bindvars)
What is the correct syntax? Variable substition works fine when I use WHERE… etc. but not with table names. I guess I have to separate ":usertable" somehow…

Database adapters rarely support using parameters for anything that isn't a 'value' (something that needs quoting). Either use string formatting (dodgy, you run the risk of a sql injection) or use a library like SQLAlchemy that let's you produce valid SQL using Python code.
If you are certain your usertable value is sane (checked against a list of existing table names, for example), the following would work:
query = 'select * from some.{usertable}.userinfo'.format(usertable=usertable)

You cannot bind an object name in Oracle, only a literal. Oracle does, however, have an inbuilt package dbms_assert, to help prevent SQL injection when using dynamic object names. The most useful function in your case is probably sql_object_name, which:
"... verifies that the input parameter string is a qualified SQL
identifier of an existing SQL object."
For instance you could do the following in cx_Oracle.
object_name = cursor.callfunc('sys.dbms_assert.sql_object_name'
, cx_Oracle.string, ['usertable'])
It raises ORA-44002, if the name is invalid, which you can capture in cx_Oracle, or if everything's fine continue as Martijn has suggested.
I would recommend reading Oracle's guide to guarding against SQL injection.

Perhaps it's a bit late to reply, but I was dealing with the same thing 2 days ago.
The solution is, as Martjin says, to format the query.
query = f'select * from {tableName}'
Hope it helps someone as it helped me.