Connect to a Database

Make a direct connection to a database and create a cursor. cnxn = pyodbc.connect('DRIVER={SQL Server};SERVER=localhost;DATABASE=testdb;UID=me;PWD=pass') cursor = cnxn.cursor()

Make a connection using a DSN. Since DSNs usually don't store passwords, you'll probably need to provide the PWD keyword. cnxn = pyodbc.connect('DSN=test;PWD=password') cursor = cnxn.cursor()

There are lots of options when connecting, so see the connect function and ConnectionStrings for more details.

Selecting Some Data

Select Basics

All SQL statements are executed using the cursor.execute function. If the statement returns rows, such as a select statement, you can retreive them using the Cursor fetch functions (fetchone, fetchall, fetchmany). If there are no rows, fetchone will return None; fetchall and fetchmany will both return empty lists. cursor.execute("select user_id, user_name from users") row = cursor.fetchone() if row: print row

Row objects are similar to tuples, but they also allow access to columns by name:

cursor.execute("select user_id, user_name from users") row = cursor.fetchone() print 'name:', row[1] # access by column index print 'name:', row.user_name # or access by name

The fetchone function returns None when all rows have been retrieved. while 1: row = cursor.fetchone() if not row: break print 'id:', row.user_id

The fetchall function returns all remaining rows in a list. If there are no rows, an empty list is returned. (If there are a lot of rows, this will use a lot of memory. Unread rows are stored by the database driver in a compact format and are often sent in batches from the database server. Reading in only the rows you need at one time will save a lot of memory.)

cursor.execute("select user_id, user_name from users") rows = cursor.fetchall() for row in rows: print row.user_id, row.user_name

If you are going to process the rows one at a time, you can use the cursor itself as an interator: cursor.execute("select user_id, user_name from users"): for row in cursor: print row.user_id, row.user_name

Since cursor.execute always returns the cursor, you can simplify this even more: for row in cursor.execute("select user_id, user_name from users"): print row.user_id, row.user_name

A lot of SQL statements don't fit on one line very easily, so you can always use triple quoted strings: cursor.execute(""" select user_id, user_name from users where last_logon < '2001-01-01' and bill_overdue = 'y' """)

Parameters

ODBC supports parameters using a question mark as a place holder in the SQL. You provide the values for the question marks by passing them after the SQL:

cursor.execute(""" select user_id, user_name from users where last_logon < ? and bill_overdue = ? """, '2001-01-01', 'y')

This is safer than putting the values into the string because the parameters are passed to the database separately, protecting against SQL injection attacks. It is also be more efficient if you execute the same SQL repeatedly with different parameters. The SQL will be prepared only once. (pyodbc only keeps the last statement prepared, so if you switch between statements, each will be prepared multiple times.)

The Python DB API specifies that parameters should be passed in a sequence, so this is also supported by pyodbc:

cursor.execute(""" select user_id, user_name from users where last_logon < ? and bill_overdue = ? """, ['2001-01-01', 'y'])

cursor.execute("select count(*) as user_count from users where age > ?", 21) row = cursor.fetchone() print '%d users' % row.user_count

Inserting Data

To insert data, pass the insert SQL to Cursor.execute, along with any parameters necessary:

cursor.execute("insert into products(id, name) values ('pyodbc', 'awesome library')") cnxn.commit()

cursor.execute("insert into products(id, name) values (?, ?)", 'pyodbc', 'awesome library') cnxn.commit()

Note the calls to cnxn.commit(). You must call commit or your changes will be lost! When the connection is closed, any pending changes will be rolled back. This makes error recovery very easy, but you must remember to call commit.

Updating and Deleting

Updating and deleting work the same way, pass the SQL to execute. However, you often want to know how many records were affected when updating and deleting, in which case you can use the cursor.rowcount value:

cursor.execute("delete from products where id <> ?", 'pyodbc') print cursor.rowcount, 'products deleted' cnxn.commit()

Since execute always returns the cursor, you will sometimes see code like this. (Notice the rowcount on the end.)

deleted = cursor.execute("delete from products where id <> 'pyodbc'").rowcount cnxn.commit()

Note the calls to cnxn.commit(). You must call commit or your changes will be lost! When the connection is closed, any pending changes will be rolled back. This makes error recovery very easy, but you must remember to call commit.

Tips and Tricks

Since single quotes are valid in SQL, use double quotes to surround your SQL:

deleted = cursor.execute("delete from products where id <> 'pyodbc'").rowcount

If you are using triple quotes, you can use either:

deleted = cursor.execute(""" delete from products where id <> 'pyodbc' """).rowcount

Some databases (e.g. SQL Server) do not generate column names for calculations, in which case you need to access the columns by index. You can also use the 'as' keyword to name columns (the "as user_count" in the SQL below).

row = cursor.execute("select count(*) as user_count from users").fetchone() print '%s users' % row.user_count

If there is only 1 value you need, you can put the fetch of the row and the extraction of the first column all on one line:

count = cursor.execute("select count(*) from users").fetchone()[0] print '%s users' % count

This will not work if the first column can be NULL! In that case, fetchone() will return None and you'll get a cryptic error about NoneType not supporting indexing. If there is a default value, often you can is ISNULL or coalesce to convert NULLs to default values directly in the SQL:

maxid = cursor.execute("select coalesce(max(id), 0) from users").fetchone()[0]

In this example, coalesce(max(id), 0) causes the selected value to be 0 if max(id) returns NULL.