Download Link to Slides

SQL – PYTHON AND DATABASES
HOW TO REPRESENT A DATABASE IN PYTHON
• Project 2 Requirement:
• Needs to match the SQLite input and output identically (for the subset of SQL used in the
projects)
• The rest of the implementation content is entirely just suggestions on one way
to solve the problem.
SQLITE TO PYTHON
SQLite type
Python type
NULL
None
INTEGER
int
REAL
float
TEXT
str
BLOB
bytes
You can ignore the existence of BLOBs for the projects
TYPE CHECKING
• Most databases require that if a column is declared to have a type, each
element in that column must conform or be converted.
• SQLite doesn't mind mismatched types.
• It won't matter for this course as all SQL in tests will not have type mismatches
• Example of a type mismatch:
• CREATE TABLE x (name TEXT);
• INSERT INTO x VALUES (5);
ROWS
• You can represent a row in a table as a python tuple:
• INSERT INTO x VALUES (1, 1.0, 'I''m a string', NULL);
• (1, 1.0, "I'm a string", None)
• If a row has only a single value, make sure it is still in a tuple:
• INSERT INTO x VALUES ('One is the loneliest number');
• ("One is the loneliest number",)
•
NOT ("One is the loneliest number")
TABLE
• A table can be represented by a class in python with two parts:
• The schema (table name, column names and types)
• The rows
• Rows aren't necessary unique, and row order doesn't matter, but you need to
be able to iterate over the rows to get their content.
• You need to be able to add and remove rows easily
WHAT DATA STRUCTURE COULD HOLD THE ROWS
OF A TABLE?
1.
2.
3.
4.
set
list
bag (multiset)
tuple
DATABASE
• A database can contain 0 or more tables.
• It needs to be able to find tables by their name.
• You can use class that contains a dictionary, mapping the tables' names to the
table instances
PROJECT API
•
Like python's sqlite3 module:
import project
conn = project.connect("filename.db")
conn.execute("CREATE TABLE myTable (name TEXT);")
conn.execute("INSERT INTO myTable VALUES ('Josh');")
rows = conn.execute("SELECT * FROM myTable;")
for row in rows:
print(row)
CONNECT
• connect is a module function that takes a filename and results a connection
object
• The filename will be used in later projects that need your database to
write its data to a file. For project 2, it should be ignored.
• The object returned should be able to access the database to pass queries
and return results with an execute method.
EXECUTE
• The execute method on the Connection object takes a string (query) and
returns an iterable object or None, depending on if the query returns rows
from the database.
• One possible implementation:
• Do the query, then
•
•
If not a SELECT query, return None
Otherwise return a list of rows (tuples), may be empty if no rows exist
HOW TO WRITE THE EXECUTE METHOD?
• Each query is guaranteed to be a legal SQL statement, for Project 2 there
are only a few statements to handle:
• CREATE TABLE ...
• INSERT INTO ...
• SELECT ...
• Caveat: whitespace is allowed before, after, and within queries
• Spaces, \n, \t, \r