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297 lines (225 loc) · 11.2 KB
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"""
Created on Sun Jan 26 16:40:04 2020
@author: Neeraj
Description: This file contains an impelementation of basic SQL operations in Python.
Reference: Chapter 24 Databse and SQL
"""
users = [[0, "Hero", 0],
[1, "Dunn", 2],
[2, "Sue", 3],
[3, "Chi", 3]]
from typing import Tuple, Sequence, List, Any, Callable, Dict, Iterator
from collections import defaultdict
# A few aliases we will use later
Row = Dict[str, Any] # A database row
WhereClause = Callable[[Row], bool] # Predicate for a single row
HavingClause = Callable[[List[Row]], bool] # Predicate over multiple rows
class Table:
def __init__(self, columns: List[str], types: List[type]) -> None:
assert len(columns) == len(types), "# of columns must be = # of types"
self.columns = columns # Number of columns
self.types = types # Data types of each column
self.rows: List[Row] = [] # (no data yet)
def col2type(self, col: str) -> type:
idx = self.columns.index(col) # Find the index of the column
return self.types[idx] # and return its type
def insert(self, values: list) -> None:
# check for right number of values
if len(values) != len(self.types):
raise ValueError(f"You need to provide {len(self.types)} values")
# Check for right type of the values
for value, typ3 in zip(values, self.types):
if not isinstance(value, typ3) and value is not None:
raise TypeError(f"Expected type {typ3} but got {value}")
# Add the corresponding dict as a row
self.rows.append(dict(zip(self.columns, values)))
def __getitem__(self, idx: int) -> Row:
return self.rows[idx]
def __iter__(self) -> Iterator[Row]:
return iter(self.rows)
def __len__(self) -> int:
return len(self.rows)
def __repr__(self):
"""Pretty representation of the table: columns then rows"""
rows = "\n".join(str(row) for row in self.rows)
return f"{self.columns}\n{rows}"
def update(self, updates: Dict[str, Any],
predicate: WhereClause = lambda row: True):
# First make sure that the updates have valid names and types
for column, new_value in updates.items():
if column not in self.columns:
raise ValueError(f"Invalid column: {column}")
typ3 = self.col2type(column)
if not isinstance(new_value, typ3) and new_value is not None:
raise TypeError(f"expected type {typ3}, but got {new_value}")
# Now update
for row in self.rows:
if predicate(row):
for column, new_value in updates.items():
row[column] = new_value
def delete(self, predicate: WhereClause = lambda row: True) -> None:
"Delete all rows matching predicate"
self.rows = [row for row in self.rows if not predicate(row)]
def select(self,
keep_columns: List[str] = None,
additional_columns: Dict[str, Callable] = None) -> 'Table':
if keep_columns is None: # If no columns specified
keep_columns = self.columns # Return no columns
if additional_columns is None:
additional_columns = {}
# New column names and types
new_columns = keep_columns + list(additional_columns.keys())
keep_types = [self.col2type(col) for col in keep_columns]
# This is how to get the return type from a type annotation.
# It will crash if 'calculation' doesn't have a return type.
add_types = [calculation.__annotations__['return']
for calculation in additional_columns.values()]
# Create a new table for results
new_table = Table(new_columns, keep_types + add_types)
for row in self.rows:
new_row = [row[column] for column in keep_columns]
for column_name, calculation in additional_columns.items():
new_row.append(calculation(row))
new_table.insert(new_row)
return new_table
def where(self, predicate: WhereClause = lambda row: True) -> 'Table':
"""Return only the rows that satisfy the supplied predicate"""
where_table = Table(self.columns, self.types)
for row in self.rows:
if predicate(row):
values = [row[column] for column in self.columns]
where_table.insert(values)
return where_table
def limit(self, num_rows: int) -> 'Table':
"""Return only the first 'num_rows' rows"""
limit_table = Table(self.columns, self.types)
for i, row in enumerate(self.rows):
if i >= num_rows:
break
values = [row[column] for column in self.columns]
limit_table.insert(values)
return limit_table
def group_by(self,
group_by_columns: List[str],
aggregates: Dict[str, Callable],
having: HavingClause = lambda group: True) -> 'Table':
grouped_rows = defaultdict(list)
# Populate groups
for row in self.rows:
key = tuple(row[column] for column in group_by_columns)
grouped_rows[key].append(row)
# Result table consists of group_by columns and aggregates
new_columns = group_by_columns + list(aggregates.keys())
group_by_types = [self.col2type(col) for col in group_by_columns]
aggregate_types = [agg.__annotations__['return']
for agg in aggregates.values()]
result_table = Table(new_columns, group_by_types + aggregate_types)
for key, rows in grouped_rows.items():
if having(rows):
new_row = list(key)
for aggregate_name, aggregate_fn in aggregates.items():
new_row.append(aggregate_fn(rows))
result_table.insert(new_row)
return result_table
def order_by(self, order: Callable[[Row], Any]) -> 'Table':
new_table = self.select() # make a copy
new_table.rows.sort(key = order)
return new_table
def join(self, other_table: 'Table', left_join: bool = False) -> 'Table':
join_on_columns = [c for c in self.columns
if c in other_table.columns] # columns in
additional_columns = [c for c in other_table.columns # columns only in
if c not in join_on_columns] # right table
# all columns from left table + additional_columns from right table
new_columns = self.columns + additional_columns
new_types = self.types + [other_table.col2type(col)
for col in additional_columns]
join_table = Table(new_columns, new_types)
for row in self.rows:
def is_join(other_row):
return all(other_row[c] == row[c] for c in join_on_columns)
other_rows = other_table.where(is_join).rows
# Each other row that matches this one produces a result row.
for other_row in other_rows:
join_table.insert([row[c] for c in self.columns] +
[other_row[c] for c in additional_columns])
# If no rows match and it's a left join, output with Nones.
if left_join and not other_rows:
join_table.insert([row[c] for c in self.columns] +
[None for c in additional_columns])
return join_table
users = Table(['user_id', 'name', 'num_friends'], [int, str, int])
users.insert([0, "Hero", 0])
users.insert([1, "Dunn", 2])
users.insert([2, "Sue", 3])
users.insert([3, "Chi", 3])
users.insert([4, "Thor", 3])
users.insert([5, "Clive", 2])
users.insert([6, "Hicks", 3])
users.insert([7, "Devin", 2])
users.insert([8, "Kate", 2])
users.insert([9, "Klein", 3])
users.insert([10, "Jen", 1])
users.update({'num_friends': 5}, # set num_friends == 1
lambda row: row['user_id'] == 1) # in rows where user_id = 1
# Select * FROM users
all_users = users.select()
two_users = users.limit(2)
just_ids = users.select(keep_columns = ["user_id"])
dunn_ids = (users.where(lambda row: row["name"] == "Dunn")
.select(keep_columns = ["user_id"]))
def name_length(row) -> int:
return len(row["name"])
name_lengths = users.select(keep_columns = [],
additional_columns = {"name_length": name_length})
def min_user_id(rows) -> int:
return min(row["user_id"] for row in rows)
def length(rows) -> int:
return len(rows)
stats_by_length = (users
.select(additional_columns = {"name_length": name_length})
.group_by(group_by_columns = ["name_length"],
aggregates = {"min_user_id": min_user_id,
"num_users": length}))
return row["name"][0] if row["name"] else ""
def average_num_friends(rows: List[Row]) -> float:
return sum(row["num_friends"] for row in rows)/ len(rows)
def enough_friends(rows: List[Row]) -> bool:
return average_num_friends(rows) > 1
avg_friends_by_letter = (users
.select(additional_columns = {'first_letter': first_letter_of_name})
.group_by(group_by_columns = ['first_letter'],
aggregates = {"avg_num_friends": average_num_friends},
having = enough_friends))
def sum_user_ids(rows: List[Row]) -> int:
return sum(row["user_id"] for row in rows)
user_id_sum = (
users
.where(lambda row: row["user_id"] > 1)
.group_by(group_by_columns=[],
aggregates={ "user_id_sum" : sum_user_ids })
)
friendliest_letters = (avg_friends_by_letter
.order_by(lambda row: -row["avg_num_friends"])
.limit(4))
user_interests = Table(['user_id', 'interest'], [int, str])
user_interests.insert([0, "SQL"])
user_interests.insert([0, "NoSQL"])
user_interests.insert([2, "SQL"])
user_interests.insert([2, "MySQL"])
sql_users = (users
.join(user_interests)
.where(lambda row: row["interest"] == "SQL")
.select(keep_columns = ["name"]))
def count_interests(rows: List[Row]) -> int:
"""Counts how many rows have non-None interests"""
return len([row for row in rows if row["interest"] is not None])
user_interest_counts = (users
.join(user_interests, left_join = True)
.group_by(group_by_columns = ["user_id"],
aggregates = {"num_interests": count_interests}))
likes_sql_user_ids = (user_interests
.where(lambda row: row["interest"] == "SQL")
.select(keep_columns = ['user_id']))
likes_sql_user_ids.group_by(group_by_columns = [],
aggregates = {'min_user_id': min_user_id})
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