Введение в очистку данных

Очистка данных в базах данных PostgreSQL

Darryl Reeves, Ph.D.

Industry Assistant Professor, New York University

Зачем очищать данные?

Данные бывают «грязными»
Перед анализом их часто нужно очистить
Полезно использовать ограничения типов столбцов
Курс посвящён случаям, когда защитные подходы недоступны или неприменимы

Очистка строковых данных

Строковые данные: распространены, гибки и часто «грязные»

 name                            | grade |            inspection_type              | census_tract | ... 
 --------------------------------+-------+-----------------------------------------+--------------+-----
 ...                             | ...   | ...                                     | ...          | ...
 EMPANADAS MONUMENTAL            | B     | Cycle Inspection  /  Re-inspection      | 26900        | ...
 ALPHONSO'S PIZZERIA & TRATTORIA | A     | Cycle Inspection  /  Initial Inspection | 202          | ...
 THE SPARROW TAVERN              | A     | Cycle Inspection  /  Initial Inspection | 12500        | ...
 BURGER KING                     | A     | Cycle Inspection  /  Re-inspection      | 86400        | ...
 ASTORIA PIZZA                   | B     | Cycle Inspection  /  Re-inspection      | 6300         | ...
 ...                             | ...   | ...                                     | ...          | ...

Очистка строковых данных

 name                            | grade |            inspection_type              | census_tract | ... 
 --------------------------------+-------+-----------------------------------------+--------------+-----
 ...                             | ...   | ...                                     | ...          | ...
 EMPANADAS MONUMENTAL            | B     | Cycle Inspection  /  Re-inspection      | 26900        | ...
 ALPHONSO'S PIZZERIA & TRATTORIA | A     | Cycle Inspection  /  Initial Inspection | 202          | ...
 THE SPARROW TAVERN              | A     | Cycle Inspection  /  Initial Inspection | 12500        | ...
 BURGER KING                     | A     | Cycle Inspection  /  Re-inspection      | 86400        | ...
 ASTORIA PIZZA                   | B     | Cycle Inspection  /  Re-inspection      | 6300         | ...
 ...                             | ...   | ...                                     | ...          | ...

Нормализовать регистр в name
Убрать лишние пробелы вокруг разделителя в inspection_type
Привести значения census_tract к единой длине

Очистка строковых данных

 name                            | grade |            inspection_type              | census_tract | ... 
 --------------------------------+-------+-----------------------------------------+--------------+-----
 ...                             | ...   | ...                                     | ...          | ...
 EMPANADAS MONUMENTAL            | B     | Cycle Inspection  /  Re-inspection      | 26900        | ...
 ALPHONSO'S PIZZERIA & TRATTORIA | A     | Cycle Inspection  /  Initial Inspection | 202          | ...
 THE SPARROW TAVERN              | A     | Cycle Inspection  /  Initial Inspection | 12500        | ...
 BURGER KING                     | A     | Cycle Inspection  /  Re-inspection      | 86400        | ...
 ASTORIA PIZZA                   | B     | Cycle Inspection  /  Re-inspection      | 6300         | ...
 ...                             | ...   | ...                                     | ...          | ...

 name                            | grade |           inspection_type             | census_tract | ... 
 --------------------------------+-------+---------------------------------------+--------------+-----
 ...                             | ...   | ...                                   | ...          | ...
 Empanadas Monumental            | B     | Cycle Inspection / Re-inspection      | 026900       | ...
 Alphonso'S Pizzeria & Trattoria | A     | Cycle Inspection / Initial Inspection | 000202       | ...
 The Sparrow Tavern              | A     | Cycle Inspection / Initial Inspection | 012500       | ...
 Burger King                     | A     | Cycle Inspection / Re-inspection      | 086400       | ...
 Astoria Pizza                   | B     | Cycle Inspection / Re-inspection      | 006300       | ...
 ...                             | ...   | ...                                   | ...          | ...

Функция INITCAP()

INITCAP(input_string) — исправление регистра

SELECT INITCAP('HELLO FRIEND!');

Hello Friend!

Функция REPLACE()

REPLACE(input_string, to_replace, replacement) — замена одного текстового значения другим

SELECT REPLACE('180 Main Street', 'Street', 'St');

180 Main St

Функция LPAD()

LPAD(input_string, length [, fill_value]) — добавление символов в начало строки

SELECT LPAD('123', 7, 'X');

XXXX123

Составляем запрос для очистки строк

SELECT
  INITCAP(name) as name,
  grade,
  REPLACE(inspection_type, '  /  ', ' / ') as inspection_type, 
  LPAD(census_tract, 6, '0') as census_tract
FROM
  restaurant_inspection;

 name                            | grade |           inspection_type             | census_tract | ... 
 --------------------------------+-------+---------------------------------------+--------------+-----
 ...                             | ...   | ...                                   | ...          | ...
 Empanadas Monumental            | B     | Cycle Inspection / Re-inspection      | 026900       | ...
 Alphonso'S Pizzeria & Trattoria | A     | Cycle Inspection / Initial Inspection | 000202       | ...
 The Sparrow Tavern              | A     | Cycle Inspection / Initial Inspection | 012500       | ...
 Burger King                     | A     | Cycle Inspection / Re-inspection      | 086400       | ...
 Astoria Pizza                   | B     | Cycle Inspection / Re-inspection      | 006300       | ...
 ...                             | ...   | ...                                   | ...          | ...

Давайте потренируемся!

Очистка данных в базах данных PostgreSQL