Úvod do čištění dat

Cleaning Data in PostgreSQL Databases

Darryl Reeves, Ph.D.

Industry Assistant Professor, New York University

Proč je čištění dat důležité?

Data jsou nepořádná
Před analýzou je často nutné je vyčistit
Pomáhají omezení datových typů sloupců
Kurz se zaměřuje na případy, kdy defenzivní přístupy nejsou dostupné

Čištění textových dat

Textová data: hojná, flexibilní a často nepořádná

 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         | ...
 ...                             | ...   | ...                                     | ...          | ...

Čištění textových dat

 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         | ...
 ...                             | ...   | ...                                     | ...          | ...

Omezit velikost písmen ve sloupci name
Odstranit nadbytečné mezery u oddělovače ve sloupci inspection_type
Sjednotit délku hodnot ve sloupci census_tract

Čištění textových dat

 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       | ...
 ...                             | ...   | ...                                   | ...          | ...

Funkce INITCAP()

INITCAP(input_string) – oprava velikosti písmen

SELECT INITCAP('HELLO FRIEND!');

Hello Friend!

Funkce REPLACE()

REPLACE(input_string, to_replace, replacement) – nahrazení jedné textové hodnoty jinou

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

180 Main St

Funkce LPAD()

LPAD(input_string, length [, fill_value]) – doplnění textu na začátek řetězce

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

XXXX123

Sestavení dotazu pro čištění řetězců

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       | ...
 ...                             | ...   | ...                                   | ...          | ...

Pojďme cvičit!

Cleaning Data in PostgreSQL Databases