Splitting column data

Limpeza de dados em bancos de dados PostgreSQL

Darryl Reeves, Ph.D.

Industry Assistant Professor, New York University

Splitting columns

 camis    | inspection_date |                    violation                    | ... 
 ---------+-----------------+-------------------------------------------------+-----
 ...      | ...             | ...                                             | ...
 50038736 | 03/29/2018      | 09B Thawing procedures                          | ...
 50033304 | 12/18/2019      | 02B Hot food item not held at or above 140º ... | ...
 50081658 | 12/13/2018      | 06F Wiping cloths soiled or not stored in sa... | ...
 50033733 | 02/12/2019      | 10B Plumbing not properly installed or maint... | ...
 40559634 | 08/22/2017      | 04N Filth flies or food/refuse/sewage-associ... | ...
 ...      | ...             | ...                                             | ...

Finding substring starting position with STRPOS()

STRPOS(source_string, search_string)

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string.

SELECT
  STRPOS('09B Thawing procedures', ' ');

Finding substring starting position with STRPOS()

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string.

SELECT
  STRPOS('09B Thawing procedures', '?');

Finding substring starting position with STRPOS()

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string.

SELECT
  STRPOS('09B Thawing procedures', ' ');

Extracting a substring using SUBSTRING()

SUBSTRING(source_string FROM start_pos FOR num_chars)

Extracting a substring using SUBSTRING()

SUBSTRING('Homerun' FROM 1 FOR 4) → Home

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string.

SELECT
  SUBSTRING(
      '09B Thawing procedures'
      FROM 1
      FOR STRPOS('09B Thawing procedures', ' ') - 1
  );

09B

Extracting a substring using SUBSTRING()

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string. A pair of blue arrows indicate where the violation description is present spanning just this part of the violation.

Requirements:

Violation description start position
Number of characters in the description

SELECT
  STRPOS('09B Thawing procedures', ' ') + 1;

Calculating the length of a string with LENGTH()

A diagram of a violation string "09B Thawing procedures" The diagram includes index numbers underneath showing 1 at the 0 in text, 4 at the first space, and 22 at the s ending the string. The string "number of characters?" is above the violation description portion of the violation string.

LENGTH(string) → INTEGER

SELECT LENGTH('hello')

Calculating the length of a string with LENGTH()

LENGTH('09B Thawing procedures') → 22

STRPOS('09B Thawing procedures', ' ') → 4

LENGTH('09B Thawing procedures') - STRPOS('09B Thawing procedures', ' ') → 18

LENGTH('Thawing procedures') → 18

Calculating the length of a string with LENGTH()

SELECT 
    LENGTH('09B Thawing procedures') - 
    STRPOS('09B Thawing procedures', ' ');

Putting the pieces together

SELECT 
    SUBSTRING(
      '09B Thawing procedures'
      FROM
        STRPOS('09B Thawing procedures', ' ')
        + 1
      FOR
        LENGTH('09B Thawing procedures')
        - STRPOS('09B Thawing procedures', ' ')
    );

Thawing procedures

Splitting the violation column

SELECT 
    camis, 
    inspection_date, 

    SUBSTRING(
      violation 
      FROM 1 
      FOR STRPOS(violation, ' ') - 1
    ) AS violation_code, 

    SUBSTRING(
      violation 
      FROM STRPOS(violation, ' ') + 1 
      FOR LENGTH(violation) - STRPOS(violation, ' ')
    ) AS violation_description 
FROM 
    restaurant_inspection;

Splitting the violation column

 camis    | inspection_date |                    violation                    | ... 
 ---------+-----------------+-------------------------------------------------+-----
 ...      | ...             | ...                                             | ...
 50038736 | 03/29/2018      | 09B Thawing procedures                          | ...
 50033304 | 12/18/2019      | 02B Hot food item not held at or above 140º ... | ...
 50081658 | 12/13/2018      | 06F Wiping cloths soiled or not stored in sa... | ...
 50033733 | 02/12/2019      | 10B Plumbing not properly installed or maint... | ...
 40559634 | 08/22/2017      | 04N Filth flies or food/refuse/sewage-associ... | ...
 ...      | ...             | ...                                             | ...

Splitting the violation column

 camis    | inspection_date | violation_code |            violation_description            | ... 
 ---------+-----------------+----------------+---------------------------------------------+-----
 ...      | ...             | ...            | ...                                         | ...
 50038736 | 03/29/2018      | 09B            | Thawing procedures                          | ...
 50033304 | 12/18/2019      | 02B            | Hot food item not held at or above 140º ... | ...
 50081658 | 12/13/2018      | 06F            | Wiping cloths soiled or not stored in sa... | ...
 50033733 | 02/12/2019      | 10B            | Plumbing not properly installed or maint... | ...
 40559634 | 08/22/2017      | 04N            | Filth flies or food/refuse/sewage-associ... | ...
 ...      | ...             | ...            | ...                                         | ...

Let's practice!

Limpeza de dados em bancos de dados PostgreSQL