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SQL - PostgreSQL String Manipulation.md

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SQL - PostgreSQL String Manipulation

Source: https://gist.github.com/chillahwhale/1f27869ece7f757e0242da94ae696e8e

Contents

  • [[#LOWER()|LOWER()]]
  • [[#UPPER()|UPPER()]]
  • [[#INITCAP()|INITCAP()]]
  • [[#LENGTH()|LENGTH()]]
  • [[#TRIM()|TRIM()]]
  • [[#SUBSTRING()|SUBSTRING()]]
  • Concatenation
  • [[#REPLACE()|REPLACE()]]
  • [[#COALESCE()|COALESCE()]]
  • Appendix: Links

LOWER()

This is the same as the .lower() method for strings in Python used to convert every letter in a string to lower case

Example: Convert all letters of the string HeLlO, wOrLd! to lower case:

SELECT LOWER('HeLlO, wOrLd!')

UPPER()

For completeness, this is the same as the .upper() method for strings in Python used to capitalize every letter in a string

Example: Capitalize all letters of the string Hello World

SELECT UPPER('Hello, world!')

INITCAP()

This is the same as the .capitalize() method for strings in Python that is used to convert the first letter to upper case.

SELECT INITCAP(first_name), INITCAP(department)
FROM employees
WHERE first_name ILIKE 'an%'

LENGTH()

This is the same as the len() function in Python. However, since we don't have lists or tuples in SQL, this is only applicable to objects with characters.

SELECT INITCAP(first_name), INITCAP(department)
FROM employees
WHERE department = 'Sports' AND LENGTH(first_name) >= 6

TRIM()

This is the same as the .strip() method for strings in Python that eliminates leading and trailing white spaces.

Example: Write a query that strips out the white space from the string ' Hello, world! '

SELECT TRIM('     Hello, world!     ')

SUBSTRING()

2-Areas/MOCs/Python doesn't have a function that extracts a sub-string since we can just do it by directly indexing through the string.

If you're familiar with 2-Areas/MOCs/R though, then you'll recognize this is similar to the substr() function.

Syntax for this function:

SELECT SUBSTRING(string_column FROM <start_position> FOR <num_characters_ahead>)

Or

SELECT SUBSTRING(string_column, <start_position>, <num_characters_ahead>)

Example #1:

SELECT SUBSTRING('Hello there, friend! Hehe.' FROM 1 FOR 5)

OR

SELECT SUBSTRING('Hello there, friend! Hehe.', 1, 5)

will return 'Hello'

Example #2:

SELECT SUBSTRING('Hello there, friend! Hehe.' FROM 14)

OR

SELECT SUBSTRING('Hello there, friend! Hehe.', 14)

will return 'friend! Hehe.

Concatenation

This is the equivalent of string concatenation in Python using +. The + in 2-Areas/MOCs/Python is replaced by || in PostgreSQL.

Alternatively, you can use the CONCAT() function.

Example: Write a query that prints every employees's full name (i.e. first name then last name)

SELECT INITCAP(e.first_name) || ' ' || INITCAP(e.last_name)
FROM employees e

EXERCISE: Write a query that automatically generates the sentence <employee first name> works in the <department> department.

Answer:

SELECT INITCAP(e.first_name) || ' twerks in ' || INITCAP(e.department) AS "where u twerk?"
FROM employees e;
-- OR
SELECT CONCAT(INITCAP(e.first_name), ' twerks in ', INITCAP(e.department)) AS "where u twerk?"
FROM employees e;

REPLACE()

This is the equivalent of the .replace() method for strings in Python and the gsub() function in R.

Example: Replace the 'Jewelry' department with 'Bling' in the employee table.

SELECT first_name,
       REPLACE(department, 'Jewelry', 'Bling') AS new_dept
FROM employees

Does the function work when replacing NULL values though? Try this and let me know what you see

SELECT first_name,
       REPLACE(email, NULL, 'missing') AS new_email
FROM employees

COALESCE()

This is an extremely powerful function that lets us handle missing values on a column-by-column basis.

The syntax is pretty straight forward for this one:

COALESCE(<column_name>, <fill_value>)

EXERCISE: Write a query that prints every employees's first name in one column and their email in another, but make sure to replace all NULL emails with ¯\_(ツ)_/¯.

Answer:

SELECT first_name,
       COALESCE(email, '`¯\_(ツ)_/¯`') AS new_email
FROM employees;

Appendix: Links

Backlinks:

list from [[SQL - PostgreSQL String Manipulation]] AND -"Changelog"