How to Replicate MySQL's ORDER BY FIELD() Function in PostgreSQL?

Simulating MySQL's ORDER BY FIELD() in Postgresql

When transitioning from MySQL to PostgreSQL, developers may encounter the absence of the ORDER BY FIELD() function in PostgreSQL. This function provides a convenient way to sort records based on a specified list of values.

Problem Statement:

A Rails application utilizes SQL that includes the following ordering clause:

SELECT * FROM `currency_codes` ORDER BY FIELD(code, 'GBP', 'EUR', 'BBD', 'AUD', 'CAD', 'USD') DESC, name ASC

This clause aims to sort the records based on the specified list of currency codes, with additional ordering by the name field. However, PostgreSQL does not support this syntax.

Solution:

To simulate the behavior of MySQL's ORDER BY FIELD() in PostgreSQL, you can use a combination of the CASE statement and the ORDER BY clause. Here's how:

SELECT * FROM currency_codes
ORDER BY
CASE
  WHEN code='USD' THEN 1
  WHEN code='CAD' THEN 2
  WHEN code='AUD' THEN 3
  WHEN code='BBD' THEN 4
  WHEN code='EUR' THEN 5
  WHEN code='GBP' THEN 6
  ELSE 7
END,
name;

In this approach:

• The CASE statement evaluates the code field and assigns a corresponding priority to each currency code listed in the FIELD() function in MySQL.
• The ORDER BY clause uses this priority to sort the records first, followed by the sorting based on the name field.

This method provides a convenient way to simulate the behavior of MySQL's ORDER BY FIELD() in PostgreSQL and ensures that records are sorted as desired.

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