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How to Use Order By Case With an Alias Column on Postgresql?

3 minutes read

When using the ORDER BY clause with a CASE statement on a PostgreSQL database, it is important to keep in mind that you cannot reference column aliases directly in the ORDER BY clause. Instead, you will need to repeat the entire CASE statement in the ORDER BY clause.


For example, suppose you have a query that calculates a new column called "status" based on some conditions using a CASE statement, like this:


SELECT id, name, age, CASE WHEN age < 18 THEN 'Minor' WHEN age >= 18 AND age < 65 THEN 'Adult' ELSE 'Senior' END AS status FROM users;


If you want to order the results by the "status" column, you will need to repeat the entire CASE statement in the ORDER BY clause like this:


SELECT id, name, age, CASE WHEN age < 18 THEN 'Minor' WHEN age >= 18 AND age < 65 THEN 'Adult' ELSE 'Senior' END AS status FROM users ORDER BY CASE WHEN age < 18 THEN 'Minor' WHEN age >= 18 AND age < 65 THEN 'Adult' ELSE 'Senior' END;


By repeating the entire CASE statement in the ORDER BY clause, you are ensuring that the results are ordered correctly based on the conditions specified in the CASE statement.


What is the syntax for using a CASE statement in the ORDER BY clause?

The syntax for using a CASE statement in the ORDER BY clause is as follows:

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SELECT column1, column2
FROM table_name
ORDER BY 
CASE 
    WHEN condition1 THEN column_name1
    WHEN condition2 THEN column_name2
    ELSE column_name3
END;


In this syntax:

  • condition1, condition2, etc., are the conditions to be evaluated in the CASE statement.
  • column_name1, column_name2, etc., are the columns to be used as the criteria for ordering the result set.
  • column1, column2, etc., are the columns to be selected in the query.
  • table_name is the name of the table from which the data is being retrieved.


What is the purpose of using the CASE statement in an ORDER BY clause?

The purpose of using the CASE statement in an ORDER BY clause is to customize the sorting logic for the result set. By using the CASE statement, you can define specific conditions to dictate the order in which the data is displayed. This allows for more control and flexibility in how the data is sorted, as you can specify different sorting criteria based on certain conditions or parameters.


What is the equivalent of the CASE statement in other databases?

In most databases, the equivalent of the CASE statement is the following:

  • In MySQL: IF statement
  • In SQL Server: CASE statement
  • In Oracle: CASE statement
  • In PostgreSQL: CASE statement
  • In SQLite: CASE statement


Each of these implementations allows for conditional logic to be applied to a query, with different syntax and features specific to the database system.


How to use a CASE statement to sort data in descending order in PostgreSQL?

In PostgreSQL, you can use a CASE statement within an ORDER BY clause to sort data in descending order. Here's an example of how to do this:

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SELECT column1, column2
FROM table_name
ORDER BY
  CASE 
    WHEN condition1 THEN column1 
    WHEN condition2 THEN column2 
    ELSE column3 
  END DESC;


In this example, we are ordering the data based on conditions specified in the CASE statement. The data will be sorted in descending order based on the value returned by the CASE statement.


You can replace condition1, condition2, column1, column2, and column3 with your actual conditions and column names. The DESC keyword at the end of the statement indicates that the data should be sorted in descending order.


Remember to adjust the query as per your specific requirements and data.

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