PostgreSQL ORDER BY

 PostgreSQL ORDER BY

Summary: in this tutorial, you will learn how to sort the result set returned from the SELECTstatement by using the PostgreSQL ORDER BY clause.

Introduction to PostgreSQL ORDER BY clause

When you query data from a table, the SELECT the statement returns rows in an unspecified order. To sort the rows of the result set, you use the ORDER BY clause in the SELECT statement.

The ORDER BY the clause allows you to sort rows returned by a SELECT a clause in ascending or descending order based on a sort expression.

The following illustrates the syntax of the ORDER BY clause:

SELECT
	select_list
FROM
	table_name
ORDER BY
	sort_expression1 [ASC | DESC],
        ...
	sort_expressionN [ASC | DESC];

In this syntax:

• First, specify a sort expression, which can be a column or an expression, that you want to sort after the ORDER BY keywords. If you want to sort the result set based on multiple columns or expressions, you need to place a comma (,) between two columns or expressions to separate them.
• Second, you use the ASC option to sort rows in ascending order and the DESC option to sort rows in descending order. If you omit the ASC or DESC option, the ORDER BY uses ASC by default.

PostgreSQL evaluates the clauses in the SELECT statement in the following order: FROM, SELECT, and ORDER BY:

Due to the order of evaluation, if you have a column alias in the SELECT clause, you can use it in the ORDER BY clause.

Let’s take some examples of using the PostgreSQL ORDER BY clause.

PostgreSQL ORDER BY examples

We will use the customer the table in the sample database for the demonstration.

1) Using PostgreSQL ORDER BY clause to sort rows by one column

The following query uses the ORDER BY clause to sort customers by their first names in ascending order:

SELECT
	first_name,
	last_name
FROM
	customer
ORDER BY
	first_name ASC;

Since the ASC the option is the default, you can omit it in the ORDER BY a clause like this:

SELECT
	first_name,
	last_name
FROM
	customer
ORDER BY
	first_name;

2) Using PostgreSQL ORDER BY clause to sort rows by one column in descending order

The following statement selects the first name and last name from the customer table and sorts the rows by values in the last name column in descending order:

SELECT
       first_name,
       last_name
FROM
       customer
ORDER BY
       last_name DESC;

3) Using PostgreSQL ORDER BY clause to sort rows by multiple columns

The following statement selects the first name and last name from the customer table and sorts the rows by the first name in ascending order and last name in descending order:

SELECT
	first_name,
	last_name
FROM
	customer
ORDER BY
	first_name ASC,
	last_name DESC;

In this example, the ORDER BY clause sorts rows by values in the first name column first. And then it sorts the sorted rows by values in the last name column.

As you can see clearly from the output, two customers with the same first name Kelly has the last name sorted in descending order?

4) Using PostgreSQL ORDER BY clause to sort rows by expressions

The LENGTH() the function accepts a string and returns the length of that string.

The following statement selects the first names and their lengths. It sorts the rows by the lengths of the first names:

SELECT 
	first_name,
	LENGTH(first_name) len
FROM
	customer
ORDER BY 
	len DESC;

Because the ORDER BY a clause is evaluated after the SELECT clause, the column alias len is available and can be used in the ORDER BY clause.

PostgreSQL ORDER BY clause and NULL

In the database world, NULL is a marker that indicates the missing data or the data is unknown at the time of recording.

When you sort rows that contains NULL, you can specify the order of NULL with other non-null values by using the NULLS FIRST or NULLS LAST option of the ORDER BY clause:

ORDER BY sort_expresssion [ASC | DESC] [NULLS FIRST | NULLS LAST]

The NULLS FIRST option places NULL before other non-null values and the NULL LAST option places NULL after other non-null values.

Let’s create a table for the demonstration.

-- create a new table
CREATE TABLE sort_demo(
	num INT
);

-- insert some data
INSERT INTO sort_demo(num)
VALUES(1),(2),(3),(null);

Note that you don’t need to understand the CREATE TABLE and INSERT statements. You just need to execute it from pgAdmin or psql to create the sort_demo table and insert data into it.

The following query returns data from the sort_demo table:

SELECT num
FROM sort_demo
ORDER BY num;

In this example, the ORDER BY clause sorts values in the num column of the sort_demo the table in ascending order. It places NULL after other values.

So if you use the ASC option, the ORDER BY clause uses the NULLS LAST option by default. Therefore, the following query returns the same result:

SELECT num
FROM sort_demo
ORDER BY num NULLS LAST;

To place NULL before other non-null values, you use the NULLS FIRST option:

SELECT num
FROM sort_demo
ORDER BY num NULLS FIRST;

The following statement sorts values in the num column of the sort_demo the table in descending order:

SELECT num
FROM sort_demo
ORDER BY num DESC;

As you can see clearly from the output, the ORDER BY clause with the DESC option uses the NULLS FIRST by default.

To reverse the order, you can use the NULLS LAST option:

SELECT num
FROM sort_demo
ORDER BY num DESC NULLS LAST;

Summary

• Use the ORDER BY clause in the SELECT statement to sort rows.
• Use the ASC option to sort rows in ascending order and DESC option to sort rows in descending order. The ORDER BY clause uses the ASC option by default.
• Use NULLS FIRST and NULLS LAST options to explicitly specify the order of NULL with other non-null values.