Rounding Numbers in PostgreSQL

PostgreSQL provides several ways to round numbers, but understanding the nuances of these methods is crucial for achieving the desired results, particularly when dealing with floating-point numbers. This tutorial will cover the common approaches and best practices for rounding in PostgreSQL.

Understanding Rounding Functions

PostgreSQL offers the round() function for rounding numbers. However, the behavior of round() depends on the data type of the number you’re rounding.

• round(numeric): When applied to a numeric value, round() accepts two arguments: the number to round and the precision (number of decimal places). For example, round(3.14159, 2) will return 3.14.
• round(double precision): When applied to a double precision (or float8) value, round() only accepts one argument: the number to round. In this case, it rounds to the nearest integer. round(3.14159) will return 3.

This difference in behavior can lead to unexpected results when working with floating-point numbers if you intend to round to a specific number of decimal places.

Rounding Floating-Point Numbers to a Specific Precision

To round a double precision number to a specific number of decimal places, you must first cast it to the numeric data type. This allows you to utilize the two-argument form of round().

SELECT round(3.14159::numeric, 2); -- Returns 3.14

The ::numeric is a type cast operator in PostgreSQL. It converts the double precision value to numeric before applying the round() function.

Rounding for Display: Using to_char()

While round() is suitable for numerical calculations, when formatting numbers for display to users, the to_char() function provides more control and flexibility. to_char() converts a number to a text string with a specified format.

SELECT to_char(3.14159, 'FM999999999.00'); -- Returns '3.14'

• FM: This prefix removes leading spaces from the output.
• 999999999: Specifies the maximum number of digits to display before the decimal point. The 9 acts as a placeholder.
• .00: Specifies that you want to display two decimal places.

to_char() also performs rounding as part of the formatting process. It’s particularly useful when you need to control the presentation of numbers without changing their underlying numerical values.

Choosing the Right Method

• Use round(numeric, precision) for rounding numbers in calculations where you need a precise numerical result.
• Use to_char(number, format) for formatting numbers for display purposes, controlling the presentation without affecting the underlying value.