In PostgreSQL, the trunc() function truncates a number.

More specifically, it truncates the number either toward zero, or to a specific number of decimal places.

Syntax

The trunc() function can be used in either of the following ways:

trunc(dp or numeric)
trunc(v numeric, s int)

The first syntax truncates the number toward zero.

The second syntax truncates it to a specified number of decimal places. In this case, v is the number, and s is the number of decimal places.

Example of First Syntax

Here’s an example that uses the first syntax.

SELECT trunc(342.49);

Result:

342

Example of Second Syntax

Here’s an example that uses the second syntax.

SELECT trunc(342.49, 1);

Result:

342.4

Adding Fractional Seconds

Here’s an example of specifying a number of decimal places when the number doesn’t include any.

SELECT trunc(342, 3);

Result:

342.000

Negative Numbers

Here’s an example that uses a negative number.

SELECT round(-4.5);

Result:

-5

Negative Decimal Places

Here’s what happens when the second argument is a negative value.

SELECT trunc(342, -2);

Result:

300

Trunc() vs Round()

In some cases, the trunc() function might appear to work in a similar way to round(). But they are quite different functions.

The round() function rounds the number. The number is rounded to the nearest integer or to the specified number of decimal places (depending on whether you give it one argument or two arguments).

The trunc() function on the other hand simply truncates the number toward zero or to the specified decimal place. No rounding is performed.

Here’s an example to demonstrate the difference.

SELECT 
  round(342.49, 1),
  trunc(342.49, 1);

Result:

round | trunc
-------+-------
342.5 | 342.4