There are several ways to format a number as a percentage in SQL, depending on the DBMS being used.

Here are examples of adding a percent sign to a number in some of the most popular DBMSs.

MySQL

In MySQL, we can use the CONCAT() function to concatenate the number and the percent sign:

SELECT CONCAT(3.75, '%');

Result:

3.75%

See Format a Number as a Percentage in MySQL for more.

SQL Server

SQL Server’s FORMAT() function provides us with the P format specifier, which is used to display a percent sign in the formatted number:

SELECT FORMAT(.27, 'P') Result;

Result:

+----------+
| Result   |
|----------|
| 27.00%   |
+----------+

See 4 Ways to Convert a Number to a Percentage in SQL Server for more.

Oracle

Oracle Database has a TO_CHAR(number) function that we can use to format a number with. We can then use CONCAT() to concatenate the number and the percent sign:

SELECT CONCAT(TO_CHAR(18, 'fm00D00'), '%')
FROM DUAL;

Result:

18.00%

See Format a Number as a Percentage in Oracle for more.

PostgreSQL

In PostgreSQL, we can use the TO_CHAR() function to format a number as a percentage.

Here are various examples to demonstrate some of the things we can do with this function:

SELECT 
    TO_CHAR(7, 'fm00%') AS "1",
    TO_CHAR(7, 'fm99%') AS "2",
    TO_CHAR(7.4567, 'fm0D00%') AS "3",
    TO_CHAR(7, 'fm0D00%') AS "4";

Result:

+-----+----+-------+-------+
|  1  | 2  |   3   |   4   |
+-----+----+-------+-------+
| 07% | 7% | 7.46% | 7.00% |
+-----+----+-------+-------+

See 3 Ways to Format a Number as a Percentage in PostgreSQL for more.

MariaDB

MariaDB has a CONCAT() function to do the job:

SELECT CONCAT(7.45, '%');

Result:

7.45%

See Add a Percent Sign to a Number in MariaDB for more.

SQLite

With SQLite, we can use the PRINTF() function:

SELECT PRINTF('%2d%%', 17);

Result:

17%

We can alternatively concatenate the percent sign and the number if required. See 2 Ways to Add a Percent Sign to a Number in SQLite for an example.

Update: SQLite 3.38.0 (released 22 Feb 2022) renamed the PRINTF() function to FORMAT(). The original PRINTF() name is retained as an alias for backwards compatibility.

So the above example can be changed to this:

SELECT FORMAT('%2d%%', 17);

Result:

17%