MySQL in a Nutshell, 2nd Edition

Name

UNHEX()

Synopsis

UNHEX(string)

This function converts hexadecimal numbers to their character equivalents. It reverses the results of the HEX() function and is available as of version 4.1.2 of MySQL.

To illustrate its use, suppose that in a table we have a column with a binary character in the data; specifically, tabs were entered through a web interface using an API. However, the column is a VARCHAR data type. The problem is that when the data is retrieved, we want to line up all the results in our display by counting the length of each column, and a tab keeps the display from lining up vertically. So we want to fix the data. We can use UNHEX() to locate rows containing the binary character and then replace it with spaces instead:

UPDATE students
SET comments = REPLACE(comments, UNHEX(09), SPACE(4))
WHERE LOCATE(UNHEX(09), comments);

We’ve looked at an ASCII chart and seen that a tab is represented by the hexadecimal number 09. Knowing that bit of information, in the WHERE clause we’re passing that value to UNHEX() to return the binary character for a tab, yielding the search string with which LOCATE() will search the column comments. If it doesn’t find a tab in the column for a row, it will return 0. Those rows will not be included in the search results. The ones that do contain tabs will have a value of 1 or greater and therefore will be included in the results. Using UNHEX() along with REPLACE() in the SET clause, we replace all tabs found with four spaces.