This came in very handy for me at one point because I needed to have a certain field in each row 14 characters long, but half of them were only 12 characters long.

In MySQL:

UPDATE `table` SET `field` = CONCAT(`field`, ‘1’) WHERE LEN(`field`) = 13;

In MSSQL:

UPDATE `table` SET `field` = CONCAT(`field`, ‘1’) WHERE LENGTH(`field`) = 13;