SQl Syntax Help

[kronnos]

Hi,

I wanted to set a certain text for a user profile filed if it is currently blank.
So i want to try something like

UPDATE userfield SET field5 = REPLACE(field5,'IF THIS IS BLANK','REPLACE WITH NEW TEXT');

I dont know what to use in "IF THIS IS BLANK" because if I leave that spot blank, nothing happened. So should I use NULL in that place or completely rewrite the line so it says not replace but set?

thanks!