SQL Error (1215): Cannot add foreign key constraint

Question

CREATE TABLE `profilePic` (
    `ClientID` VARCHAR(255) NOT NULL,
    PRIMARY KEY (`ClientID`),
    CONSTRAINT `FK__user_details` FOREIGN KEY (`ClientID`) REFERENCES `us         


        

Answer 1

When trying to create a foreign key via HeidiSQL, you get a warning as soon as the selected column data types don't match. I added this warning to HeidiSQL's table designer due to the non-intuitive message from the server ("Cannot add foreign key constraint")

The selected foreign column do not match the source columns data type and unsigned flag. This will give you an error message when trying to save this change. Please compare yourself: