Commenting System

SELECT tbl_register.register_username AS register_username, tbl_comment.comment_heading AS comment_heading, tbl_comment.comment_content AS comment_content, tbl_comment.comment_mreference AS comment_mreference FROM tbl_commentINNER JOIN tbl_registerON tbl_register.register_username = CASEWHEN tbl_comment.comment_mreference REGEXP '^[0-9]{1,5}$' THEN tbl_comment.comment_mreferenceELSE 0END