CREATE TABLE captchas (
    capid       INT UNSIGNED NOT NULL AUTO_INCREMENT,
    TYPE        enum('image','audio'),
    issuetime   INT UNSIGNED NOT NULL DEFAULT 0,
    answer      CHAR(10),
    userid      INT UNSIGNED NOT NULL DEFAULT 0,
    anum        SMALLINT UNSIGNED NOT NULL,
    INDEX(TYPE,issuetime),
    INDEX(userid),
    PRIMARY KEY(capid)
)