Témata, vzkazy - mazání

CREATE TABLE temata (
    tid INTEGER UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    tema VARCHAR(100) NOT NULL
)

CREATE TABLE vzkaz (
    vid INTEGER UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    datum VARCHAR(40) NOT NULL,
    vzkaz VARCHAR(1000) NOT NULL,
    uzivatel VARCHAR(20) NOT NULL,
    id_tematu INTEGER NOT NULL,
    FOREIGN KEY (id_tematu) REFERENCES temata (tid) ON DELETE CASCADE
)

$sql = "DELETE FROM temata WHERE tid ='$id'";