mysql count多个表的写法 多表联合查询的count用法

SELECT (   
SELECT count( * )    
FROM `phpcms_content`    
) AS num, i.contentid   
FROM `phpcms_content` t, `phpcms_c_news` i   
WHERE t.contentid =3   
AND t.contentid = i.contentid   
LIMIT 0 , 30