sql语句的逻辑问题。。。

语句出错了，如下：
$_sql = "SELECT COUNT(*) FROM cms_content c,cms_nav n WHERE c.nav=n.id AND c.nav IN (SELECT id FROM cms_nav WHERE pid='$this->nav') OR c.nav='$this->nav'";

模型就是
SELECT * FROM tablename WHERE 条件1 AND 条件2 OR 条件3；

现在逻辑关系应该是(1 and 2) or 3;

我想要 1 and (2 or 3),请问怎样才能实现呢？ （我用括号或花括号框上2、3都不行。。）

SELECT * FROM tablename WHERE 条件1 AND 条件2 OR 条件1 AND 条件3；
如果条件1 是必须的话 就是这样。

试试 SELECT * FROM tablename WHERE (条件1 AND 条件2) OR ((条件1 AND 条件3)；

SELECT * FROM tablename WHERE (条件1 AND 条件2) OR (条件1 AND 条件3)

SELECT * FROM tablename WHERE (条件1 AND 条件2) OR (条件1 AND 条件3)

$_sql = "SELECT COUNT(*) FROM cms_content c,cms_nav n WHERE c.nav=n.id AND （c.nav IN (SELECT id FROM cms_nav WHERE pid='$this->nav') OR c.nav='$this->nav'）";

SELECT COUNT(* ) FROM cms_content c,cms_nav n
WHERE (c.nav=n.id AND c.nav IN (SELECT id FROM cms_nav WHERE pid='$this->nav') )
OR (c.nav=n.id AND c.nav='$this->nav')

我觉得3楼说的挺有道理，顶一下