<转>mysql 树查询语句
原链接：http://skudy.iteye.com/blog/1434254
求下级所有节点

SELECT  
    b.*  
FROM  
    tree AS a,  
    tree AS b  
WHERE  
    a.id= b.pid  
AND(a.id= ? OR a.pi= ?) 

求所有子节点

SELECT  
    *  
FROM  
    tree t  
WHERE  
    NOT EXISTS(  
        SELECT  
            1  
        FROM  
            tree  t1,  
            tree t2  
        WHERE  
            t1.id= t2.pid  
        AND t1.id= t.id  
    ) 

求所有根节点

SELECT  
    *  
FROM  
    tree t  
WHERE  
    NOT EXISTS(  
        SELECT  
            1  
        FROM  
            tree t1,  
            tree t2  
        WHERE  
            t1.id= t2.pid  
        AND t1.id= t.pid  
    )