根据父节点编号查询子节点名称，孙节点个数

新建一个部门表

DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '标识列',
  `name` varchar(255) NOT NULL COMMENT '名称',
  `pid` int(11) NOT NULL COMMENT '父级编号',
  `num` int(11) NOT NULL DEFAULT '100' COMMENT '排序号',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=19 DEFAULT CHARSET=utf8 COMMENT='部门表';

-- ----------------------------
-- Records of dept
-- ----------------------------
INSERT INTO `dept` VALUES ('1', '总部', '0', '100');
INSERT INTO `dept` VALUES ('2', '市场', '1', '100');
INSERT INTO `dept` VALUES ('3', '研发', '1', '100');
INSERT INTO `dept` VALUES ('4', '财务', '1', '100');
INSERT INTO `dept` VALUES ('5', '设计', '3', '100');
INSERT INTO `dept` VALUES ('6', '运维', '3', '100');
INSERT INTO `dept` VALUES ('7', '测试', '3', '100');
INSERT INTO `dept` VALUES ('8', '开发', '3', '100');
INSERT INTO `dept` VALUES ('9', '人事', '1', '100');
INSERT INTO `dept` VALUES ('10', '行政', '1', '100');
INSERT INTO `dept` VALUES ('11', '营销', '2', '100');
INSERT INTO `dept` VALUES ('12', '策划', '2', '100');
INSERT INTO `dept` VALUES ('13', '售后', '2', '100');
INSERT INTO `dept` VALUES ('14', 'Java', '8', '100');
INSERT INTO `dept` VALUES ('15', '大数据', '8', '100');
INSERT INTO `dept` VALUES ('16', '.Net', '8', '100');
INSERT INTO `dept` VALUES ('17', '美工', '5', '100');
INSERT INTO `dept` VALUES ('18', '前端', '5', '100');

顶级的pid的值为0

需求是根据父节点编号查询子节点名称及其孙节点个数：要求一条sql语句完成

-- 根据父节点编号,查询子节点名称，及其孙节点个数
        SELECT
            id,
            dept1.pid,
            name,
            ifnull(children,0) children
FROM ( SELECT
            id,
            pId,
            name
from dept
where pid = 3 ) dept1
LEFT JOIN (SELECT pid,count(*) children FROM dept 
where pid in (SELECT id FROM dept where pid = 3)
GROUP BY pid) dept2
on dept1.id = dept2.pid