浅谈MySQL中的group by

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user`  (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '用户名',
  `nation` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '民族',
  `age` int(11) NULL DEFAULT NULL COMMENT '年龄',
  `height` double NULL DEFAULT NULL COMMENT '身高',
  `sex` smallint(6) NULL DEFAULT NULL COMMENT '性别',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES (1, '李子捌', '汉族', 18, 180, 1);
INSERT INTO `user` VALUES (2, '张三', '回族', 20, 175, 1);
INSERT INTO `user` VALUES (3, '李四', '维吾尔族', 45, 168, 0);
INSERT INTO `user` VALUES (4, '王五', '蒙古族', 18, 177, 1);
INSERT INTO `user` VALUES (5, '赵六', '汉族', 16, 184, 0);
INSERT INTO `user` VALUES (6, '田七', '维吾尔族', 27, 192, 1);

mysql> select * from user;
+----+--------+----------+------+--------+------+
| id | name   | nation   | age  | height | sex  |
+----+--------+----------+------+--------+------+
|  1 | 李子捌 | 汉族     |   18 |    180 |    1 |
|  2 | 张三   | 回族     |   20 |    175 |    1 |
|  3 | 李四   | 维吾尔族 |   45 |    168 |    0 |
|  4 | 王五   | 蒙古族   |   18 |    177 |    1 |
|  5 | 赵六   | 汉族     |   16 |    184 |    0 |
|  6 | 田七   | 维吾尔族 |   27 |    192 |    1 |
+----+--------+----------+------+--------+------+
6 rows in set (0.00 sec)

mysql> select nation, count(*) from user group by nation;
+----------+----------+
| nation   | count(*) |
+----------+----------+
| 汉族     |        2 |
| 回族     |        1 |
| 维吾尔族 |        2 |
| 蒙古族   |        1 |
+----------+----------+
4 rows in set (0.00 sec)

mysql> select nation, count(*) as nation_num  from user where sex = 0 group by nation;
+----------+------------+
| nation   | nation_num |
+----------+------------+
| 维吾尔族 |          1 |
| 汉族     |          1 |
+----------+------------+
2 rows in set (0.00 sec)

mysql> select nation, count(*) as nation_num  from user group by nation where nation = '汉族';
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where nation = '汉族'' at line 1

vmysql> select nation, count(*) as nation_num  from user group by nation having nation = '汉族';
+--------+------------+
| nation | nation_num |
+--------+------------+
| 汉族   |          2 |
+--------+------------+
1 row in set (0.00 sec)

mysql> select nation, count(*) as nation_num  from user group by nation having nation != '汉族' order by nation_num desc;
+----------+------------+
| nation   | nation_num |
+----------+------------+
| 维吾尔族 |          2 |
| 回族     |          1 |
| 蒙古族   |          1 |
+----------+------------+
3 rows in set (0.00 sec)

mysql> select nation, count(*) as nation_num  from user group by nation having nation != '汉族' order by nation_num desc limit 2;
+----------+------------+
| nation   | nation_num |
+----------+------------+
| 维吾尔族 |          2 |
| 回族     |          1 |
+----------+------------+
2 rows in set (0.00 sec)

mysql> select nation, max(height) as nation_num  from user group by nation with rollup;
+----------+------------+
| nation   | nation_num |
+----------+------------+
| 回族     |        175 |
| 汉族     |        184 |
| 维吾尔族 |        192 |
| 蒙古族   |        177 |
| NULL     |        192 |
+----------+------------+
5 rows in set (0.00 sec)

mysql> select nation, avg(height) as nation_num  from user group by nation with rollup;
+----------+--------------------+
| nation   | nation_num         |
+----------+--------------------+
| 回族     |                175 |
| 汉族     |                182 |
| 维吾尔族 |                180 |
| 蒙古族   |                177 |
| NULL     | 179.33333333333334 |
+----------+--------------------+
5 rows in set (0.00 sec)

mysql> select nation, count(*) as nation_num  from user group by nation with rollup;
+----------+------------+
| nation   | nation_num |
+----------+------------+
| 回族     |          1 |
| 汉族     |          2 |
| 维吾尔族 |          2 |
| 蒙古族   |          1 |
| NULL     |          6 |
+----------+------------+
5 rows in set (0.00 sec)