mysql> set names gbk;
Query OK, 0 rows affected (0.03 sec)
mysql> use test
Database changed
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| account |
| class |
| goods |
| m1 |
| member |
| mian |
| salary |
| stu |
| test |
| test2 |
| test3 |
| test4 |
| test5 |
| test6 |
| test7 |
+----------------+
15 rows in set (0.69 sec)
mysql> #查询出最贵的商品的价格
mysql> #即shop_price最大的值查询出来
mysql> select max(shop_price) from goods;
+-----------------+
| max(shop_price) |
+-----------------+
| 5999.00 |
+-----------------+
1 row in set (0.00 sec)
mysql> # 查询出最便宜的商品的价格
mysql> select min(shop_price) from goods;
+-----------------+
| min(shop_price) |
+-----------------+
| 18.00 |
+-----------------+
1 row in set (0.00 sec)
mysql> # 查询出发布最早的商品,即goods_id最小的值
mysql> select min(goods_id) from goods;
+---------------+
| min(goods_id) |
+---------------+
| 1 |
+---------------+
1 row in set (0.00 sec)
mysql> #goods_number是指库存量.
mysql> #店主想统计一下,本店一共库存多少件商品
mysql> #应该把每一行上的goods_number字段 加起来
mysql> select sum(goods_number) from goods;
+-------------------+
| sum(goods_number) |
+-------------------+
| 314 |
+-------------------+
1 row in set (0.02 sec)
mysql> #店主想查看自己店里所有商品的平均价格
mysql> #avg求平均
mysql> select avg(shop_price) from goods;
+-----------------+
| avg(shop_price) |
+-----------------+
| 1232.541563 |
+-----------------+
1 row in set (0.02 sec)
mysql> #店主想统计一下,本店一共有多种商品(不是多少个)
mysql> #用count()函数来数.
mysql> select count(*) from goods;
+----------+
| count(*) |
+----------+
| 32 |
+----------+
1 row in set (0.00 sec)
mysql> select count(1) from goods;
+----------+
| count(1) |
+----------+
| 32 |
+----------+
1 row in set (0.00 sec)
mysql> create table test8 (
-> id int,
-> name varchar(20)
-> )engine myisam charset utf8;
Query OK, 0 rows affected (0.08 sec)
mysql> insert into test8
-> values
-> (1,'lisi');
Query OK, 1 row affected (0.05 sec)
mysql> insert into test8
-> values
-> (2,null);
Query OK, 1 row affected (0.00 sec)
mysql> select * from test8;
+------+------+
| id | name |
+------+------+
| 1 | lisi |
| 2 | NULL |
+------+------+
2 rows in set (0.00 sec)
mysql> select count(*) from test8;
+----------+
| count(*) |
+----------+
| 2 |
+----------+
1 row in set (0.00 sec)
mysql> select count(name) from test8;
+-------------+
| count(name) |
+-------------+
| 1 |
+-------------+
1 row in set (0.00 sec)
mysql> insert into test8 values (null,null);
Query OK, 1 row affected (0.00 sec)
mysql> select * from test8;
+------+------+
| id | name |
+------+------+
| 1 | lisi |
| 2 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)
mysql> select count(*) from test8;
+----------+
| count(*) |
+----------+
| 3 |
+----------+
1 row in set (0.00 sec)
mysql> select count(id) from test8;
+-----------+
| count(id) |
+-----------+
| 2 |
+-----------+
1 row in set (0.00 sec)
mysql> select count(name) from test8;
+-------------+
| count(name) |
+-------------+
| 1 |
+-------------+
1 row in set (0.00 sec)
mysql> select count(1) from test8;
+----------+
| count(1) |
+----------+
| 3 |
+----------+
1 row in set (0.00 sec)
mysql> #上面是5个统计函数,单独使用,意义不大,要和分组配合起来使用.
mysql> #帮我计算一下,第3个栏目下所有商品的库存量之和.sum(goods_num)
mysql> select sum(goods_number) from goods
-> where cat_id=3;
+-------------------+
| sum(goods_number) |
+-------------------+
| 203 |
+-------------------+
1 row in set (0.00 sec)
mysql> #帮我计算第4个栏目下所有商品的库存量之和 sum(goods_number)
mysql> select sum(goods_number) from goods
-> where cat_id=4;
+-------------------+
| sum(goods_number) |
+-------------------+
| 4 |
+-------------------+
1 row in set (0.00 sec)
mysql> #帮我计算一次计算完,每个栏目下的库存量之和
mysql> #gourp分组查询
mysql> select sum(goods_number) from goods
-> group by cat_id;
+-------------------+
| sum(goods_number) |
+-------------------+
| 0 |
| 203 |
| 4 |
| 8 |
| 61 |
| 23 |
| 4 |
| 9 |
| 2 |
+-------------------+
9 rows in set (0.00 sec)
mysql> select cat_id,sum(goods_number) from goods
-> group by cat_id;
+--------+-------------------+
| cat_id | sum(goods_number) |
+--------+-------------------+
| 2 | 0 |
| 3 | 203 |
| 4 | 4 |
| 5 | 8 |
| 8 | 61 |
| 11 | 23 |
| 13 | 4 |
| 14 | 9 |
| 15 | 2 |
+--------+-------------------+
9 rows in set (0.00 sec)
mysql> #分了组再统计,这样意义更大.
mysql> #看下面的语句对不对?
mysql> select goods_id,sum(goods_number) from goods;
+----------+-------------------+
| goods_id | sum(goods_number) |
+----------+-------------------+
| 1 | 314 |
+----------+-------------------+
1 row in set (0.00 sec)
mysql> #上个语句执行了,为什么把goods_id取出来
mysql> #答:对于sql标准来说,这个语句是错误的,不能执行的.
mysql> #但是 在mysql中可以这么干
mysql> #而且,goods_id是把第一次出现的值取出来
mysql> #按cat_id分组,计算每个栏目下的商品的平均价格
mysql> select cat_id,avg(shop_price) from goods
-> gruop by cat_id;
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 'by cat_id' at line 2
mysql> select cat_id,avg(shop_price) from goods
-> group by cat_id;
+--------+-----------------+
| cat_id | avg(shop_price) |
+--------+-----------------+
| 2 | 823.330000 |
| 3 | 1746.066667 |
| 4 | 2031.000000 |
| 5 | 3700.000000 |
| 8 | 75.333333 |
| 11 | 31.000000 |
| 13 | 33.500000 |
| 14 | 54.000000 |
| 15 | 70.000000 |
+--------+-----------------+
9 rows in set (0.02 sec)
mysql> #回顾之前做过的一道题
mysql> #查询本店价比市场价省的钱,并且要求省钱200元以上的取出来
mysql> select goods_id,market_price,shop_price,(market_price-shop_price) as discount
-> from goods
-> where discount >200;
ERROR 1054 (42S22): Unknown column 'discount' in 'where clause'
mysql> select goods_id,market_price,shop_price,(market_price-shop_price) as discount
-> from goods
-> where 1
-> having discount > 200;
+----------+--------------+------------+----------+
| goods_id | market_price | shop_price | discount |
+----------+--------------+------------+----------+
| 1 | 1665.60 | 1388.00 | 277.60 |
| 9 | 2757.60 | 2298.00 | 459.60 |
| 10 | 1593.60 | 1328.00 | 265.60 |
| 13 | 1573.20 | 1311.00 | 262.20 |
| 14 | 3150.00 | 2625.00 | 525.00 |
| 17 | 2760.00 | 2300.00 | 460.00 |
| 18 | 3453.60 | 2878.00 | 575.60 |
| 21 | 2400.00 | 2000.00 | 400.00 |
| 22 | 7198.80 | 5999.00 | 1199.80 |
| 23 | 4440.00 | 3700.00 | 740.00 |
| 24 | 2400.00 | 2000.00 | 400.00 |
| 31 | 1604.39 | 1337.00 | 267.39 |
| 32 | 3612.00 | 3010.00 | 602.00 |
| 33 | 1479.60 | 1233.00 | 246.60 |
+----------+--------------+------------+----------+
14 rows in set (0.00 sec)
mysql> #查询出每种商品所积压的货款
mysql> #提示:商品*价格
mysql> select goods_id,shop_price,goods_number,shop_price*goods_number as hk
-> from goods;
+----------+------------+--------------+-----------+
| goods_id | shop_price | goods_number | hk |
+----------+------------+--------------+-----------+
| 1 | 1388.00 | 1 | 1388.00 |
| 4 | 58.00 | 17 | 986.00 |
| 3 | 68.00 | 24 | 1632.00 |
| 5 | 20.00 | 8 | 160.00 |
| 6 | 42.00 | 15 | 630.00 |
| 7 | 100.00 | 20 | 2000.00 |
| 8 | 399.00 | 1 | 399.00 |
| 9 | 2298.00 | 4 | 9192.00 |
| 10 | 1328.00 | 7 | 9296.00 |
| 11 | 1300.00 | 1 | 1300.00 |
| 12 | 983.00 | 8 | 7864.00 |
| 13 | 1311.00 | 8 | 10488.00 |
| 14 | 2625.00 | 1 | 2625.00 |
| 15 | 788.00 | 3 | 2364.00 |
| 16 | 823.33 | 0 | 0.00 |
| 17 | 2300.00 | 1 | 2300.00 |
| 18 | 2878.00 | 1 | 2878.00 |
| 19 | 858.00 | 12 | 10296.00 |
| 20 | 280.00 | 12 | 3360.00 |
| 21 | 2000.00 | 40 | 80000.00 |
| 22 | 5999.00 | 1 | 5999.00 |
| 23 | 3700.00 | 8 | 29600.00 |
| 24 | 2000.00 | 100 | 200000.00 |
| 25 | 48.00 | 2 | 96.00 |
| 26 | 19.00 | 2 | 38.00 |
| 27 | 95.00 | 2 | 190.00 |
| 28 | 45.00 | 0 | 0.00 |
| 29 | 90.00 | 0 | 0.00 |
| 30 | 18.00 | 9 | 162.00 |
| 31 | 1337.00 | 1 | 1337.00 |
| 32 | 3010.00 | 4 | 12040.00 |
| 33 | 1233.00 | 1 | 1233.00 |
+----------+------------+--------------+-----------+
32 rows in set (0.00 sec)
mysql> #查询该店积压的总货款
mysql> #每个商品积压货款之和
mysql> #sum哪个列呀?
mysql> select sum(goods_number*shop_price) from goods
-> ;
+------------------------------+
| sum(goods_number*shop_price) |
+------------------------------+
| 399853.00 |
+------------------------------+
1 row in set (0.00 sec)
mysql> #再深入,查询每个栏目下,积压的货款
mysql> #只需要再分组即可
mysql> select sum(goods_number*shop_price) from goods
-> \c
mysql> select cat_id,sum(goods_number*shop_price) from goods
-> group by cat_id;
+--------+------------------------------+
| cat_id | sum(goods_number*shop_price) |
+--------+------------------------------+
| 2 | 0.00 |
| 3 | 356235.00 |
| 4 | 8124.00 |
| 5 | 29600.00 |
| 8 | 4618.00 |
| 11 | 790.00 |
| 13 | 134.00 |
| 14 | 162.00 |
| 15 | 190.00 |
+--------+------------------------------+
9 rows in set (0.00 sec)
mysql> #查询出栏目的积压货款金额,且筛选出积压金额>20000的栏目
mysql> #用到了group与having
mysql> select cat_id,sum(goods_number*shop_price) from goods
-> \c
mysql> select cat_id,sum(goods_number*shop_price) as zhk from goods
-> group by cat_id
-> having zhk>20000;
+--------+-----------+
| cat_id | zhk |
+--------+-----------+
| 3 | 356235.00 |
| 5 | 29600.00 |
+--------+-----------+
2 rows in set (0.00 sec)
mysql> #查询本店价比市场价省的钱,且筛选出省钱200以上的商品.
mysql> #用where和having分别来实现
mysql> #先用where
mysql> select goods_id,market_price,shop_price,market_price-shop_price as discount
-> from goods
-> where (market_price-shop_price)>200;
+----------+--------------+------------+----------+
| goods_id | market_price | shop_price | discount |
+----------+--------------+------------+----------+
| 1 | 1665.60 | 1388.00 | 277.60 |
| 9 | 2757.60 | 2298.00 | 459.60 |
| 10 | 1593.60 | 1328.00 | 265.60 |
| 13 | 1573.20 | 1311.00 | 262.20 |
| 14 | 3150.00 | 2625.00 | 525.00 |
| 17 | 2760.00 | 2300.00 | 460.00 |
| 18 | 3453.60 | 2878.00 | 575.60 |
| 21 | 2400.00 | 2000.00 | 400.00 |
| 22 | 7198.80 | 5999.00 | 1199.80 |
| 23 | 4440.00 | 3700.00 | 740.00 |
| 24 | 2400.00 | 2000.00 | 400.00 |
| 31 | 1604.39 | 1337.00 | 267.39 |
| 32 | 3612.00 | 3010.00 | 602.00 |
| 33 | 1479.60 | 1233.00 | 246.60 |
+----------+--------------+------------+----------+
14 rows in set (0.00 sec)
mysql> select goods_id,market_price,shop_price,market_price-shop_price as discount
-> from goods
-> having discount > 200;
+----------+--------------+------------+----------+
| goods_id | market_price | shop_price | discount |
+----------+--------------+------------+----------+
| 1 | 1665.60 | 1388.00 | 277.60 |
| 9 | 2757.60 | 2298.00 | 459.60 |
| 10 | 1593.60 | 1328.00 | 265.60 |
| 13 | 1573.20 | 1311.00 | 262.20 |
| 14 | 3150.00 | 2625.00 | 525.00 |
| 17 | 2760.00 | 2300.00 | 460.00 |
| 18 | 3453.60 | 2878.00 | 575.60 |
| 21 | 2400.00 | 2000.00 | 400.00 |
| 22 | 7198.80 | 5999.00 | 1199.80 |
| 23 | 4440.00 | 3700.00 | 740.00 |
| 24 | 2400.00 | 2000.00 | 400.00 |
| 31 | 1604.39 | 1337.00 | 267.39 |
| 32 | 3612.00 | 3010.00 | 602.00 |
| 33 | 1479.60 | 1233.00 | 246.60 |
+----------+--------------+------------+----------+
14 rows in set (0.00 sec)
mysql> create table result (
-> name varchar(20),
-> subject varchar(20),
-> score tinyint
-> )enigne myisam charset utf8;
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 'enigne myisam charset utf8' at line 5
mysql> create table result (
-> name varchar(20),
-> subject varchar(20),
-> score tinyint
-> )engine myisam charset utf8;
Query OK, 0 rows affected (0.11 sec)
mysql> desc result;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| subject | varchar(20) | YES | | NULL | |
| score | tinyint(4) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
3 rows in set (0.06 sec)
mysql> insert into result
-> values
-> ('张三','数学',90),
-> ('张三','语文',50),
-> ('张三','地理',40),
-> ('李四','语文',55),
-> ('李四','政治',45),
-> ('王五','政治',30);
Query OK, 6 rows affected (0.03 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from result;
+------+---------+-------+
| name | subject | score |
+------+---------+-------+
| 张三 | 数学 | 90 |
| 张三 | 语文 | 50 |
| 张三 | 地理 | 40 |
| 李四 | 语文 | 55 |
| 李四 | 政治 | 45 |
| 王五 | 政治 | 30 |
+------+---------+-------+
6 rows in set (0.01 sec)
mysql> show create table result;
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| result | CREATE TABLE `result` (
`name` varchar(20) DEFAULT NULL,
`subject` varchar(20) DEFAULT NULL,
`score` tinyint(4) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8 |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> #第一种错误
mysql> select name,avg(scocre) from result
-> where score<60 group by name having count(*)>=2;
ERROR 1054 (42S22): Unknown column 'scocre' in 'field list'
mysql> select name,avg(score) from result
-> where score<60 group by name having count(*)>=2;
+------+------------+
| name | avg(score) |
+------+------------+
| 张三 | 45.0000 |
| 李四 | 50.0000 |
+------+------------+
2 rows in set (0.00 sec)
mysql> select name,avg(score) from result
-> where score<60 group by name;
+------+------------+
| name | avg(score) |
+------+------------+
| 张三 | 45.0000 |
| 李四 | 50.0000 |
| 王五 | 30.0000 |
+------+------------+
3 rows in set (0.00 sec)
mysql> select * from result;
+------+---------+-------+
| name | subject | score |
+------+---------+-------+
| 张三 | 数学 | 90 |
| 张三 | 语文 | 50 |
| 张三 | 地理 | 40 |
| 李四 | 语文 | 55 |
| 李四 | 政治 | 45 |
| 王五 | 政治 | 30 |
+------+---------+-------+
6 rows in set (0.00 sec)
mysql> select name,avg(score) from result
-> group by name having count(score<60) >= 2;
+------+------------+
| name | avg(score) |
+------+------------+
| 张三 | 60.0000 |
| 李四 | 50.0000 |
+------+------------+
2 rows in set (0.00 sec)
mysql> #第二种错误
mysql> select name,avg(score),count(score<60) as gks
-> from result
-> group by name having gks>=2;
+------+------------+-----+
| name | avg(score) | gks |
+------+------------+-----+
| 张三 | 60.0000 | 3 |
| 李四 | 50.0000 | 2 |
+------+------------+-----+
2 rows in set (0.00 sec)
mysql> #暴露他的错误
mysql> insert into result
-> values
-> ('赵六','语文',100),
-> ('赵六','数学',99),
-> ('赵六','品德',98);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from result;
+------+---------+-------+
| name | subject | score |
+------+---------+-------+
| 张三 | 数学 | 90 |
| 张三 | 语文 | 50 |
| 张三 | 地理 | 40 |
| 李四 | 语文 | 55 |
| 李四 | 政治 | 45 |
| 王五 | 政治 | 30 |
| 赵六 | 语文 | 100 |
| 赵六 | 数学 | 99 |
| 赵六 | 品德 | 98 |
+------+---------+-------+
9 rows in set (0.00 sec)
mysql> select name,avg(score),count(score<60) as gks
-> from result
-> group by name having gks>=2;
+------+------------+-----+
| name | avg(score) | gks |
+------+------------+-----+
| 张三 | 60.0000 | 3 |
| 李四 | 50.0000 | 2 |
| 赵六 | 99.0000 | 3 |
+------+------------+-----+
3 rows in set (0.00 sec)
mysql> select count(1) from result;
+----------+
| count(1) |
+----------+
| 9 |
+----------+
1 row in set (0.03 sec)
mysql> select count(0) from result;
+----------+
| count(0) |
+----------+
| 9 |
+----------+
1 row in set (0.00 sec)
mysql> #正解
mysql> #第1步:我们就查询所有的平均分
mysql> select name,avg(score) from result group by name;
+------+------------+
| name | avg(score) |
+------+------------+
| 张三 | 60.0000 |
| 李四 | 50.0000 |
| 王五 | 30.0000 |
| 赵六 | 99.0000 |
+------+------------+
4 rows in set (0.03 sec)
mysql> #下一步,再想办法计算出每个人挂科的情况
mysql> select name,subject,score,score<60 as g from result;
+------+---------+-------+------+
| name | subject | score | g |
+------+---------+-------+------+
| 张三 | 数学 | 90 | 0 |
| 张三 | 语文 | 50 | 1 |
| 张三 | 地理 | 40 | 1 |
| 李四 | 语文 | 55 | 1 |
| 李四 | 政治 | 45 | 1 |
| 王五 | 政治 | 30 | 1 |
| 赵六 | 语文 | 100 | 0 |
| 赵六 | 数学 | 99 | 0 |
| 赵六 | 品德 | 98 | 0 |
+------+---------+-------+------+
9 rows in set (0.00 sec)
mysql> #如上,挂科数目就是g的sum结果
mysql> #综合上面两个语句
mysql> select name,avg(score),sum(score<60)) as gks
-> from result
-> group by name;
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 ') as gks
from result
group by name' at line 1
mysql> select name,avg(score),sum(score<60) as gks
-> from result
-> group by name;
+------+------------+------+
| name | avg(score) | gks |
+------+------------+------+
| 张三 | 60.0000 | 2 |
| 李四 | 50.0000 | 2 |
| 王五 | 30.0000 | 1 |
| 赵六 | 99.0000 | 0 |
+------+------------+------+
4 rows in set (0.00 sec)
mysql> #每个人的平均分及挂科数已算出,再having筛选一下即可.
mysql> select name,avg(score),sum(score<60) as gks
-> from result
-> group by name having gks>=2;
+------+------------+------+
| name | avg(score) | gks |
+------+------------+------+
| 张三 | 60.0000 | 2 |
| 李四 | 50.0000 | 2 |
+------+------------+------+
2 rows in set (0.00 sec)
mysql> #这一个思路是逆向思维,先查出所有人的平均,再筛选.
mysql> #如果正常的考虑,我们可能会这样做:
mysql> #先找出谁的挂科数>=2,找到这些人,再求这些人的平均分
mysql> #先找挂科数>=2的那些人
mysql> select name from result where score<60;
+------+
| name |
+------+
| 张三 |
| 张三 |
| 李四 |
| 李四 |
| 王五 |
+------+
5 rows in set (0.01 sec)
mysql> select name,score from result where score<60;
+------+-------+
| name | score |
+------+-------+
| 张三 | 50 |
| 张三 | 40 |
| 李四 | 55 |
| 李四 | 45 |
| 王五 | 30 |
+------+-------+
5 rows in set (0.00 sec)
mysql> select name,count(1),score from result where score<60;
+------+----------+-------+
| name | count(1) | score |
+------+----------+-------+
| 张三 | 5 | 50 |
+------+----------+-------+
1 row in set (0.00 sec)
mysql> select name,count(1),score from result where score<60
-> group by name;
+------+----------+-------+
| name | count(1) | score |
+------+----------+-------+
| 张三 | 2 | 50 |
| 李四 | 2 | 55 |
| 王五 | 1 | 30 |
+------+----------+-------+
3 rows in set (0.00 sec)
mysql> select name,count(1) from result where score<60
-> group by name;
+------+----------+
| name | count(1) |
+------+----------+
| 张三 | 2 |
| 李四 | 2 |
| 王五 | 1 |
+------+----------+
3 rows in set (0.00 sec)
mysql> select name from result
-> where score<60 and count(1)>=2
-> group by name;
ERROR 1111 (HY000): Invalid use of group function
mysql> select name,count(1) as gks from result
-> where score<60
-> group by name
-> having gks>=2;
+------+-----+
| name | gks |
+------+-----+
| 张三 | 2 |
| 李四 | 2 |
+------+-----+
2 rows in set (0.00 sec)
mysql> select name from (select name,count(1) as gks from result
-> where score<60
-> group by name
-> having gks>=2);
ERROR 1248 (42000): Every derived table must have its own alias
mysql> select name from (select name,count(1) as gks from result
-> where score<60
-> group by name
-> having gks>=2) as tmp;
+------+
| name |
+------+
| 张三 |
| 李四 |
+------+
2 rows in set (0.00 sec)
mysql> #最终3层嵌套的子查询
mysql> select name,avg(score)
-> from result
-> where name in (select name from (select name,count(1) as gks from result
-> where score<60
-> group by name
-> having gks>=2) as tmp)
-> group by name;
+------+------------+
| name | avg(score) |
+------+------------+
| 张三 | 60.0000 |
| 李四 | 50.0000 |
+------+------------+
2 rows in set (0.00 sec)
mysql> #这个例子也做出来了,但是用了子查询,且嵌套,非常麻烦.
mysql> #如果group having用的熟悉,一条sql就完成了. 不必嵌套子查询
mysql> exit
