mysql select

select 查询：

赋值：赋值不能应用在where中，因为where操作的是磁盘上的文件，可以应用在having筛选中。

　　例：select (market_price-shop_price) as jiesheng from goods;

一、条件查询：where(针对存在磁盘上的数据文件发挥作用)

　　select 列名1,列名2... from 表名 where 条件

　　1.条件表达式表达式为真，则取出该行

　　2.比较运算符 =，!= , <>(不等于) , <= , >=

　　3.like,not like ,in ,not in , between,and

　　　　字符匹配：'%'匹配多个字符，'_'匹配任意单个字符

　　4. is null,is not null

示例a：在goods表中取出goods大于30的产品，并显示出goods_id,goods_name,market_price

mysql> select goods_id,goods_name,market_price from goods where goods_id >30;
+----------+-----------------+--------------+
| goods_id | goods_name      | market_price |
+----------+-----------------+--------------+
|       31 | 摩托罗拉e8      |      1604.39 |
|       32 | 诺基亚n85       |      3612.00 |
+----------+-----------------+--------------+
2 rows in set (0.08 sec)

示例b:在goods表中查找goods_name以"诺基亚"开头的行，并打印

mysql> select goods_id,goods_name from goods where goods_name like "诺基亚%";
+----------+----------------------------------------+
| goods_id | goods_name                             |
+----------+----------------------------------------+
|        4 | 诺基亚n85原装充电器                    |
|        3 | 诺基亚原装5800耳机                     |
|        7 | 诺基亚n85原装立体声耳机hs-82           |
|        9 | 诺基亚e66                              |
|       13 | 诺基亚5320 xpressmusic                 |
|       14 | 诺基亚5800xm                           |
|       23 | 诺基亚n96                              |
|       32 | 诺基亚n85                              |
+----------+----------------------------------------+
8 rows in set (0.00 sec)

示例c：在goods表中查找goods_name为"诺基亚"开头后面跟着三个字符的行，并打印

mysql> select goods_id,goods_name from goods where goods_name like "诺基亚___";
+----------+--------------+
| goods_id | goods_name   |
+----------+--------------+
|        9 | 诺基亚e66    |
|       23 | 诺基亚n96    |
|       32 | 诺基亚n85    |
+----------+--------------+
3 rows in set (0.00 sec)

二、 分组：

　　group by

　　一般要配合5个统计函数使用:max(),min(),sum(),avg(),count()

avg():求平均值

mysql> select avg(shop_price) from goods;
+-----------------+
| avg(shop_price) |
+-----------------+
|     1232.526774 |
+-----------------+
1 row in set (0.09 sec)

max():求最大

min():求最小

sum():求和

count(*):统计行数

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 |     2297.000000 |
|      5 |     3700.000000 |
|      8 |       75.333333 |
|     11 |       31.000000 |
|     13 |       33.500000 |
|     14 |       54.000000 |
|     15 |       70.000000 |
+--------+-----------------+
9 rows in set (0.00 sec)

mysql> select cat_id,count(*) from goods group by cat_id;
+--------+----------+
| cat_id | count(*) |
+--------+----------+
|      2 |        1 |
|      3 |       15 |
|      4 |        3 |
|      5 |        1 |
|      8 |        3 |
|     11 |        2 |
|     13 |        2 |
|     14 |        2 |
|     15 |        2 |
+--------+----------+
9 rows in set (0.00 sec)

三、 having 筛选：

• 掌握having查询，理解having与where的不同

1.筛选出market_price高于shop_price 200以上的商品

mysql> select goods_name,(market_price-shop_price) as jiesheng from goods having jiesheng > 200;
+---------------------------+----------+
| goods_name                | jiesheng |
+---------------------------+----------+
| kd876                     |   277.60 |
| 诺基亚e66                 |   459.60 |
| 索爱c702c                 |   265.60 |
| 诺基亚5320 xpressmusic    |   262.20 |
| 诺基亚5800xm              |   525.00 |
| 夏新n7                    |   460.00 |
| 夏新t5                    |   575.60 |
| 金立 a30                  |   400.00 |
| 多普达touch hd            |  1199.80 |
| 诺基亚n96                 |   740.00 |
| p806                      |   400.00 |
| 摩托罗拉e8                |   267.39 |
| 诺基亚n85                 |   602.00 |
+---------------------------+----------+
13 rows in set (0.00 sec)

where条件查询是操作磁盘上文件，不能处理内存中的数据，例如赋值操作。

having筛选是是处理内存中的数据，如果where 和having 都存在时，having必须在where后面。

习题：where-having-group综合习题

要求：查询出2门及2门以上不及格者的平均成绩

+----------+-----------+-------+
| name     | subject   | score |
+----------+-----------+-------+
| zhangsan | math      |    90 |
| zhangsan | language  |    50 |
| zhangsan | geography |    40 |
| lisi     | language  |    55 |
| lisi     | politics  |    45 |
| wangwu   | politics  |    30 |
| zhangsan | english   |   100 |
+----------+-----------+-------+

正确答案：

select name,subject,score,avg(score),sum(score<60) as f from report group by name having f>=2;

+----------+----------+-------+------------+------+
| name     | subject  | score | avg(score) | f    |
+----------+----------+-------+------------+------+
| lisi     | language |    55 |    50.0000 |    2 |
| zhangsan | math     |    90 |    70.0000 |    2 |
+----------+----------+-------+------------+------+

错误案例：

select name,subject,score,avg(score),count(score<60) as f from report group by name having g f<=2; （count用来数行号的，不判断条件）

+--------+----------+-------+------------+---+
| name   | subject  | score | avg(score) | f |
+--------+----------+-------+------------+---+
| lisi   | language |    55 |    50.0000 | 2 |
| wangwu | politics |    30 |    30.0000 | 1 |
+--------+----------+-------+------------+---+

四、 排序：

order by

• 降序 desc
• 升序 asc(默认是asc)
• 多列排序

select goods_id,cat_id,goods_name,shop_price from goods order by cat_id asc,shop_price desc,goods_id asc;

+----------+--------+----------------------------------------+------------+
| goods_id | cat_id | goods_name                             | shop_price |
+----------+--------+----------------------------------------+------------+
|       16 |      2 | 恒基伟业g101                           |     823.33 |
|       22 |      3 | 多普达touch hd                         |    5999.00 |
|       32 |      3 | 诺基亚n85                              |    3010.00 |
|       17 |      3 | 夏新n7                                 |    2300.00 |
|        9 |      3 | 诺基亚e66                              |    2298.00 |
|       21 |      3 | 金立 a30                               |    2000.00 |
|       24 |      3 | p806                                   |    2000.00 |
|       31 |      3 | 摩托罗拉e8                             |    1337.00 |
|       10 |      3 | 索爱c702c                              |    1328.00 |
|       13 |      3 | 诺基亚5320 xpressmusic                 |    1311.00 |
|       11 |      3 | 索爱c702c                              |    1300.00 |
|       12 |      3 | 摩托罗拉a810                           |     983.00 |
|       19 |      3 | 三星sgh-f258                           |     858.00 |
|       15 |      3 | 摩托罗拉a810                           |     788.00 |
|        8 |      3 | 飞利浦9@9v                             |     399.00 |
|       20 |      3 | 三星bc01                               |     280.00 |
|       18 |      4 | 夏新t5                                 |    2878.00 |
|       14 |      4 | 诺基亚5800xm                           |    2625.00 |
|        1 |      4 | kd876                                  |    1388.00 |
|       23 |      5 | 诺基亚n96                              |    3700.00 |
|        7 |      8 | 诺基亚n85原装立体声耳机hs-82           |     100.00 |
|        3 |      8 | 诺基亚原装5800耳机                     |      68.00 |
|        4 |      8 | 诺基亚n85原装充电器                    |      58.00 |
|        6 |     11 | 胜创kingmax内存卡                      |      42.00 |
|        5 |     11 | 索爱原装m2卡读卡器                     |      20.00 |
|       25 |     13 | 小灵通/固话50元充值卡                  |      48.00 |
|       26 |     13 | 小灵通/固话20元充值卡                  |      19.00 |
|       29 |     14 | 移动100元充值卡                        |      90.00 |
|       30 |     14 | 移动20元充值卡                         |      18.00 |
|       27 |     15 | 联通100元充值卡                        |      95.00 |
|       28 |     15 | 联通50元充值卡                         |      45.00 |
+----------+--------+----------------------------------------+------------+

五、 limit 限制取出条目：

　　limit 偏移量 条数

习题：查找最新的商品（goods_id最大为最新）

select goods_id,goods_name,shop_price from goods order by goods_id desc limit 0,1;

+----------+--------------+------------+
| goods_id | goods_name   | shop_price |
+----------+--------------+------------+
|       32 | 诺基亚n85    |    3010.00 |
+----------+--------------+------------+