十八哥mysql复习集合

mysql复习

一:复习前的准备

1:确认你已安装wamp

2:确认你已安装ecshop,并且ecshop的数据库名为shop



二	基础知识:

1.数据库的连接

mysql -u -p -h

-u 用户名

-p 密码

-h host主机



2:库级知识

2.1 显示数据库: show databases;

2.2 选择数据库: use dbname;

2.3 创建数据库: create database dbname charset utf8;

2.3 删除数据库: drop database dbname;



3: 表级操作:

3.1 显示库下面的表

show tables;



3.2 查看表的结构: 

desc tableName;



3.3 查看表的创建过程: 

show create table  tableName;



3.4 创建表:

　create table tbName (

列名称1　列类型　[列参数]　[not null default ],

....列2...

....

列名称N　列类型　[列参数]　[not null default ]

)engine myisam/innodb charset utf8/gbk





3.4的例子:

create table user (

    id int auto_increment,

    name varchar(20) not null default '',

    age tinyint unsigned not null default 0,

   index id (id)

   )engine=innodb charset=utf8;

注:innodb是表引擎,也可以是myisam或其他,但最常用的是myisam和innodb,

charset 常用的有utf8,gbk;





3.5 修改表

3.5.1	修改表之增加列:

alter table tbName 

add 列名称１　列类型　[列参数]　[not null default ]　#(add之后的旧列名之后的语法和创建表时的列声明一样)



3.5.2	修改表之修改列

alter table tbName

change 旧列名  新列名  列类型　[列参数]　[not null default ]

(注:旧列名之后的语法和创建表时的列声明一样)



3.5.3	修改表之减少列:

alter table tbName 

drop 列名称;





3.5.4	修改表之增加主键

alter table tbName add primary key(主键所在列名);

例:alter table goods add primary key(id)

该例是把主键建立在id列上



3.5.5	修改表之删除主键

alter table tbName　drop primary key;



3.5.6	修改表之增加索引

alter table tbName add [unique|fulltext] index 索引名(列名);



3.5.7	修改表之删除索引

alter table tbName drop index 索引名;



3.5.8	清空表的数据

truncate tableName;



4:列类型讲解

列类型:

        整型:tinyint (0~255/-128~127) smallint (0~65535/-32768~32767) mediumint int bigint (参考手册11.2)

        参数解释:

        unsigned 无符号(不能为负)  zerofill 0填充  M 填充后的宽度

        举例:tinyint unsigned;

             tinyint(6) zerofill;   

数值型

        浮点型:float double

        格式:float(M,D)  unsigned\zerofill;





字符型

        char(m) 定长

        varchar(m)变长

        text



列          实存字符i        实占空间            利用率



char(M)      0<=i<=M            M                i/m<=100%



varchar(M)    0<=i<=M          i+1,2             i/i+1/2<100%

    



               year       YYYY	范围:1901~2155. 可输入值2位和4位(如98,2012)

日期时间类型   date       YYYY-MM-DD 如:2010-03-14

               time       HH:MM:SS	如:19:26:32

               datetime   YYYY-MM-DD  HH:MM:SS 如:2010-03-14 19:26:32

               timestamp  YYYY-MM-DD  HH:MM:SS 特性:不用赋值,该列会为自己赋当前的具体时间 







5:增删改查基本操作



5.1 插入数据 

	insert into 表名(col1,col2,……) values(val1,val2……); -- 插入指定列

	insert into 表名 values (,,,,); -- 插入所有列

	insert into 表名 values	-- 一次插入多行 

	(val1,val2……),

	(val1,val2……),

	(val1,val2……);





5.3修改数据

	update tablename 

	set 

	col1=newval1,  

	col2=newval2,

	...

	...

	colN=newvalN

	where 条件;



5.4，删除数据    delete from tablenaeme where 条件;



5.5，    select     查询



  （1）  条件查询   where  a. 条件表达式的意义，表达式为真，则该行取出

			   b.  比较运算符  = ，!=，< > <=  >=

                           c.  like , not like ('%'匹配任意多个字符,'_'匹配任意单个字符) 

				in , not in , between and

                           d. is null , is not null			

  （2）  分组       group by 

			一般要配合5个聚合函数使用:max,min,sum,avg,count

  （3）  筛选       having

  （4）  排序       order by

  （5）  限制       limit







6:	连接查询



6.1， 左连接

	.. left join .. on

	table A left join table B on tableA.col1 = tableB.col2 ; 

  例句:

  select 列名 from table A left join table B on tableA.col1 = tableB.col2

2.  右链接: right join

3.  内连接:  inner join



左右连接都是以在左边的表的数据为准,沿着左表查右表.

内连接是以两张表都有的共同部分数据为准,也就是左右连接的数据之交集.



7	子查询

  where 型子查询:内层sql的返回值在where后作为条件表达式的一部分

  例句: select * from tableA where colA = (select colB from tableB where ...);

  

  from 型子查询:内层sql查询结果,作为一张表,供外层的sql语句再次查询

  例句:select * from (select * from ...) as tableName where ....



  

8: 字符集

  客服端sql编码 character_set_client

  服务器转化后的sql编码 character_set_connection

  服务器返回给客户端的结果集编码     character_set_results

  快速把以上3个变量设为相同值: set names 字符集



   存储引擎 engine=1\2

  1 Myisam  速度快 不支持事务 回滚

  2 Innodb  速度慢 支持事务,回滚

  

  ①开启事务          start transaction

  ②运行sql;          

  ③提交,同时生效\回滚 commit\rollback



  触发器 trigger

  监视地点:表

  监视行为:增 删 改

  触发时间:after\before

  触发事件:增 删 改





  创建触发器语法

	create trigger tgName

	after/before insert/delete/update 

	on tableName

	for each row

	sql; -- 触发语句

	

  删除触发器:drop trigger tgName;





 索引

 提高查询速度,但是降低了增删改的速度,所以使用索引时,要综合考虑.

 索引不是越多越好,一般我们在常出现于条件表达式中的列加索引.

 值越分散的列，索引的效果越好



 索引类型

 primary key主键索引

 index 普通索引

 unique index 唯一性索引

 fulltext index 全文索引





综合练习:

连接上数据库服务器

创建一个gbk编码的数据库

建立商品表和栏目表,字段如下:



商品表:goods

goods_id　--主键,

goods_name -- 商品名称

cat_id  -- 栏目id

brand_id -- 品牌id

goods_sn -- 货号

goods_number -- 库存量

shop_price  -- 价格

goods_desc　--商品详细描述



栏目表:category

cat_id --主键 

cat_name -- 栏目名称

parent_id -- 栏目的父id







建表完成后,作以下操作:

删除goods表的goods_desc 字段,及货号字段

并增加字段:click_count  -- 点击量



在goods_name列上加唯一性索引

在shop_price列上加普通索引

在clcik_count列上加普通索引

删除click_count列上的索引





对goods表插入以下数据:

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+

| goods_id | goods_name                   | cat_id | brand_id | goods_sn  | goods_number | shop_price | click_count |

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+

|        1 | KD876                        |      4 |        8 | ECS000000 |           10 |    1388.00 |           7 |

|        4 | 诺基亚N85原装充电器          |      8 |        1 | ECS000004 |           17 |      58.00 |           0 |

|        3 | 诺基亚原装5800耳机           |      8 |        1 | ECS000002 |           24 |      68.00 |           3 |

|        5 | 索爱原装M2卡读卡器           |     11 |        7 | ECS000005 |            8 |      20.00 |           3 |

|        6 | 胜创KINGMAX内存卡            |     11 |        0 | ECS000006 |           15 |      42.00 |           0 |

|        7 | 诺基亚N85原装立体声耳机HS-82 |      8 |        1 | ECS000007 |           20 |     100.00 |           0 |

|        8 | 飞利浦9@9v                   |      3 |        4 | ECS000008 |           17 |     399.00 |           9 |

|        9 | 诺基亚E66                    |      3 |        1 | ECS000009 |           13 |    2298.00 |          20 |

|       10 | 索爱C702c                    |      3 |        7 | ECS000010 |            7 |    1328.00 |          11 |

|       11 | 索爱C702c                    |      3 |        7 | ECS000011 |            1 |    1300.00 |           0 |

|       12 | 摩托罗拉A810                 |      3 |        2 | ECS000012 |            8 |     983.00 |          14 |

|       13 | 诺基亚5320 XpressMusic       |      3 |        1 | ECS000013 |            8 |    1311.00 |          13 |

|       14 | 诺基亚5800XM                 |      4 |        1 | ECS000014 |            4 |    2625.00 |           6 |

|       15 | 摩托罗拉A810                 |      3 |        2 | ECS000015 |            3 |     788.00 |           8 |

|       16 | 恒基伟业G101                 |      2 |       11 | ECS000016 |            0 |     823.33 |           3 |

|       17 | 夏新N7                       |      3 |        5 | ECS000017 |            1 |    2300.00 |           2 |

|       18 | 夏新T5                       |      4 |        5 | ECS000018 |            1 |    2878.00 |           0 |

|       19 | 三星SGH-F258                 |      3 |        6 | ECS000019 |            0 |     858.00 |           7 |

|       20 | 三星BC01                     |      3 |        6 | ECS000020 |           13 |     280.00 |          14 |

|       21 | 金立 A30                     |      3 |       10 | ECS000021 |           40 |    2000.00 |           4 |

|       22 | 多普达Touch HD               |      3 |        3 | ECS000022 |            0 |    5999.00 |          15 |

|       23 | 诺基亚N96                    |      5 |        1 | ECS000023 |            8 |    3700.00 |          17 |

|       24 | P806                         |      3 |        9 | ECS000024 |          148 |    2000.00 |          36 |

|       25 | 小灵通/固话50元充值卡        |     13 |        0 | ECS000025 |            2 |      48.00 |           0 |

|       26 | 小灵通/固话20元充值卡        |     13 |        0 | ECS000026 |            2 |      19.00 |           0 |

|       27 | 联通100元充值卡              |     15 |        0 | ECS000027 |            2 |      95.00 |           0 |

|       28 | 联通50元充值卡               |     15 |        0 | ECS000028 |            0 |      45.00 |           0 |

|       29 | 移动100元充值卡              |     14 |        0 | ECS000029 |            0 |      90.00 |           0 |

|       30 | 移动20元充值卡               |     14 |        0 | ECS000030 |            9 |      18.00 |           1 |

|       31 | 摩托罗拉E8                   |      3 |        2 | ECS000031 |            1 |    1337.00 |           5 |

|       32 | 诺基亚N85                    |      3 |        1 | ECS000032 |            1 |    3010.00 |           9 |

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+







三	查询知识

注:以下查询基于ecshop网站的商品表(ecs_goods)

在练习时可以只取部分列,方便查看.



1: 基础查询 where的练习:



查出满足以下条件的商品

1.1:主键为32的商品

select goods_id,goods_name,shop_price 

     from ecs_goods

     where goods_id=32;

1.2:不属第3栏目的所有商品

select goods_id,cat_id,goods_name,shop_price  from ecs_goods

     where cat_id!=3;



1.3:本店价格高于3000元的商品



 select goods_id,cat_id,goods_name,shop_price  from ecs_goods

     where shop_price >3000;



1.4:本店价格低于或等于100元的商品

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where shop_price <=100;



1.5:取出第4栏目或第11栏目的商品(不许用or)

select goods_id,cat_id,goods_name,shop_price  from ecs_goods

     where cat_id in (4,11);





1.6:取出100<=价格<=500的商品(不许用and)

select goods_id,cat_id,goods_name,shop_price  from ecs_goods

     where shop_price between 100 and 500;





1.7:取出不属于第3栏目且不属于第11栏目的商品(and,或not in分别实现)

select goods_id,cat_id,goods_name,shop_price  from ecs_goods     where cat_id!=3 and cat_id!=11;



select goods_id,cat_id,goods_name,shop_price  from ecs_goods     where cat_id not in (3,11);







1.8:取出价格大于100且小于300,或者大于4000且小于5000的商品()

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where shop_price>100 and shop_price <300 or shop_price >4000 and shop_price <5000;







1.9:取出第3个栏目下面价格<1000或>3000,并且点击量>5的系列商品

select goods_id,cat_id,goods_name,shop_price,click_count from ecs_goods where

cat_id=3 and (shop_price <1000 or shop_price>3000) and click_count>5;



1.10:取出第1个栏目下面的商品(注意:1栏目下面没商品,但其子栏目下有)

select goods_id,cat_id,goods_name,shop_price,click_count from ecs_goods

     where cat_id in (2,3,4,5);



1.11:取出名字以"诺基亚"开头的商品

select goods_id,cat_id,goods_name,shop_price  from ecs_goods     where goods_name like '诺基亚%';





1.12:取出名字为"诺基亚Nxx"的手机

select goods_id,cat_id,goods_name,shop_price  from ecs_goods  

   where goods_name like '诺基亚N__';





1.13:取出名字不以"诺基亚"开头的商品

select goods_id,cat_id,goods_name,shop_price from ecs_goos

     where goods_name not like '诺基亚%';



1.14:取出第3个栏目下面价格在1000到3000之间,并且点击量>5 "诺基亚"开头的系列商品

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where 

cat_id=3 and shop_price>1000 and shop_price <3000 and click_count>5 and goods_name like '诺基亚%';





select goods_id,cat_id,goods_name,shop_price  from ecs_goods where 

shop_price between 1000 and 3000 and cat_id=3  and click_count>5 and goods_name like '诺基亚%';





一道面试题

有如下表和数组

把num值处于[20,29]之间,改为20

num值处于[30,39]之间的,改为30



mian表

+------+

| num  |

+------+

|    3 |

|   12 |

|   15 |

|   25 |

|   23 |

|   29 |

|   34 |

|   37 |

|   32 |

|   45 |

|   48 |

|   52 |

+------+



练习题:

把good表中商品名为'诺基亚xxxx'的商品,改为'HTCxxxx',

提示:大胆的把列看成变量,参与运算,甚至调用函数来处理 .

substring(),concat()





2	分组查询group:

2.1:查出最贵的商品的价格

select max(shop_price) from ecs_goods;



2.2:查出最大(最新)的商品编号

select max(goods_id) from ecs_goods;



2.3:查出最便宜的商品的价格

select min(shop_price) from ecs_goods;



2.4:查出最旧(最小)的商品编号

select min(goods_id) from ecs_goods;



2.5:查询该店所有商品的库存总量

select sum(goods_number) from ecs_goods;



2.6:查询所有商品的平均价

 select avg(shop_price) from ecs_goods;



2.7:查询该店一共有多少种商品

 select count(*) from ecs_goods;





2.8:查询每个栏目下面

最贵商品价格

最低商品价格

商品平均价格

商品库存量

商品种类

提示:(5个聚合函数,sum,avg,max,min,count与group综合运用)

select cat_id,max(shop_price) from ecs_goods  group by cat_id;





3 having与group综合运用查询:

3.1:查询该店的商品比市场价所节省的价格

select goods_id,goods_name,market_price-shop_price as j 

     from ecs_goods ;





3.2:查询每个商品所积压的货款(提示:库存*单价)

select goods_id,goods_name,goods_number*shop_price  from ecs_goods



3.3:查询该店积压的总货款

select sum(goods_number*shop_price) from ecs_goods;



3.4:查询该店每个栏目下面积压的货款.

select cat_id,sum(goods_number*shop_price) as k from ecs_goods group by cat_id;



3.5:查询比市场价省钱200元以上的商品及该商品所省的钱(where和having分别实现)

select goods_id,goods_name,market_price-shop_price  as k from ecs_goods

where market_price-shop_price >200;



select goods_id,goods_name,market_price-shop_price  as k from ecs_goods

having k >200;



3.6:查询积压货款超过2W元的栏目,以及该栏目积压的货款

select cat_id,sum(goods_number*shop_price) as k from ecs_goods group by cat_id

having k>20000



3.7:where-having-group综合练习题

有如下表及数据

+------+---------+-------+

| 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 |

+------+---------+-------+



要求:查询出2门及2门以上不及格者的平均成绩



## 一种错误做法

mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三     | 3 |    60.0000 |

| 李四     | 2 |    50.0000 |

+------+---+------------+

2 rows in set (0.00 sec)



mysql> select name,count(score<60) as k,avg(score) from stu group by name;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三     | 3 |    60.0000 |

| 李四     | 2 |    50.0000 |

| 王五     | 1 |    30.0000 |

+------+---+------------+

3 rows in set (0.00 sec)



mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三     | 3 |    60.0000 |

| 李四     | 2 |    50.0000 |

+------+---+------------+

2 rows in set (0.00 sec)



#加上赵六后错误暴露

mysql> insert into stu 

    -> values 

    -> ('赵六','A',100),

    -> ('赵六','B',99),

    -> ('赵六','C',98);

Query OK, 3 rows affected (0.05 sec)

Records: 3  Duplicates: 0  Warnings: 0



#错误显现

mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三 | 3 |    60.0000 |

| 李四 | 2 |    50.0000 |

| 赵六 | 3 |    99.0000 |

+------+---+------------+

3 rows in set (0.00 sec)



#正确思路,先查看每个人的平均成绩

mysql> select name,avg(score) from stu 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 (0.00 sec)



mysql> # 看每个人挂科情况

mysql> select name,score < 60 from stu;

+------+------------+

| name | score < 60 |

+------+------------+

| 张三 |          0 |

| 张三 |          1 |

| 张三 |          1 |

| 李四 |          1 |

| 李四 |          1 |

| 王五 |          1 |

| 赵六 |          0 |

| 赵六 |          0 |

| 赵六 |          0 |

+------+------------+

9 rows in set (0.00 sec)



mysql> #计算每个人的挂科科目

mysql> select name,sum(score < 60) from stu group by name;

+------+-----------------+

| name | sum(score < 60) |

+------+-----------------+

| 张三 |               2 |

| 李四 |               2 |

| 王五 |               1 |

| 赵六 |               0 |

+------+-----------------+

4 rows in set (0.00 sec)



#同时计算每人的平均分

mysql> select name,sum(score < 60),avg(score) as pj from stu group by name;

+------+-----------------+---------+

| name | sum(score < 60) | pj      |

+------+-----------------+---------+

| 张三 |               2 | 60.0000 |

| 李四 |               2 | 50.0000 |

| 王五 |               1 | 30.0000 |

| 赵六 |               0 | 99.0000 |

+------+-----------------+---------+

4 rows in set (0.00 sec)



#利用having筛选挂科2门以上的.

mysql> select name,sum(score < 60) as gk ,avg(score) as pj from stu group by name having gk >=2; 

+------+------+---------+

| name | gk   | pj      |

+------+------+---------+

| 张三 |    2 | 60.0000 |

| 李四 |    2 | 50.0000 |

+------+------+---------+

2 rows in set (0.00 sec)







4:	order by 与 limit查询

4.1:按价格由高到低排序

select goods_id,goods_name,shop_price from ecs_goods order by shop_price desc;



4.2:按发布时间由早到晚排序

select goods_id,goods_name,add_time from ecs_goods order by add_time;



4.3:接栏目由低到高排序,栏目内部按价格由高到低排序

select goods_id,cat_id,goods_name,shop_price from ecs_goods

     order by cat_id ,shop_price desc;





4.4:取出价格最高的前三名商品

select goods_id,goods_name,shop_price from ecs_goods order by shop_price desc limit 3;







4.5:取出点击量前三名到前5名的商品

select goods_id,goods_name,click_count from ecs_goods order by click_count desc limit 2,3;



5	连接查询

5.1:取出所有商品的商品名,栏目名,价格

select goods_name,cat_name,shop_price from 

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id;



5.2:取出第4个栏目下的商品的商品名,栏目名,价格

select goods_name,cat_name,shop_price from 

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id

where ecs_goods.cat_id = 4;







5.3:取出第4个栏目下的商品的商品名,栏目名,与品牌名

select goods_name,cat_name,brand_name from 

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id

left join ecs_brand 

on ecs_goods.brand_id=ecs_brand.brand_id

where ecs_goods.cat_id = 4;



5.4: 用友面试题



根据给出的表结构按要求写出SQL语句。

Match 赛程表

字段名称	字段类型	描述
matchID	int	主键
hostTeamID	int	主队的ID
guestTeamID	int	客队的ID
matchResult	varchar(20)	比赛结果，如（2:0）
matchTime	date	比赛开始时间

Team 参赛队伍表

字段名称	字段类型	描述
teamID	int	主键
teamName	varchar(20)	队伍名称

Match的hostTeamID与guestTeamID都与Team中的teamID关联

查出 2006-6-1 到2006-7-1之间举行的所有比赛，并且用以下形式列出：

拜仁  2：0 不来梅 2006-6-21



mysql> select * from m;

+-----+------+------+------+------------+

| mid | hid  | gid  | mres | matime     |

+-----+------+------+------+------------+

|   1 |    1 |    2 | 2:0  | 2006-05-21 |

|   2 |    2 |    3 | 1:2  | 2006-06-21 |

|   3 |    3 |    1 | 2:5  | 2006-06-25 |

|   4 |    2 |    1 | 3:2  | 2006-07-21 |

+-----+------+------+------+------------+

4 rows in set (0.00 sec)



mysql> select * from t;

+------+----------+

| tid  | tname    |

+------+----------+

|    1 | 国安     |

|    2 | 申花     |

|    3 | 传智联队 |

+------+----------+

3 rows in set (0.00 sec)



mysql> select hid,t1.tname as hname ,mres,gid,t2.tname as gname,matime

    -> from 

    -> m left join t as t1

    -> on m.hid = t1.tid

    -> left join t as t2

    -> on m.gid = t2.tid;

+------+----------+------+------+----------+------------+

| hid  | hname    | mres | gid  | gname    | matime     |

+------+----------+------+------+----------+------------+

|    1 | 国安     | 2:0  |    2 | 申花     | 2006-05-21 |

|    2 | 申花     | 1:2  |    3 | 传智联队 | 2006-06-21 |

|    3 | 传智联队 | 2:5  |    1 | 国安     | 2006-06-25 |

|    2 | 申花     | 3:2  |    1 | 国安     | 2006-07-21 |

+------+----------+------+------+----------+------------+

4 rows in set (0.00 sec)



6	union查询

6.1:把ecs_comment,ecs_feedback两个表中的数据,各取出4列,并把结果集union成一个结果集.



6.2:3期学员碰到的一道面试题

A表:

+------+------+

| id   | num  |

+------+------+

| a    |    5 |

| b    |   10 |

| c    |   15 |

| d    |   10 |

+------+------+



B表:

+------+------+

| id   | num  |

+------+------+

| b    |    5 |

| c    |   15 |

| d    |   20 |

| e    |   99 |

+------+------+





mysql> # 合并 ,注意all的作用

mysql> select * from ta 

    -> union all

    -> select * from tb;

+------+------+

| id   | num  |

+------+------+

| a    |    5 |

| b    |   10 |

| c    |   15 |

| d    |   10 |

| b    |    5 |

| c    |   15 |

| d    |   20 |

| e    |   99 |

+------+------+



要求查询出以下效果:

+------+----------+

| id   | sum(num) |

+------+----------+

| a    |        5 |

| b    |       15 |

| c    |       30 |

| d    |       30 |

| e    |       99 |

+------+----------+



参考答案:

mysql> # sum,group求和

mysql> select id,sum(num) from (select * from ta union all select * from tb) as tmp group by id; 

+------+----------+

| id   | sum(num) |

+------+----------+

| a    |        5 |

| b    |       15 |

| c    |       25 |

| d    |       30 |

| e    |       99 |

+------+----------+

5 rows in set (0.00 sec)





7: 子查询:

7.1:查询出最新一行商品(以商品编号最大为最新,用子查询实现)

select goods_id,goods_name from 

     ecs_goods where goods_id =(select max(goods_id) from ecs_goods);





7.2:查询出编号为19的商品的栏目名称(用左连接查询和子查询分别)

7.3:用where型子查询把ecs_goods表中的每个栏目下面最新的商品取出来

select goods_id,goods_name,cat_id from ecs_goods where goods_id in (select max(goods_id) from ecs_goods group by cat_id);

7.4:用from型子查询把ecs_goods表中的每个栏目下面最新的商品取出来

select * from (select goods_id,cat_id,goods_name from ecs_goods order by goods_id desc) as t group by cat_id;









创建触发器:



 CREATE  trigger tg2

after insert on ord

for each row

update goods set goods_number=goods_number-new.num where id=new.gid



CREATE trigger tg3

after delete on ord

for each row

update goods set goods_number=good_number+old.num where id=old.gid





CREATE  trigger tg4

after update on ord

for each row

update goods set goods_number=goods_number+old.num-new.num where id=old.gid

2012-03-25更新,添加了面试案例