数据库复习秘籍

匿名 (未验证) 提交于 2019-12-03 00:32:02
练习前的准备,先创建一个test库,在test库中建立数据:
create table goods (   goods_id mediumint(8) unsigned primary key auto_increment,   goods_name varchar(120) not null default ‘‘,   cat_id smallint(5) unsigned not null default 0,   brand_id smallint(5) unsigned not null default 0,   goods_sn char(15) not null default ‘‘,   goods_number smallint(5) unsigned not null default 0,   shop_price decimal(10,2) unsigned not null default 0.00,   market_price decimal(10,2) unsigned not null default 0.00,   click_count int(10) unsigned not null default 0 ) engine=myisam default charset=utf8;  insert into `goods` values (1,kd876,4,8,ecs000000,1,1388.00,1665.60,9), (4,诺基亚n85原装充电器,8,1,ecs000004,17,58.00,69.60,0), (3,诺基亚原装5800耳机,8,1,ecs000002,24,68.00,81.60,3), (5,索爱原装m2卡读卡器,11,7,ecs000005,8,20.00,24.00,3), (6,胜创kingmax内存卡,11,0,ecs000006,15,42.00,50.40,0), (7,诺基亚n85原装立体声耳机hs-82,8,1,ecs000007,20,100.00,120.00,0), (8,飞利浦9@9v,3,4,ecs000008,1,399.00,478.79,10), (9,诺基亚e66,3,1,ecs000009,4,2298.00,2757.60,20), (10,索爱c702c,3,7,ecs000010,7,1328.00,1593.60,11), (11,索爱c702c,3,7,ecs000011,1,1300.00,0.00,0), (12,摩托罗拉a810,3,2,ecs000012,8,983.00,1179.60,13), (13,诺基亚5320 xpressmusic,3,1,ecs000013,8,1311.00,1573.20,13), (14,诺基亚5800xm,4,1,ecs000014,1,2625.00,3150.00,6), (15,摩托罗拉a810,3,2,ecs000015,3,788.00,945.60,8), (16,恒基伟业g101,2,11,ecs000016,0,823.33,988.00,3), (17,夏新n7,3,5,ecs000017,1,2300.00,2760.00,2), (18,夏新t5,4,5,ecs000018,1,2878.00,3453.60,0), (19,三星sgh-f258,3,6,ecs000019,12,858.00,1029.60,7), (20,三星bc01,3,6,ecs000020,12,280.00,336.00,14), (21,金立 a30,3,10,ecs000021,40,2000.00,2400.00,4), (22,多普达touch hd,3,3,ecs000022,1,5999.00,7198.80,16), (23,诺基亚n96,5,1,ecs000023,8,3700.00,4440.00,17), (24,p806,3,9,ecs000024,100,2000.00,2400.00,35), (25,小灵通/固话50元充值卡,13,0,ecs000025,2,48.00,57.59,0), (26,小灵通/固话20元充值卡,13,0,ecs000026,2,19.00,22.80,0), (27,联通100元充值卡,15,0,ecs000027,2,95.00,100.00,0), (28,联通50元充值卡,15,0,ecs000028,0,45.00,50.00,0), (29,移动100元充值卡,14,0,ecs000029,0,90.00,0.00,0), (30,移动20元充值卡,14,0,ecs000030,9,18.00,21.00,1), (31,摩托罗拉e8 ,3,2,ecs000031,1,1337.00,1604.39,5), (32,诺基亚n85,3,1,ecs000032,4,3010.00,3612.00,9);    create table category (  cat_id smallint unsigned auto_increment primary key,  cat_name varchar(90) not null default ‘‘,  parent_id smallint unsigned  )engine myisam charset utf8;   INSERT INTO `category` VALUES  (1,手机类型,0), (2,CDMA手机,1), (3,GSM手机,1), (4,3G手机,1), (5,双模手机,1), (6,手机配件,0), (7,充电器,6), (8,耳机,6), (9,电池,6), (11,读卡器和内存卡,6), (12,充值卡,0), (13,小灵通/固话充值卡,12), (14,移动手机充值卡,12), (15,联通手机充值卡,12);   CREATE TABLE `result` (   `name` varchar(20) DEFAULT NULL,   `subject` varchar(20) DEFAULT NULL,   `score` tinyint(4) DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8;    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);   create table a ( id char(1), num int )engine myisam charset utf8;  insert into a values (a,5),(b,10),(c,15),(d,10);  create table b ( id char(1), num int )engine myisam charset utf8;  insert into b values (b,5),(c,15),(d,20),(e,99);     create table m(      mid int,      hid int,      gid int,      mres varchar(10),      matime date )engine myisam charset utf8;  create table t (      tid int,      tname varchar(20) )engine myisam charset utf8;      insert into m      values      (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);     insert into t      values      (1,国安),      (2,申花),      (3,布尔联队);    create table mian ( num int) engine myisam; insert into mian values  (3), (12), (15), (25), (23), (29), (34), (37), (32);   create table user ( uid int primary key auto_increment, name varchar(20) not null default ‘‘, age smallint unsigned not null default 0 ) engine myisam charset utf8;     create table boy (     hid char(1),      bname varchar(20)  )engine myisam charset utf8;     insert into boy (bname,hid)      values      (,A),      (杨过,B),      (陈冠希,C);     create table girl (     hid char(1),      gname varchar(20)      )engine myisam charset utf8;     insert into girl(gname,hid)      values      (小龙女,B),      (张柏芝,C),      (死宅女,D);
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基础知识: 1.数据库的连接 mysql -h -u -p -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%‘;   1.15 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 | +------+  1.16 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 | 公益联队 | +------+----------+ 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 | 公益联队 | 2006-06-21 | |    3 | 公益联队 | 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 | +------+------+   要求查询出以下效果: +------+----------+ | id   |    num   | +------+----------+ | a    |        5 | | b    |       15 | | c    |       30 | | d    |       30 | | e    |       99 | +------+----------+  create table a ( id char(1), num int )engine myisam charset utf8;  insert into a values (‘a‘,5),(‘b‘,10),(‘c‘,15),(‘d‘,10);  create table b ( id char(1), num int )engine myisam charset utf8;  insert into b values (‘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 | +------+------+  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    |       30 | | 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; 7.5 用exists型子查询,查出所有有商品的栏目 select * from category where exists (select * from goods where goods.cat_id=category.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 

  



原文:https://www.cnblogs.com/tianqizhi/p/9214318.html

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