mysql查询

Posted 2020-10-13 see-young

一、mysql查询的五种子句

 

        where(条件查询)、having（筛选）、group by（分组）、order by（排序）、limit（限制结果数）

 

        1、where常用运算符：

            比较运算符

                > ,  < ,=  , != （< >）,>=   ,   <=  

                in(v1,v2..vn)  

                between v1 and v2    在v1至v2之间（包含v1,v2）

            逻辑运算符

                not ( ! )  逻辑非

                or ( || )    逻辑或

                and ( && )  逻辑与

 

                where price>=3000 and price <= 5000 or price >=500 and price <=1000

                取500-1000或者3000-5000的值

                where price not between 3000 and 5000

                不在3000与5000之间的值

 

            模糊查询

                like 像

                通配符:

                %  任意字符

                _   单个字符

                    where goods_name like ‘诺基亚%‘

                    where goods_name like ‘诺基亚N__‘

 

         2、group by 分组

                一般情况下group需与统计函数（聚合函数）一起使用才有意义

                如：select goods_id,goods_name,cat_id,max(shop_price) from goods group by cat_id;

                        这里取出来的结果中的good_name是错误的！因为shop_price使用了max函数，那么它是取最大的，而语句中使用了group by 分组，那么goods_name并没有使用聚合函数，它只是cat_id下的第一个商品，并不会因为shop_price改变而改变

                mysql中的五种统计函数：

                （1）max：求最大值

                    select max(goods_price) from goods

                      这里会取出最大的价格的值，只有值

                        #查询每个栏目下价格最高的

                        select cat_id,max(goods_price) from goos group by cat_id;

                        #查出价格最高的商品编号

                        select goods_id,max(goods_price) from goods group by goods_id;

                        

                        

                （2）min：求最小值

                （3）sum：求总数和

                        #求商品库存总和

                        select sum(goods_number) from goods;

                （4）avg：求平均值

                        #求每个栏目的商品平均价格

                        select cat_id,avg(goods_price) from goods group by cat_id;

                （5）count：求总行数

                        #求每个栏目下商品种类

                        select cat_id,count(*) from goods group by cat_id;

 

                   ###要把每个字段名当成变量来理解，它可以进行运算###

                        例：查询本店每个商品价格比市场价低多少；

                        select goods_id,goods_name,goods_price-market_price from goods;

                            查询每个栏目下面积压的货款

                        select cat_id,sum(goods_price*goods_number) from goods group by cat_id;

 

                    ###可以用as来给计算结果取个别名###

                        select cat_id,sum(goods_price * goods_number)  as hk from goods group by cat_id

                        不仅列名可以取别名，表单也可以取别名

 

            3、having 与where 的异同点

 

                    having与where类似，可以筛选数据，where后的表达式怎么写，having后就怎么写

                    where针对表中的列发挥作用，查询数据

                    having对查询结果中的列发挥作用，筛选数据

                    #查询本店商品价格比市场价低多少钱，输出低200元以上的商品

                    select goods_id,good_name,market_price - shop_price as s from goods having s>200 ;

                    //这里不能用where因为s是查询结果，而where只能对表中的字段名筛选

                    如果用where的话则是：

                    select goods_id,goods_name from goods where market_price - shop_price > 200;

 

                    #同时使用where与having

                    select cat_id,goods_name,market_price - shop_price as s from goods where cat_id = 3 having s > 200;

                    #查询积压货款超过2万元的栏目，以及该栏目积压的货款

                    select cat_id,sum(shop_price * goods_number) as t from goods group by cat_id having s > 20000

                    #查询两门及两门以上科目不及格的学生的平均分

                          思路：

                            #先计算所有学生的平均分

                             select name,avg(score) as pj from stu group by name;

                            #查出所有学生的挂科情况

                            select name,score<60 from stu;

                                    #这里score<60是判断语句，所以结果为真或假，mysql中真为1假为0

                            #查出两门及两门以上不及格的学生

                            select name,sum(score<60) as gk from stu group by name having gk > 1;

                            #综合结果

                            select name,sum(score<60) as gk,avg(score) as pj from stu group by name having gk >1;

                4、order by

                    （1） order by price  //默认升序排列

                    （2）order by price desc //降序排列

                    （3）order by price asc //升序排列，与默认一样

                    （4）order by rand() //随机排列，效率不高

                        #按栏目号升序排列，每个栏目下的商品价格降序排列

                        select * from goods where cat_id !=2 order by cat_id,price desc;

                5、limit

                    limit [offset,] N

                    offset 偏移量，可选，不写则相当于limit 0,N

                    N     取出条目

 

                    #取价格第4-6高的商品

                    select good_id,goods_name,goods_price from goods order by good_price desc limit 3,3;

                    

            ###查询每个栏目下最贵的商品

                思路：

                        #先对每个栏目下的商品价格排序

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

                        #上面的查询结果中每个栏目的第一行的商品就是最贵的商品

                        #把上面的查询结果理解为一个临时表[存在于内存中]【子查询】

                        #再从临时表中选出每个栏目最贵的商品

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

                        #这里使用group by cat_id是因为临时表中每个栏目的第一个商品就是最贵的商品，而group by前面没有使用聚合函数，所以默认就取每个分组的第一行数据，这里以cat_id分组

 

                 良好的理解模型：

                    1、where后面的表达式，把表达式放在每一行中，看是否成立

                    2、字段(列)，理解为变量，可以进行运算（算术运算和逻辑运算）  

                    3、 取出结果可以理解成一张临时表

  二、mysql子查询

        1、where型子查询

                （把内层查询结果当作外层查询的比较条件）

                #不用order by 来查询最新的商品

                select goods_id,goods_name from goods where goods_id = (select max(goods_id) from goods);

                #取出每个栏目下最新的产品(goods_id唯一)

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

 

        2、from型子查询

                (把内层的查询结果供外层再次查询)

                #用子查询查出挂科两门及以上的同学的平均成绩

                    思路：

                        #先查出哪些同学挂科两门以上

                        select name,count(*) as gk from stu where score < 60 having gk >=2;

                        #以上查询结果，我们只要名字就可以了，所以再取一次名字

                        select name from (select name,count(*) as gk from stu having gk >=2) as t;

                        #找出这些同学了，那么再计算他们的平均分

                        select name,avg(score) from stu where name in (select name from (select name,count(*) as gk from stu having gk >=2) as t) group by name;

 

        3、exists型子查询

                （把外层查询结果拿到内层，看内层的查询是否成立）

                #查询哪些栏目下有商品，栏目表category,商品表goods

                    select cat_id,cat_name from category where exists(select * from goods where goods.cat_id = category.cat_id);

   三、union的用法

              （把两次或多次的查询结果合并起来，要求查询的列数一致，推荐查询的对应的列类型一致，可以查询多张表，多次查询语句时如果列名不一样，则取第一次的列名！如果不同的语句中取出的行的每个列的值都一样，那么结果将自动会去重复，如果不想去重复则要加all来声明，即union all）

           ## 现有表a如下

                id  num

                a    5

                b    10

                c    15

                d    10

            表b如下

                id  num

                b    5

                c    10

                d    20

                e    99

            求两个表中id相同的和

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

            //以上查询结果在本例中的确能正确输出结果，但是，如果把tb中的b的值改为10以查询结果的b的值就是10了，因为ta中的b也是10，所以union后会被过滤掉一个重复的结果，这时就要用union all

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

                

            #取第4、5栏目的商品，按栏目升序排列，每个栏目的商品价格降序排列，用union完成

            select goods_id,goods_name,cat_id,shop_price from goods where cat_id=4 union select goods_id,goods_name,cat_id,shop_price from goods where cat_id=5 order by cat_id,shop_price desc;

            【如果子句中有order by 需要用( ) 包起来，但是推荐在最后使用order by，即对最终合并后的结果来排序】

            #取第3、4个栏目，每个栏目价格最高的前3个商品，结果按价格降序排列

             (select goods_id,goods_name,cat_id,shop_price from goods where cat_id=3 order by shop_price desc limit 3) union  (select goods_id,goods_name,cat_id,shop_price from goods where cat_id=4 order by shop_price desc limit 3) order by shop_price desc;

            

    四、左连接，右连接，内连接

 

                现有表a有10条数据，表b有8条数据，那么表a与表b的笛尔卡积是多少？

                    select * from ta,tb   //输出结果为8*10=80条

                  

            1、左连接

               以左表为准，去右表找数据，如果没有匹配的数据，则以null补空位，所以输出结果数>=左表原数据数

 

                语法：select n1,n2,n3 from ta left join tb on ta.n1= ta.n2 [这里on后面的表达式，不一定为=，也可以>，<等算术、逻辑运算符]【连接完成后，可以当成一张新表来看待，运用where等查询】

                 #取出价格最高的五个商品，并显示商品的分类名称

                select goods_id,goods_name,goods.cat_id,cat_name,shop_price from goods left join category on goods.cat_id = category.cat_id order by  shop_price desc limit 5;        

           2、右连接

                a left join b 等价于 b right join a

                推荐使用左连接代替右连接

                语法：select n1,n2,n3 from ta right join tb on ta.n1= ta.n2

           3、内连接

                查询结果是左右连接的交集，【即左右连接的结果去除null项后的并集（去除了重复项）】

                mysql目前还不支持 外连接（即左右连接结果的并集,不去除null项）

                语法：select n1,n2,n3 from ta inner join tb on ta.n1= ta.n2

        #########

                 例：现有表a

                        name  hot

                         a        12

                         b        10

                         c        15

                    表b:

                        name   hot

                          d        12

                          e        10

                          f         10

                          g        8

                    表a左连接表b，查询hot相同的数据

                    select a.*,b.* from a left join b on a.hot = b.hot

                    查询结果：

                        name  hot   name  hot

                          a       12     d       12

                          b       10     e       10

                          b       10     f        10

                          c       15     null    null

                    从上面可以看出，查询结果表a的列都存在，表b的数据只显示符合条件的项目        

                      再如表b左连接表a，查询hot相同的数据

                        select a.*,b.* from b left join a on a.hot = b.hot

                        查询结果为：

                        name  hot   name  hot

                          d       12     a       12

                          e        10    b       10

                          f        10     b      10

                          g        8     null    null

                    再如表a右连接表b，查询hot相同的数据

                        select a.*,b.* from a right join b on a.hot = b.hot

                        查询结果和上面的b left join a一样

                ###练习，查询商品的名称，所属分类，所属品牌

                    select goods_id,goods_name,goods.cat_id,goods.brand_id,category.cat_name,brand.brand_name from goods left join category on goods.cat_id = category.cat_id left join brand on goods.brand_id = brand.brand_id limit 5;

                    理解：每一次连接之后的结果都可以看作是一张新表

 

                ###练习，现创建如下表

create table m(

id int,

zid int,

kid int,

res varchar(10),

mtime date

) charset utf8;

insert into m values

(1,1,2,‘2:0‘,‘2006-05-21‘),

(2,3,2,‘2:1‘,‘2006-06-21‘),

(3,1,3,‘2:2‘,‘2006-06-11‘),

(4,2,1,‘2:4‘,‘2006-07-01‘);

create table t

(tid int,tname varchar(10)) charset utf8;

insert into t values

(1,‘申花‘),

(2,‘红牛‘),

(3,‘火箭‘);

　　

 

 要求按下面样式打印2006-0601至2006-07-01期间的比赛结果

                        样式：

                            火箭   2:0    红牛  2006-06-11

 

                        查询语句为：

                select zid,t1.tname as t1name,res,kid,t2.tname as t2name,mtime from m left join t as t1 on m.zid = t1.tid  

 left join t as t2 on m.kid = t2.tid where mtime between ‘2006-06-01‘ and ‘2006-07-01‘;

                    总结：可以对同一张表连接多次，以分别取多次数据