SQL语言的设计与编写（下）

--SELECT查询精讲

概要：

SELECT[ALL | DISTINCT] #distinct 明显的，清楚的，有区别的

{*|table.*|[table.]field1[asalias1][,[table.]field2[as alias2]][.....]} #alias 别名，化名

FROM 表名

[WHERE...]

[GROUPBY...]

[HAVING...]

[ORDERBY ...]

[LIMITcount]

使用SELECT查询语言，目的是可以按用户的想法将数据查出来，将结果返回！

1、字段要列出要查询的字段

e.g. selectname,price from products;

selectprice,name from products;

select* from products;

selectproducts.* from products; #单表其实不需要使用表名

2、可以为每个字段起个别名【后面会用到（关键字，多表查询）】【表也可起别名（多表查询）】

e.g. selectname as bookname,price as bookprice from products;#使用别名；也可不加as；注意别名中有空格时，需要加单引号；

3、使用distinct作用与整个记录，取消重复的数据，只返回一个，而不是单独的一列

e.g. selectdistinct price ‘book price‘ from products;

4、在SQL语句中使用表达式的列（可以使用算术运算符，条件运算符，逻辑运算符...）

e.g. select1+2*3;

select8%5

updateproducts set num = num + 1 where id = 22;

selectname,price,price*0.7 as ‘discount price‘ from products where id <=15;

5、WHERE可以在SELECT/UPDATE/DELETE中

a）可使用的逻辑运算符号（将多个条件组合）

&&/AND ||/OR !/NOT

b)可使用的比较运算符号

=#判断是否相等，与程序中的==作用相同

<=>#判断是否相等，与=一致，但可以用于与NULL比较

!=/ <> #不等号

<

<=

>

>=

c）程序中没有的运算符

ISNULL #与‘<=>NULL‘ 相等

ISNOT NULL

BETWEENAND

e.g. select* from products where id between 10 and 20;

与 “select* from products where id >= 10 && id <= 20;”作用相同

NOTBETWEEN AND

IN

e.g. select* from products where id in(5,10,15,20);

updateproducts set num = 77 where id in(5,10,15,20);

deletefrom products where id in(5,10);

d）模糊查询

LIKE _（任意一个字符）和%（0个或多个任意字符）两个通配符号

e.g. select* from products where name like ‘______‘; #查找任意名字为6个字符的数据

select* from products where name like ‘%java%‘; #查询名字中包含有java的数据

NOTLIKE

e.g. select* from products where name not like ‘%java%‘; #查询名字中不包含java字样的数据。

REGEXP/RLIKE【正则表达式】 #RegExp 正则表达式

e.g. select* from products where name regexp ‘^java‘; #查找所有以java开头的数据

select* from products where name regexp ‘s$‘; #查找所有以s结尾的数据

6、多表查询（连接查询），比较常用 #ambiguous

e.g. selectcats.name,products.name from cats,products;

selectc.name cname,c.desn cdesn,p.name pname,p.price,p.desn pdesn,p.numfrom carts c,products as p;#将A表中的记录与B表中的记录依次匹配，得到A*B种结果【笛卡尔乘积】，该结果是没有意义的。

selectc.name cname,c.desn cdesn,p.name pname,p.price,p.desn pdesn,p.numfrom carts c,products as p where c.id=p.cid;

selectc.name cname,c.desn cdesn,p.name pname,p.price,p.desn pdesn,p.numfrom carts c,products as p where c.id=p.cid and c.id=3;

selecta.id aid,a.name aname,b.id bid,b.name bname from cats a,catsb; #将单表分为多表，进行查询

selecta.id aid,a.name aname,b.id bid,b.name bname from cats a,cats b wherea.pid = b.id;

7、嵌套查询子查询

e.g. select* from products where cid in(select id from carts where name regexp‘^j‘) ;

select* from products where cid in(select id from carts where name like‘j%‘); #作用相同

8、orderby 字段 [asc正序]desc倒序

e.g. select* from order by name;

select* from order by price; #按价格非递减排序

select* from order by price desc; #非递增排序

select* from where cid > 5 order by price desc; #与where结合使用

9、limitcount【限制显示个数】

e.g. select* from limit 7;

select* from order by id desc limit 7;

select* from where id < 10 order by id desc limit 7;

select* from where id > 14 order by id asc limit 0,1; #limit0,1表示从第0个开始取，取1个

10、groupby 字段【分组】

常用函数：

count() #一个字段的总数

sum()

avg()#平均值

max()

min()

e.g. selectcount(*),sum(price),avg(price),max(price),min(price) from products;

selectcid,count(price),sum(price),avg(price),max(price),min (price) fromproducts group by cid;

selectcid,count(price),sum(price),avg(price),max(price),min (price) fromproducts group by cid having avg(price) > 50; #加having条件，与where类似

#having必须与gropby结合才能使用