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
