top N percent (),
with ties order by,top ,
distinct 2.Count()
count(*) ,avg,max,min,sum
3.Group by ,:,,Having group by where ,where,having.
4.RCube Compute Compute by 5. : 1 inner join 2 on :select distinct (lastname+' '+firstname) as name,orderid from employees as e inner join orders as o on e.employeeid=o.employeeid
:,:table1,table1table2,table2Table1 left join table2 on expression
Table1 left outer join table2 on expression
:table2,table2table1,table1Table1 right join table2 on expression
Table1 right outer table2 on expression
:,.
Table1 full join table2 on expression
Table1expression
:where,table1,table2,Table1 cross join table2
6.Union [all],all
.
7.
DML,.32,,.
:
,where.
,.
先外层查询再内层查询再外层再内层
:select lastname,firstname from employees as e where '1998-1-1' in (select orderdate from orders as o where o.employeeid=e.employeeid)
exists(not exists),,true ,false
:select lastname,firstname from employees as e where exists (select * from orders as o where o.employeeid=e.e.employeeid and o.orderdate='1998-1-1')
where.
,.
:select lastname,firstname from employees where employeesid in (select employeesid from orders where orders.orderdate='1998-1-1')
子查询可以作为派生的?/p>
可以作ؓ表达?/p>
Select productname,unitprice,(unitprice-(select avg(unitprice) from products) ) as diff from products where productname='chai'
模拟联接子句
having
包含命oQ?/span>require("文g?/span>");
include("文g?/span>");
注释Q?/span>//,#,/**/
输出命oQ?/span>echo("字符串或者是变量?/span>");print("字符?/span>");
变量的定义：$变量名＝字符Ԍ数字Q数l；
数组变量的定义以及引用：$数组名＝array("数组元素","数组元素");
$数组?/span>[数组下标];
变量名区分大写Q变量名不用声明cd?/p>
字符串连接：$变量?/span>.$变量?/span>
q算W号Q＋Q－Q?/span>*,/,%,++,--
逻辑q算W：<,>,<=,>=,==,!=,&&,and,||,or,xor,!
程控制
If(expr){
statement
}
If(expr){
statement
}else{
statement
}
If(expr){
statement
}elseif{
statement
}
For(expr1;expr2;expr3){
statement
}
While(expr){
statement
}
Switch(expr){
case expr1:
statement;
break;
case expr2:
statement;
break;
default:
statement;
break;
}
数据库管?/p>
phpMyadmin 理
数据库操?/p>
打开一个数据库q接Q?/span>$link=mysql_connect("服务器名"Q“用户名”，“密码?/span>) or die("can't connect db:".mysql_error());
关闭数据库连接：mysql_close($link);
选择数据库：mysql_select_db("数据库名",$link) or die("could't open db:".mysql_error());
d数据表数据：$result=mysql_query("sql语句").die("eror:".mysql_error());如果?/span>selectq回新的ID?/span>,如果sql语句?/span>update ,insert,deleteq回boolean?/span>
q回cd量：$rs=mysql_fetch_object($result);把查询结果放到类变量$rs里，如果查询l果为空则类变量的gؓflase.
Session的用：
使用session前：session_start();
生成session:$_session['session的名U?/span>']="session?/span>";
清除session:$_session['session的名U?/span>']="";
得到面表单值或url传|$_Post['字段?/span>']Q?/span>$_Get['字段?/span>'];Ҏ表单Ҏ选择取值办法。取url的值用$_GET['字段?/span>'];
int b=10;
则a==b是true?BR>但不好理解的地方是：
String a=new String("foo");
String b=new String("foo");
则a==b返回false?BR>
对象变量其实是一个引用，它们的值是指向对象所在的内存地址Q而不是对象本w。a和b都用了new操作W，意味着在内存中生两个内容ؓ"foo"的字W串Q既然是“两个”，它们自然位于不同的内存地址。a和b的值其实是两个不同的内存地址的|所以?=="操作W，l果会是false。诚Ӟa和b所指的对象Q它们的内容都是"foo"Q应该是“相{”，但是==操作Wƈ不涉及到对象内容的比较?BR>对象内容的比较，正是equalsҎ做的事?BR>
看一下Object对象的equalsҎ是如何实现的Q?BR>boolean equals(Object o){
return this==o;
}
Object对象默认使用?=操作W。所以如果你自创的类没有覆盖equalsҎQ那你的cM用equals和?=会得到同Ll果。同样也可以看出QObject的equalsҎ没有辑ֈequalsҎ应该辑ֈ的目标：比较两个对象内容是否相等。因为答案应该由cȝ创徏者决定，所以Object把这个Q务留l了cȝ创徏者?BR>
看一下一个极端的c：
Class Monster{
private String content;
...
boolean equals(Object another){ return true;}
}
我覆盖了equalsҎ。这个实CD无论Monster实例内容如何Q它们之间的比较永远q回true?BR>
所以当你是用equalsҎ判断对象的内Ҏ否相{，请不要想当然。因为可能你认ؓ相等Q而这个类的作者不q样认ؓQ而类的equalsҎ的实现是׃掌握的。如果你需要用equalsҎQ或者用Q何基于散列码的集合（HashSet,HashMap,HashTableQ，请察看一下java doc以确认这个类的equals逻辑是如何实现的?BR>