c语言里所有以#开头的都是预编译指令,就是在正式编译之前,让编译器做一些预处理的工作. #ifdef DEBUG printf("variable x has value = %d\n",x); #endif #if和#endif是配对的,叫做条件编译指令,如果满足#if后面的条件,就编译#if和#endif之间的程序段,否则不编译.比如这段程序,如果常量OS_CRITICAL_METHOD的值为3就编译下面的一条语句 #ifdef 和 #if 效果是一样的,但是当你要判断复杂的条件时
1.去重语句:DELETE FROM `v_klg_item` WHERE id NOT IN (SELECT * FROM (SELECT MAX(id) FROM `v_klg_item` GROUP BY NAME) AS tmp); 2.数据库不同表之间列的复制语句:INSERT INTO crm_add (id,address,pid) SELECT id,NAME,parentId FROM labos_area;
select *,(2 * 6378.137* ASIN(SQRT(POW(SIN(PI()*(111.86141967773438-latitude)/360),2)+COS(PI()*33.07078170776367/180)* COS(latitude * PI()/180)*POW(SIN(PI()*(33.07078170776367-longitude)/360),2)))) as juli from school_base_infoorder by juli asc limit
1.extern 修饰一个变量,告诉编译器这个变量在其他地方定义,编译器不会给出变量未定义的警告. extern tells the compiler that the variable is defined somewhere else, so it doesn't complain about it being undefined.--includes.hextern int count;--main.cpp#include "includes.h"int count = 4;--