执行SQL语句---SELECT

1、通常从MySQL数据库中检索数据有4个步骤：

（1）发出查询: 用mysql_query发出查询。

（2）检索数据: 用mysql_store_result/mysql_use_result

（3）处理数据：用mysql_fetch_row, mysql_fetch_field

（4）整理所需要的数据:用mysql_free_result允许MySQL进行必要的整理工作。

2、例子：结构化的数据库查询输出

/*
* MysqlQuery2.c
*
*  Created on: Sep 8, 2013
*      Author: root
*/
#include <stdlib.h>
#include <stdio.h>
#include <mysql/mysql.h>
MYSQL my_connection;
MYSQL_RES * res_ptr;
MYSQL_ROW sqlrow;
void display_header();
void display_row(MYSQL * ptr);

int main(){
int res;
int first_row = 1;
mysql_init(&my_connection);
if(mysql_real_connect(&my_connection, "localhost", "root", "ROOT123456", "icmp", 0, NULL, 0)){
printf("Connection success.\n");
res = mysql_query(&my_connection, "select childno, fname, age from children where age > 5");
if(res){
printf("Select error:%s \n", mysql_error(&my_connection));
return -2;
}else{
res_ptr = mysql_use_result(&my_connection);
if(res_ptr){
display_header();
while((sqlrow = mysql_fetch_row(res_ptr))){
if(first_row){
display_header();
first_row = 0;
}
display_row(&my_connection);
}
if(mysql_errno(&my_connection)){
fprintf(stderr, "Retrieve error:%s\n", mysql_error(&my_connection));
return -3;
}
}
mysql_free_result(res_ptr);
}
mysql_close(&my_connection);
printf("Connection closed.\n");
}else{
fprintf(stderr, "Connection failed.\n");
if(mysql_errno(&my_connection)){
fprintf(stderr, "Connection error %d %s\n", mysql_errno(&my_connection), mysql_error(&my_connection));
return -1;
}
}
return 0;
}

void display_header(){
MYSQL_FIELD * field_ptr;
printf("Column details:\n");
while((field_ptr = mysql_fetch_field(res_ptr)) != NULL){
printf("\tName: %s\n", field_ptr->name);
printf("\tType:");
if(IS_NUM(field_ptr->type)){
printf("Numeric field\n");
}else{
switch(field_ptr->type){
case FIELD_TYPE_VAR_STRING:
printf("VARCHAR\n");break;
case FIELD_TYPE_LONG:
printf("LONG\n");break;
default:
printf("Type is %d, check in mysql_com.h\n", field_ptr->type);
}
}

printf("\tMax width %d\n", field_ptr->length);
if(field_ptr->flags & AUTO_INCREMENT_FLAG){
printf("\t Auto increments\n");
}
printf("\n");
}
}

void display_row(MYSQL * ptr){
unsigned int field_count;
field_count = 0;
while(field_count < mysql_field_count(ptr)){
printf("%s ", sqlrow[field_count]);
field_count++;
}
printf("\n");
}

运行结果：

Connection success.
Column details:
Name: childno
Type:Numeric field
Max width 11
Auto increments

Name: fname
Type:VARCHAR
Max width 30

Name: age
Type:Numeric field
Max width 11

Column details:
1 wangle 28
2 xuyehui 29
Connection closed.