Mysql数据库动态库： libmysql.dll libmysql.lib mysql.h WinSock2.h
Mysql API数据结构 (mysql.h)
MYSQL:连接数据库前，必须先创建MYSQL变量，此变量在很多Mysql API函数会用到。它包含了一些连接信息等数据。
MYSQL_RES:MYSQL_RES结构体中包含了查询结果集，也就是从数据库中查询到的数据。可以使用mysql_store_result或mysql_use_result函数获得。
MYSQL_ROW:MYSQL ROW的定义如下：typedef char **MYSQL_ROW;可见，它实际上是char **类型，指向一个字符串数组。存储一行中各段字符数组，可以通过mysql_fetch_row函数获得。
MYSQL_FIELD:MYSQL_FIELD中包含了字段名、字段类型和大小等信息。可以重复调用mysql_fetch_field函数获得所有字段的信息。
Mysql C API编程步骤
1、首先我们要包含mysql的头文件，并链接mysql动态库。
#include <WinSock2.h> // 进行网络编程需要winsock2.h
#include <mysql.h>
#pragma comment(lib, "libmysql.lib")
2、创建MYSQL变量。如：
MYSQL mysql;
3、初始化MYSQL变量。
mysql_init(&mysql);
4、调用mysql_real_connect函数连接Mysql数据库。
mysql_real_connect函数的MYSQL * STDCALL mysql_real_connect(MYSQL *mysql, const char *host,const char *user,const char *passwd,const char *db,unsigned int port,const char *unix_socket,unsigned long clientflag);
参数说明：mysql--前面定义的MYSQL变量；host--MYSQL服务器的地址；user--登录用户名；passwd--登录密码；db--要连接的数据库；port--MYSQL服务器的TCP服务端口；unix_socket--unix连接方式，为NULL时表示不使用socket或管道机制；clientflag--Mysql运行为ODBC数据库的标记，一般取0。
连接失败时该函数返回0。
5、调用mysql_real_query函数进行数据库查询。mysql_real_query函数的原型如下：
int STDCALL mysql_real_query(MYSQL *mysql, const char *q, unsigned long length);
参数说明：mysql--前面定义的MYSQL变量；q--SQL查询语句；length--查询语句的长度。
查询成功则该函数返回0。
6、通过调用mysql_store_result或mysql_use_result函数返回的MYSQL_RES变量获取查询结果数据。
两个函数的原型分别为：
MYSQL_RES * STDCALL mysql_store_result(MYSQL *mysql);
MYSQL_RES * STDCALL mysql_use_result(MYSQL *mysql);
这两个函数分别代表了获取查询结果的两种方式。第一种，调用mysql_store_result函数将从Mysql服务器查询的所有数据都存储到客户端，然后读取；第二种，调用mysql_use_result初始化检索，以便于后面一行一行的读取结果集，而它本身并没有从服务器读取任何数据，这种方式较之第一种速度更快且所需内存更少，但它会绑定服务器，阻止其他线程更新任何表，而且必须重复执行mysql_fetch_row读取数据，直至返回NULL，否则未读取的行会在下一次查询时作为结果的一部分返回，故经常我们使用mysql_store_result。
7、调用mysql_fetch_row函数读取结果集数据。
上述两种方式最后都是重复调用mysql_fetch_row函数读取数据。mysql_fetch_row函数的原型如下：
MYSQL_ROW STDCALL mysql_fetch_row(MYSQL_RES *result);
参数result就是mysql_store_result或mysql_use_result的返回值。
该函数返回MYSQL_ROW型的变量，即字符串数组，假设为row，则row[i]为第i个字段的值。当到结果集尾部时，此函数返回NULL。
8、结果集用完后，调用mysql_free_result函数释放结果集，以防内存泄露。mysql_free_result函数的原型如下：
void STDCALL mysql_free_result(MYSQL_RES *result);
9、不再查询Mysql数据库时，调用mysql_close函数关闭数据库连接。mysql_close函数的原型为：
void STDCALL mysql_close(MYSQL *sock);
例子：
1. int main() {
2. MYSQL mysql;
3. MYSQL_RES *res;
4. MYSQL_ROW row;
5. mysql_init(&mysql); // 初始化MYSQL变量
6. // 连接Mysql服务器，本例使用本机作为服务器。访问的数据库名称为"msyql"，参数中的user为你的登录用户名，***为登录密码，需要根据你的实际用户进行设置
7. if (!mysql_real_connect(&mysql, "127.0.0.1", "user", "123", "mysql", 3306, 0, 0)) {
8. cout << "mysql_real_connect failure!" << endl;
9. return 0;
10. }
11. if (mysql_real_query(&mysql, "select * from user", (unsigned long)strlen("select * from user"))){ // 查询mysql数据库中的user表
12. cout << "mysql_real_query failure!" << endl;
13. return 0;
14. } // 存储结果集
15. res = mysql_store_result(&mysql);
16. if (NULL == res) {
17. cout << "mysql_store_result failure!" << endl;
18. return 0;
19. }
20. // 重复读取行，并输出第一个字段的值，直到row为NULL
21. while (row = mysql_fetch_row(res)) {
22. cout << row[0] << endl;
23. }
24. mysql_free_result(res); // 释放结果集
25. mysql_close(&mysql); // 关闭Mysql连接
26. return 0; }
10.Char * mysql_get_client_info() 显示mysql客户端版本
MySQL client version: 5.0.38
11.int mysql_num_fields(MYSQL_RES *result) 返回结果子表中域(字段)的个数
12. MYSQL_FIELD * mysql_fetch_field(MYSQL_RES *result) 返回结果子表中的域结构体指针
13.Void mysql_real_escape_string(MYSQL* con, char* savedata, char *data, int size) 在将二进制数据(非文本)保存到数据库之前，需要转义，否则数据库不能正常保存，取出数据时，无需解转移。转义时一个字符转义后2个字符，所以savedata内存必须为data的2倍
14. unsigned long * mysql_fetch_lengths(MYSQL_RES *result) 获取结果中各个字符串的长度，返回为1维数组
一些有用的例子：
#include
#include
int main(int argc, char **argv)
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW row;
int num_fields;
int i;
conn = mysql_init(NULL);
mysql_real_connect(conn, "localhost", "zetcode", "passwd", "testdb", 0, NULL, 0);
mysql_query(conn, "SELECT * FROM writers");
result = mysql_store_result(conn);
num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
for(i = 0; i < num_fields; i++)
{
printf("%s ", row[i] ? row[i] : "NULL");
}
printf("\n");
}
mysql_free_result(result);
mysql_close(conn);
}
The example prints all names from the writers table.
$ ./select
Leo Tolstoy
Jack London
Honore de Balzac
Lion Feuchtwanger
Emile Zola
mysql_query(conn, "SELECT * FROM writers");
We execute the query, that will retrieve all names from the writers database.
result = mysql_store_result(conn);
We get the result set.
num_fields = mysql_num_fields(result);
We get the number of fields in the table.
while ((row = mysql_fetch_row(result)))
{
for(i = 0; i < num_fields; i++)
{
printf("%s ", row[i] ? row[i] : "NULL");
}
printf("\n");
}
We fetch the rows and print them to the screen.
mysql_free_result(result);
We free the resources.
Column headers
In the next example, we will retrieve data and show the their column names from the table.
For this, we will create a new table friends.
mysql> create table friends (id int not null primary key auto_increment,
name varchar(20), age int);
mysql> insert into friends(name, age) values('Tom', 25);
mysql> insert into friends(name, age) values('Elisabeth', 32);
mysql> insert into friends(name, age) values('Jane', 22);
mysql> insert into friends(name, age) values('Luke', 28);
We insert some data into the table.
#include
#include
int main(int argc, char **argv)
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW row;
MYSQL_FIELD *field;
int num_fields;
int i;
conn = mysql_init(NULL);
mysql_real_connect(conn, "localhost", "zetcode", "passwd", "testdb", 0, NULL, 0);
mysql_query(conn, "SELECT * FROM friends");
result = mysql_store_result(conn);
num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
for(i = 0; i < num_fields; i++)
{
if (i == 0) {
while(field = mysql_fetch_field(result)) {
printf("%s ", field->name);
}
printf("\n");
}
printf("%s ", row[i] ? row[i] : "NULL");
}
}
printf("\n");
mysql_free_result(result);
mysql_close(conn);
}
The example is similar to the previous one. It just adds column header names to it.
while(field = mysql_fetch_field(result)) {
printf("%s ", field->name);
}
The mysql_fetch_field() call returns a MYSQL_FIELD structure. We get the column header names from this structure.
$ ./headers
id name age
1 Tom 25
2 Elisabeth 32
3 Jane 22
4 Luke 28
And this is the output of our program.
Inserting p_w_picpaths into MySQL database
Some people prefer to put their p_w_picpaths into the database, some prefer to keep them on the file system for their applications. Technical difficulties arise when we work with millions of p_w_picpaths. Images are binary data. MySQL database has a special data type to store binary data called BLOB (Binary Large Object).
mysql> describe p_w_picpaths;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| data | mediumblob | YES | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
This is the table, that we will use in our example. It can be created by the following SQL statement.
create table p_w_picpaths(id int not null primary key, data mediumblob);
#include
#include
int main(int argc, char **argv)
{
MYSQL *conn;
int len, size;
char data[1000*1024];
char chunk[2*1000*1024+1];
char query[1024*5000];
FILE *fp;
conn = mysql_init(NULL);
mysql_real_connect(conn, "localhost", "zetcode", "passwd", "testdb", 0, NULL, 0);
fp = fopen("p_w_picpath.png", "rb");
size = fread(data, 1, 1024*1000, fp);
mysql_real_escape_string(conn, chunk, data, size);
char *stat = "INSERT INTO p_w_picpaths(id, data) VALUES('1', '%s')";
len = snprintf(query, sizeof(stat)+sizeof(chunk) , stat, chunk);
mysql_real_query(conn, query, len);
fclose(fp);
mysql_close(conn);
}
In this example, we will insert one p_w_picpath into the p_w_picpaths table. The p_w_picpath can be max 1 MB.
fp = fopen("p_w_picpath.png", "rb");
size = fread(data, 1, 1024*1000, fp);
Here we open the p_w_picpath and read it into the data array.
mysql_real_escape_string(conn, chunk, data, size);
Binary data can obtain special characters, that might cause troubles in the statements. We must escape them. The mysql_real_escape_string() puts the encoded data into the chunk array. In theory, every character might be a special character. That's why the chunk array two times as big as the data array. The function also adds a terminating null character.
char *stat = "INSERT INTO p_w_picpaths(id, data) VALUES('1', '%s')";
len = snprintf(query, sizeof(stat)+sizeof(chunk) , stat, chunk);
These two code lines prepare the MySQL query.
mysql_real_query(conn, query, len);
Finally, we execute the query.
Selecting p_w_picpaths from MySQL database
In the previous example, we have inserted an p_w_picpath into the database. In the following example, we will select the inserted p_w_picpath back from the database.
#include
#include
int main(int argc, char **argv)
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW row;
unsigned long *lengths;
FILE *fp;
conn = mysql_init(NULL);
mysql_real_connect(conn, "localhost", "zetcode", "passwd", "testdb", 0, NULL, 0);
fp = fopen("p_w_picpath.png", "wb");
mysql_query(conn, "SELECT data FROM p_w_picpaths WHERE id=1");
result = mysql_store_result(conn);
row = mysql_fetch_row(result);
lengths = mysql_fetch_lengths(result);
fwrite(row[0], lengths[0], 1, fp);
mysql_free_result(result);
fclose(fp);
mysql_close(conn);
}
In this example, we will create an p_w_picpath file from the database.
fp = fopen("p_w_picpath.png", "wb");
We open a file for writing.
mysql_query(conn, "SELECT data FROM p_w_picpaths WHERE id=1");
We select an p_w_picpath with id 1.
row = mysql_fetch_row(result);
The row contains raw data.
lengths = mysql_fetch_lengths(result);
We get the length of the p_w_picpath.
fwrite(row[0], lengths[0], 1, fp);
We create the p_w_picpath file using the fwrite() standard function call.
