mysql_num_fields与mysql_field_count的区别

mysql_num_fields和mysql_field_count都是获取结果中列的个数，但是这两个函数在使用的时候是存在区别的。

先看一段代码（上篇文章提供的数据库）：

[cpp] view plaincopyprint?

1. #include <mysql/mysql.h> 
2. #include <stdio.h> 
3. #include <stdlib.h> 
4. #include <string.h> 
5.  
6. int main() 
7. { 
8.     char out[100]; 
9.     MYSQL mysql; 
10.     MYSQL_RES* res; 
11.  
12.     int num_fields; 
13.     int count_fields; 
14.     int num_rows; 
15.  
16.     mysql_init(&mysql); 
17.     mysql_real_connect(&mysql,NULL,"root","123","ta",0,NULL,0); 
18.  
19.     fprintf(stdout,"--insert------------------------/n"); 
20.  
21.     mysql_real_query(&mysql,"INSERT INTO t1 VALUES(6,'n6')",29); 
22.  
23.     count_fields = mysql_field_count(&mysql); 
24.     sprintf(out,"insert----count_fields is %d/n",count_fields); 
25.     fprintf(stdout,out); 
26.  
27.     res = mysql_store_result(&mysql); 
28.     if(res == NULL) 
29.     { 
30.         fprintf(stdout,"insert---store return null/n"); 
31.     } 
32.     else 
33.     { 
34.         num_fields = mysql_num_fields(res); 
35.         sprintf(out,"insert---num_fields is %d/n",num_fields); 
36.         fprintf(stdout,out); 
37.     } 
38.  
39.     fprintf(stdout,"---select-----------/n"); 
40.  
41.     mysql_real_query(&mysql,"SELECT * FROM t1",16); 
42.  
43.     count_fields = mysql_field_count(&mysql); 
44.     sprintf(out,"select---count_fields is %d/n",count_fields); 
45.     fprintf(stdout,out); 
46.  
47.     res = mysql_store_result(&mysql); 
48.     if(res == NULL) 
49.     { 
50.         fprintf(stdout,"select---store return null/n"); 
51.     } 
52.     else 
53.     { 
54.         num_fields = mysql_num_fields(res); 
55.         sprintf(out,"select---num_fields is %d/n",num_fields); 
56.         fprintf(stdout,out); 
57.  
58.         num_rows = mysql_num_rows(res); 
59.         sprintf(out,"select---num_rows is %d/n",num_rows); 
60.         fprintf(stdout,out); 
61.     } 
62.  
63.     fprintf(stdout,"---select 0------/n"); 
64.  
65.     mysql_real_query(&mysql,"SELECT * FROM t1 where id = 10",30); 
66.  
67.     count_fields = mysql_field_count(&mysql); 
68.     sprintf(out,"select 0---count_fields is %d/n",count_fields); 
69.     fprintf(stdout,out); 
70.  
71.     res = mysql_store_result(&mysql); 
72.     if(res == NULL) 
73.     { 
74.         fprintf(stdout,"select 0---store return null/n"); 
75.     } 
76.     else 
77.     { 
78.         num_fields = mysql_num_fields(res); 
79.         sprintf(out,"select 0---num_fields is %d/n",num_fields); 
80.         fprintf(stdout,out); 
81.  
82.         num_rows = mysql_num_rows(res); 
83.         sprintf(out,"select 0---num_rows is %d/n",num_rows); 
84.         fprintf(stdout,out); 
85.     } 
86.  
87.     mysql_free_result(res); 
88.     mysql_close(&mysql); 
89.     exit(0); 
90. } 

#include <mysql/mysql.h>#include <stdio.h>#include <stdlib.h>#include <string.h>int main(){ char out[100]; MYSQL mysql; MYSQL_RES* res; int num_fields; int count_fields; int num_rows; mysql_init(&mysql); mysql_real_connect(&mysql,NULL,"root","123","ta",0,NULL,0); fprintf(stdout,"--insert------------------------/n"); mysql_real_query(&mysql,"INSERT INTO t1 VALUES(6,'n6')",29); count_fields = mysql_field_count(&mysql); sprintf(out,"insert----count_fields is %d/n",count_fields); fprintf(stdout,out); res = mysql_store_result(&mysql); if(res == NULL) { fprintf(stdout,"insert---store return null/n"); } else { num_fields = mysql_num_fields(res); sprintf(out,"insert---num_fields is %d/n",num_fields); fprintf(stdout,out); } fprintf(stdout,"---select-----------/n"); mysql_real_query(&mysql,"SELECT * FROM t1",16); count_fields = mysql_field_count(&mysql); sprintf(out,"select---count_fields is %d/n",count_fields); fprintf(stdout,out); res = mysql_store_result(&mysql); if(res == NULL) { fprintf(stdout,"select---store return null/n"); } else { num_fields = mysql_num_fields(res); sprintf(out,"select---num_fields is %d/n",num_fields); fprintf(stdout,out); num_rows = mysql_num_rows(res); sprintf(out,"select---num_rows is %d/n",num_rows); fprintf(stdout,out); } fprintf(stdout,"---select 0------/n"); mysql_real_query(&mysql,"SELECT * FROM t1 where id = 10",30); count_fields = mysql_field_count(&mysql); sprintf(out,"select 0---count_fields is %d/n",count_fields); fprintf(stdout,out); res = mysql_store_result(&mysql); if(res == NULL) { fprintf(stdout,"select 0---store return null/n"); } else { num_fields = mysql_num_fields(res); sprintf(out,"select 0---num_fields is %d/n",num_fields); fprintf(stdout,out); num_rows = mysql_num_rows(res); sprintf(out,"select 0---num_rows is %d/n",num_rows); fprintf(stdout,out); } mysql_free_result(res); mysql_close(&mysql); exit(0);}

执行结果是：
--insert------------------------
insert----count_fields is 0
insert---store return null
---select-----------
select---count_fields is 2
select---num_fields is 2
select---num_rows is 7
---select 0------
select 0---count_fields is 2
select 0---num_fields is 2
select 0---num_rows is 0

显然，当执行insert语句的时候，是没有结果返回的，因此列的个数为0，且mysql_store_result返回NULL。因此可以通过mysql_field_count()是否返回0来判断是否有结果返回，而不需要执行mysql_store_result来判断是否返回了NULL。我想，mysql_field_count()的效率肯定要比mysql_store_result()高。

在这种情况下，由于没有返回结果，因此mysql_store_result()返回NULL，也就是得不到res指针，于是mysql_num_fields()函数就无法执行，缺少必要的参数。

当执行第一条select语句的时候，返回了结果，因此mysql_field_count()和mysql_num_fields()都返回了正确的列的个数2，mysql_num_rows()返回了记录的条数7.

当执行第二条select语句，由于表中没有 id = 0 的记录，因此mysql_num_rows返回了0表示记录数为0，但是，我们发现mysql_store_result()并没有返回NULL，mysql_num_fields()和mysql_field_count()还是返回了2.

因此我们可以得出这样的结论：

执行结果有三种情况，第一是执行insert、update和delete这样的语句的时候，是不会有任何内容返回，因此mysql_store_result()会返回一个NULL。第二，执行select或show这样的语句时，一定会有内容返回，可以取得列信息，但是记录可以为0，也可以不为0。这就像一个表，表头一定存在，但是表中可以没有数据。

另外：

mysql_affected_rows()函数的作用是，当执行update  insert  delete 时，返回影响的行数。

mysql中文文档：

http://dev.mysql.com/doc/refman/5.1/zh/apis.html