好了,我們知道了怎麼調用 sqlite3 的C/C++的API函數接口去創建數據庫、創建表格、並插入數據,
下面我們看看怎麼在C語言中查詢數據庫中的數據。
好的,我們現編輯一段c代碼,取名爲 query.c
// name: query.c
// This prog is used to test C/C++ API for sqlite3 .It is very simple,ha !
// Author : zieckey All rights reserved.
// data : 2006/11/18
#include <stdio.h>
#include <stdlib.h>
#include "sqlite3.h"
#define _DEBUG_
int main( void )
{
sqlite3 *db=NULL;
char *zErrMsg = 0;
int rc;
rc = sqlite3_open("zieckey.db", &db); //打開指定的數據庫文件,如果不存在將創建一個同名的數據庫文件
if( rc )
{
fprintf(stderr, "Can't open database: %s
", sqlite3_errmsg(db));
sqlite3_close(db);
exit(1);
}
else printf("You have opened a sqlite3 database named zieckey.db successfully!
Congratulations! Have fun ! ^-^
");
//創建一個表,如果該表存在,則不創建,並給出提示信息,存儲在 zErrMsg 中
char *sql = " CREATE TABLE SensorData(
ID INTEGER PRIMARY KEY,
SensorID INTEGER,
SiteNum INTEGER,
Time VARCHAR(12),
SensorParameter REAL
);" ;
sqlite3_exec( db , sql , 0 , 0 , &zErrMsg );
#ifdef _DEBUG_
printf("zErrMsg = %s
", zErrMsg);
#endif
//插入數據
sql = "INSERT INTO "SensorData" VALUES(NULL , 1 , 1 , '200605011206', 18.9 );" ;
sqlite3_exec( db , sql , 0 , 0 , &zErrMsg );
sql = "INSERT INTO "SensorData" VALUES(NULL , 1 , 1 , '200605011306', 16.4 );" ;
sqlite3_exec( db , sql , 0 , 0 , &zErrMsg );
int nrow = 0, ncolumn = 0;
char **azResult; //二維數組存放結果
//查詢數據
sql = "SELECT * FROM SensorData ";
sqlite3_get_table( db , sql , &azResult , &nrow , &ncolumn , &zErrMsg );
int i = 0 ;
printf( "row:%d column=%d
" , nrow , ncolumn );
printf( "
The result of querying is :
" );
for( i=0 ; i<( nrow + 1 ) * ncolumn ; i++ )
printf( "azResult[%d] = %s
", i , azResult[i] );
//釋放掉 azResult 的內存空間
sqlite3_free_table( azResult );
#ifdef _DEBUG_
printf("zErrMsg = %s
", zErrMsg);
#endif
sqlite3_close(db); //關閉數據庫
return 0;
}
我們這裏用到了一個查詢的語句是 "SELECT * FROM SensorData " ,
在C語言中對應的函數接口是 sqlite3_get_table( db , sql , &azResult , &nrow , &ncolumn , &zErrMsg );
這個函數接口的解釋在程序中已經註釋。
下面我們編譯運行下看看,
[root@localhost temp]# export LD_LIBRARY_PATH=/usr/local/sqlite3/lib:$LD_LIBRARY_PATH
[root@localhost temp]# gcc query.c -lsqlite3 -L/usr/local/sqlite3/lib -I/usr/local/sqlite3/include
query.c:29:21: warning: multi-line string literals are deprecated
[root@localhost temp]# ./a.out
You have opened a sqlite3 database named zieckey.db successfully!
Congratulations! Have fun ! ^-^
zErrMsg = (null)
row:2 column=5
The result of querying is :
azResult[0] = ID
azResult[1] = SensorID
azResult[2] = SiteNum
azResult[3] = Time
azResult[4] = SensorParameter
azResult[5] = 1
azResult[6] = 1
azResult[7] = 1
azResult[8] = 200605011206
azResult[9] = 18.9
azResult[10] = 2
azResult[11] = 1
azResult[12] = 1
azResult[13] = 200605011306
azResult[14] = 16.4
zErrMsg = (null)
這裏我們可以看到,azResult 的前面 5 個數據正好是我們的表 SensorData 的列屬性,
之後纔是我們要查詢的數據。所以我們的程序中才有 i<( nrow + 1 ) * ncolumn 的判斷條件:
for( i=0 ; i<( nrow + 1 ) * ncolumn ; i++ )
printf( "azResult[%d] = %s ", i , azResult[i] );
輸出中有 zErrMsg = (null) 這樣的字句,這是 zErrMsg 保留的錯誤信息,
正如你所看到的,zErrMsg 爲空,表明在執行過程中沒有錯誤信息。