Sqlite 简明教程

SQLite - C/C++

在本章中,您将学习如何在 C/C++ 程序中使用 SQLite。

Installation

在 C/C++ 程序中开始使用 SQLite 之前,需要确保计算机上已设置好 SQLite 库。您可以查看 SQLite 安装章节以了解安装过程。

C/C++ Interface APIs

以下是重要的 C/C SQLite 接口例程,这些例程可满足您使用 C/C 程序处理 SQLite 数据库的要求。如果您正在寻找更复杂的应用程序,则可以查看 SQLite 官方文档。

Sr.No.

API & Description

1

sqlite3_open(const char *filename, sqlite3 **ppDb) 此例程打开与 SQLite 数据库文件的连接,并返回一个供其他 SQLite 例程使用的数据库连接对象。如果 filename 参数为 NULL 或 ':memory:',则 sqlite3_open() 将在 RAM 中创建一个内存中数据库,并且该数据库仅持续会话的持续时间。如果 filename 不为 NULL,则 sqlite3_open() 将尝试使用其值打开数据库文件。如果不存在该名称的文件,则 sqlite3_open() 将通过该名称打开一个新的数据库文件。

2

sqlite3_exec(sqlite3 , const char sql, sqlite_callback, void *data, char **errmsg) 此例程提供了一种快速简便的方法来执行由 sql 参数提供的 SQL 命令,其中可以包含多个 SQL 命令。此处,第一个参数 sqlite3 是一个开放的数据库对象,sqlite_callback 是一个回调,其中数据是第一个参数,并将返回 errmsg 以捕获该例程引发的任何错误。SQLite3_exec() 例程解析并执行 sql 参数中给出的每个命令,直到达到字符串结尾或遇到错误。

3

sqlite3_close(sqlite3 )*此例程关闭先前通过调用 sqlite3_open() 打开的数据库连接。应该在关闭连接之前完成与连接关联的所有准备好的语句。如果仍然有任何尚未完成的查询,则 sqlite3_close() 将返回 SQLITE_BUSY,并显示错误消息因未完成的语句而无法关闭。

Connect To Database

以下 C 代码片段演示了如何连接到现有数据库。如果数据库不存在,则将创建它,最后将返回一个数据库对象。

#include <stdio.h>
#include <sqlite3.h>

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;

   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stderr, "Opened database successfully\n");
   }
   sqlite3_close(db);
}

现在,让我们来编译并运行上面的程序,在当前目录中创建我们的数据库 test.db 。您可以根据您的要求更改您的路径。

$gcc test.c -l sqlite3
$./a.out
Opened database successfully

如果你打算使用 C++ 源代码,那么可以如下编译代码:

$g++ test.c -l sqlite3

此处,我们将我们的程序与 sqlite3 库链接,以向 C 程序提供所需函数。这会在你的目录中创建数据库文件 test.db,并且你会得到以下结果。

-rwxr-xr-x. 1 root root 7383 May 8 02:06 a.out
-rw-r--r--. 1 root root  323 May 8 02:05 test.c
-rw-r--r--. 1 root root    0 May 8 02:06 test.db

Create a Table

以下 C 代码段将用于在之前已创建的数据库中创建表:

#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>

static int callback(void *NotUsed, int argc, char **argv, char **azColName) {
   int i;
   for(i = 0; i<argc; i++) {
      printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
   }
   printf("\n");
   return 0;
}

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;
   char *sql;

   /* Open database */
   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stdout, "Opened database successfully\n");
   }

   /* Create SQL statement */
   sql = "CREATE TABLE COMPANY("  \
      "ID INT PRIMARY KEY     NOT NULL," \
      "NAME           TEXT    NOT NULL," \
      "AGE            INT     NOT NULL," \
      "ADDRESS        CHAR(50)," \
      "SALARY         REAL );";

   /* Execute SQL statement */
   rc = sqlite3_exec(db, sql, callback, 0, &zErrMsg);

   if( rc != SQLITE_OK ){
      fprintf(stderr, "SQL error: %s\n", zErrMsg);
      sqlite3_free(zErrMsg);
   } else {
      fprintf(stdout, "Table created successfully\n");
   }
   sqlite3_close(db);
   return 0;
}

当以上程序编译并且执行时,它会在你的 test.db 中创建 COMPANY 表,而文件的最终列表如下:

-rwxr-xr-x. 1 root root 9567 May 8 02:31 a.out
-rw-r--r--. 1 root root 1207 May 8 02:31 test.c
-rw-r--r--. 1 root root 3072 May 8 02:31 test.db

INSERT Operation

以下 C 代码段显示了你如何在以上示例中创建的 COMPANY 表中创建记录:

#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>

static int callback(void *NotUsed, int argc, char **argv, char **azColName) {
   int i;
   for(i = 0; i<argc; i++) {
      printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
   }
   printf("\n");
   return 0;
}

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;
   char *sql;

   /* Open database */
   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stderr, "Opened database successfully\n");
   }

   /* Create SQL statement */
   sql = "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) "  \
         "VALUES (1, 'Paul', 32, 'California', 20000.00 ); " \
         "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) "  \
         "VALUES (2, 'Allen', 25, 'Texas', 15000.00 ); "     \
         "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
         "VALUES (3, 'Teddy', 23, 'Norway', 20000.00 );" \
         "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
         "VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 );";

   /* Execute SQL statement */
   rc = sqlite3_exec(db, sql, callback, 0, &zErrMsg);

   if( rc != SQLITE_OK ){
      fprintf(stderr, "SQL error: %s\n", zErrMsg);
      sqlite3_free(zErrMsg);
   } else {
      fprintf(stdout, "Records created successfully\n");
   }
   sqlite3_close(db);
   return 0;
}

当以上程序编译并且执行时,它会在 COMPANY 表中创建给定的记录,并且会显示以下两行:

Opened database successfully
Records created successfully

SELECT Operation

在继续使用实际示例来获取记录之前,让我们了解一些关于回调函数的详细信息,而我们在示例中使用了此函数。此回调提供了一种从 SELECT 语句获取结果的方式。它具有以下声明:

typedef int (*sqlite3_callback)(
   void*,    /* Data provided in the 4th argument of sqlite3_exec() */
   int,      /* The number of columns in row */
   char**,   /* An array of strings representing fields in the row */
   char**    /* An array of strings representing column names */
);

如果在 sqlite_exec() 例程中以第三个参数的形式提供了以上回调,那么 SQLite 会在执行 SQL 参数内的每个 SELECT 语句时为在其中处理的每个记录调用此回调函数。

以下 C 代码段显示了你如何从在以上示例中创建的 COMPANY 表获取并显示记录:

#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>

static int callback(void *data, int argc, char **argv, char **azColName){
   int i;
   fprintf(stderr, "%s: ", (const char*)data);

   for(i = 0; i<argc; i++){
      printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
   }

   printf("\n");
   return 0;
}

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;
   char *sql;
   const char* data = "Callback function called";

   /* Open database */
   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stderr, "Opened database successfully\n");
   }

   /* Create SQL statement */
   sql = "SELECT * from COMPANY";

   /* Execute SQL statement */
   rc = sqlite3_exec(db, sql, callback, (void*)data, &zErrMsg);

   if( rc != SQLITE_OK ) {
      fprintf(stderr, "SQL error: %s\n", zErrMsg);
      sqlite3_free(zErrMsg);
   } else {
      fprintf(stdout, "Operation done successfully\n");
   }
   sqlite3_close(db);
   return 0;
}

当以上程序编译并且执行时,它会生成以下结果。

Opened database successfully
Callback function called: ID = 1
NAME = Paul
AGE = 32
ADDRESS = California
SALARY = 20000.0

Callback function called: ID = 2
NAME = Allen
AGE = 25
ADDRESS = Texas
SALARY = 15000.0

Callback function called: ID = 3
NAME = Teddy
AGE = 23
ADDRESS = Norway
SALARY = 20000.0

Callback function called: ID = 4
NAME = Mark
AGE = 25
ADDRESS = Rich-Mond
SALARY = 65000.0

Operation done successfully

UPDATE Operation

以下 C 代码段显示了我们如何使用 UPDATE 语句更新任何记录,然后从 COMPANY 表中获取和显示已更新的记录。

#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>

static int callback(void *data, int argc, char **argv, char **azColName){
   int i;
   fprintf(stderr, "%s: ", (const char*)data);

   for(i = 0; i<argc; i++) {
      printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
   }
   printf("\n");
   return 0;
}

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;
   char *sql;
   const char* data = "Callback function called";

   /* Open database */
   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stderr, "Opened database successfully\n");
   }

   /* Create merged SQL statement */
   sql = "UPDATE COMPANY set SALARY = 25000.00 where ID=1; " \
         "SELECT * from COMPANY";

   /* Execute SQL statement */
   rc = sqlite3_exec(db, sql, callback, (void*)data, &zErrMsg);

   if( rc != SQLITE_OK ) {
      fprintf(stderr, "SQL error: %s\n", zErrMsg);
      sqlite3_free(zErrMsg);
   } else {
      fprintf(stdout, "Operation done successfully\n");
   }
   sqlite3_close(db);
   return 0;
}

当以上程序编译并且执行时,它会生成以下结果。

Opened database successfully
Callback function called: ID = 1
NAME = Paul
AGE = 32
ADDRESS = California
SALARY = 25000.0

Callback function called: ID = 2
NAME = Allen
AGE = 25
ADDRESS = Texas
SALARY = 15000.0

Callback function called: ID = 3
NAME = Teddy
AGE = 23
ADDRESS = Norway
SALARY = 20000.0

Callback function called: ID = 4
NAME = Mark
AGE = 25
ADDRESS = Rich-Mond
SALARY = 65000.0

Operation done successfully

DELETE Operation

以下 C 代码段显示了你如何使用 DELETE 语句删除任何记录,然后从 COMPANY 表中获取和显示其余的记录。

#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>

static int callback(void *data, int argc, char **argv, char **azColName) {
   int i;
   fprintf(stderr, "%s: ", (const char*)data);

   for(i = 0; i<argc; i++) {
      printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
   }
   printf("\n");
   return 0;
}

int main(int argc, char* argv[]) {
   sqlite3 *db;
   char *zErrMsg = 0;
   int rc;
   char *sql;
   const char* data = "Callback function called";

   /* Open database */
   rc = sqlite3_open("test.db", &db);

   if( rc ) {
      fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
      return(0);
   } else {
      fprintf(stderr, "Opened database successfully\n");
   }

   /* Create merged SQL statement */
   sql = "DELETE from COMPANY where ID=2; " \
         "SELECT * from COMPANY";

   /* Execute SQL statement */
   rc = sqlite3_exec(db, sql, callback, (void*)data, &zErrMsg);

   if( rc != SQLITE_OK ) {
      fprintf(stderr, "SQL error: %s\n", zErrMsg);
      sqlite3_free(zErrMsg);
   } else {
      fprintf(stdout, "Operation done successfully\n");
   }
   sqlite3_close(db);
   return 0;
}

当以上程序编译并且执行时,它会生成以下结果。

Opened database successfully
Callback function called: ID = 1
NAME = Paul
AGE = 32
ADDRESS = California
SALARY = 20000.0

Callback function called: ID = 3
NAME = Teddy
AGE = 23
ADDRESS = Norway
SALARY = 20000.0

Callback function called: ID = 4
NAME = Mark
AGE = 25
ADDRESS = Rich-Mond
SALARY = 65000.0

Operation done successfully