C language operation database (SQLite3) call interface function

C First, we need to have a database , So we need to create a database ,
stay linux In the environment sqlite3 In mode , By the way, write the database name at the end ：

First, define a database pointer type , And then call the database. The interface function :sqlite3_open(“ Database name ”, Pointer address )”, The functionality ： Open a database , If the database file does not exist , Automatically create , The command to open or create the database will be cached , It will not be executed until the database is actually called . And let the pointer defined above point to the database .ret Variable is used to receive the return value of this interface function , The return value of this function is zero SQLITE_OK Its value is 0, Indicates that the opening is successful , If not , Then you have to report an error . Let's judge
At this point, we will open the database and complete the operation , We will start to create tables in the database , Because there is no table where to insert the data , good , To create table.
Here is a judgment on the return value of the create table function , If the return value is not SQLITE_OK This will result in unsuccessful table creation .

The above figure shows the function of creating a table , The function parameter is to pass a pointer to the database , The red mark 1 The string is the string used to create the table sql sentence , These three words are added here , This is to ensure that the existing errors in this table will not be reported when the code is executed in the future , This sentence means , establish mytable Table of , If the table exists, it will not be created , Just use it , If it doesn't exist, create it . The red mark 2 Is to call the database interface function , This function can do several things .
The function interface prototype is ：
int sqlite3_exec(sqlite3 *db, const char *sql, sqlite3_callback callback, void *,char **errmsg);
db: Called database
sql:sql sentence
callback: Callback function , Every successful execution sql Statement is executed once callback function
void *: Parameters passed to the callback function （ Here, when you need to use , You need to cast ）
errmsg: error message

After creating the database and tables above , We will start to insert data later , Pay attention here , When inserting, the field names should be in the same order as when creating the table in the previous step .

The image above sql What is defined is an array , When creating tables sql What is defined is a character pointer , Note that these two usages are different, although they both represent sql sentence , The one that creates the table points directly to sql sentence , In fact, it refers to the first address of the string , here sql It's for sprintf() Inside the function . This function takes the third parameter , Put it in the second parameter , Then put the second parameter in the first parameter , So here we need to use an array to take this string of characters , This is why this step is necessary , This is because we have to input the data we want to input , So this step is necessary .
Interface functions are also used later sqlite3_exec（） function , This function can be used to create tables , insert data , Delete data , Display the data
The function here is similar to that of creating tables , Is to insert data into this database , If successful, proceed to the next step , If unsuccessful, an error message is returned .

After the insertion is completed, you can view , Look at the picture below ：

The figure above shows that the data in the table will use the callback function parameter , there display It's a callback function , hinder (void *)&flag For callback function , The reason for taking the address here is that it needs to be changed flag Value , So send the address , See the following figure for this callback function ：
The callback function here is to display the output table Data in .
After the display, delete the data in the database .
This is similar to the one used above ,sql The sentence has changed , Change some more hints
Here's how to use sqlite3_get_table() The function interface

What we're going to use is sqlite_get_table() The prototype of the function is ：
int sqlite3_get_table(
sqlite3 *db, // Handle to the open database
const char *zSql, // Executes SQL sentence
char pazResult, // The result is written to the char
int *pnRow, // Number of rows in the result set
int *pnColumn, // Number of columns in the result set
char **pzErrmsg // error message
);

And then perform the output display operation .
Okay , Almost that much , end . Paste the source code .

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

// Call the database interface API


// Create table 
int create_table(sqlite3 *pdb)
{
    
    char *sql = NULL;     
    char *errmsg = NULL;
    int ret;

    sql = "create table if not exists mytable (id integer primary key,name text);";

    ret = sqlite3_exec(pdb,sql,NULL,NULL,&errmsg);

    if(SQLITE_OK != ret)
    {
    
        printf("create table error! %s\n",errmsg);
        return -1;
    }
    else
    {
    
        return SQLITE_OK;
    }
}

// insert data 
int insert_record(sqlite3 *pdb)
{
    
    char sql[100];
    char *errmsg = NULL;
    int ret;
    int id;
    char name[20];

    printf("please input id and name: \n");

    scanf("%d",&id);
    scanf("%s",name);

    // take id and name Print in string , Save in sql in 
    sprintf(sql,"insert into mytable (id,name) values (%d,'%s');",id, name);
    
    ret = sqlite3_exec(pdb,sql,NULL,NULL,&errmsg);

    if(SQLITE_OK != ret)
    {
    
        perror("insert error!");
        printf("insert record 1 error! %s\n",errmsg);
        return -1;
    }
   /* else { return SQLITE_OK; }*/

    return SQLITE_OK;
}

// Callback function , The function name and parameter name are variable , But the return value type , Parameter type , The number and position of parameters cannot be changed .
// Callback function  1 The fourth parameter in the callback function  2 The column number of the table 
// 3 An array of pointers to query results  4 An array of pointers to the header name 
int display(void *para, int ncol, char *col_val[], char **col_name)
{
    
    int i;
    int *flag = NULL;

    flag = (int *)para;

    if(0 == *flag)
    {
    
        *flag = 1;
        // Number of output columns 
        printf("column number is: %d\n", ncol);

        for(i = 0; i < ncol; i++)
        {
    
            printf("%10s",col_name[i]);
        }

        printf("\n");
    }

    for(i = 0; i < ncol; i++)
    {
    
        printf("%10s",col_val[i]);
    }

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

// Show 
int inquire_uscb(sqlite3 *pdb)
{
    
    char *sql = NULL;
    char *errmsg = NULL;
    int ret;
    int flag = 0;

    sql = "select * from mytable;";

    // database , Statement string , Callback function , User entered parameters , Finally, it is passed to the callback function for use , error message 
    ret = sqlite3_exec(pdb,sql,display,(void *)&flag,&errmsg);

    if(SQLITE_OK != ret)
    {
    
        printf("select error! %s\n",errmsg);

        return -1;
    }
    else
    {
    
        return SQLITE_OK;
    }

}

// Delete 
int delete_record(sqlite3 *pdb)
{
    
    char sql[100];
    char *errmsg = NULL;
    int ret;
    int id;

    printf("please input delete id:\n");
    scanf("%d",&id);

    sprintf(sql,"delete from mytable where id = %d;", id);

    ret = sqlite3_exec(pdb,sql,NULL,NULL,&errmsg);

    if(SQLITE_OK != ret)
    {
    
        printf("delete error! %s\n",errmsg);

        return -1;
    }
    else
    {
    
        return SQLITE_OK;
    }
}
// Show 
int inquire_nocb(sqlite3 *pdb)
{
    
    char *sql = NULL;
    char ** ret_val = NULL;
    char *errmsg = NULL;
    int nrow;
    int ncol;
    int ret;
    int i;

    sql = "select * from mytable";

    ret = sqlite3_get_table(pdb, sql, &ret_val, &nrow, &ncol, &errmsg);

    if(SQLITE_OK == ret)
    {
    
        printf("nrow = %d ncol = %d\n",nrow,ncol);

        for(i = 0; i < (nrow + 1)* ncol; i++ )
        {
    
            printf("%10s",ret_val[i]);

            if((i + 1) % ncol == 0)
            {
    
                printf("\n");
            }
        }
    }
    else
    {
    
        sqlite3_free_table(ret_val);

        return -1;
    }


     sqlite3_free_table(ret_val);
}


int main(int argc, char **argv)
{
    
    sqlite3 *pdb;  // Define a database pointer 
    int ret;       // Pick up sqlite3_open() The return value of the function 

    ret = sqlite3_open("mydatabase.db",&pdb);  // Open database , Let the pointer point to the database 
    
    if(ret != SQLITE_OK)  // Judge whether the database is opened successfully 
    {
    
        // Prompt open failed and return error description information 
        printf("open database fail! %s\n",sqlite3_errmsg(pdb));
        exit(1);
    }
    else
    {
    
        printf("open database successfully！\n");
    }
    
    // The above is to open the database 

    // After opening the database, create a table 
    if(SQLITE_OK == create_table(pdb))
    {
    
        printf("create table success!\n");
    }
    else
    {
    
        sqlite3_close(pdb);
        return 0;
    }

    // The above is to create a table , Later, we need to insert data into it 
    
    if(0 != insert_record(pdb))
    {
    
        sqlite3_close(pdb);
        exit(-1);
    }
 
   
    // The above is the insert data , It should be shown later 
    // Show  
    inquire_uscb(pdb);

    // Delete 
    delete_record(pdb);

    // Show 
    inquire_nocb(pdb);

    // Close the database 
    sqlite3_close(pdb);
    
    return 0;
}