Embedded database -- SQLite

Database notes

1. A database file is similar to a EXCEL file

There are multiple tables in the database file （tables）, Be similar to EXCEL Medium SHEET.
In each table , Store something similar to EXCEL Medium 2 Dimension table data .
Every column in every table , Call a field .
Every row in every table , Call a record .

2. Command line （ With . The first order ）

• .tables View the table under the current database file
• .schema View the table structure under the current database file （ Fields and types of tables ）

3. Basic SQL sentence ( Is a syntax rule that all databases follow , A command that ends with a semicolon )

3.1 Create a table , You need to know the name of the table , Field name , The type of each field

    create table  The name of the table （ Field name 1  Field 1 type  ,  Field name 2  Field 2 type  , ...）;
    EG:create table  student(id text , name text , age text ,  score text);
create table  user(id text ,name text, pwd text);

3.2 insert data

    INSERT INTO  The name of the table  VALUES (value1, value2, value3, ...);
    EG:  SERT INTO student VALUES("1000", "zhangsan", "18", "90");

3.3 View the data in a table

	SELECT * FROM  The name of the table  （WHERE  Conditions ）;
	EG:SELECT * FROM student;
	SELECT * FROM student WHERE id = "1000";
	SELECT name,score FROM student WHERE id = "1000";

3.4 Delete table （ Be similar to EXCEL Zhongba whole SHEET Delete all ）

DROP TABLE  The name of the table ;

3.5 Delete data from table

DELETE FROM  The name of the table  (WHERE  Conditions );
EG:DELETE FROM  student WHERE name ="zhangsan";

3.6 Modify the data in the table

UPDATE  The name of the table  SET  Field 1 = "" AND  Field 2 = "" ...(WHERE ) ;
EG: UPDATE student SET score =“80” WHERE name ="zhangsan";

4 .API Use

sqlite3_open

The name of the first parameter database , The first 2 Parameters sqlite3 * db Represents a handle to the database ;

	int sqlite3_exec();　 Execute one SQL sentence .
		sqlite3*,                                  /* An open database */
		const char *sql,                           /* SQL to be evaluated SQL sentence */
		int (*callback)(void*,int,char**,char**),  /* Callback function   Callback function */
		void *,                                    /* 1st argument to callback  The parameters of the callback function */
		char **errmsg                              /* Error msg written here  error message */
	/*
	callback Will automatically be called many times , Every record ,callback Will be automatically called once .
	notused : The incoming parameters of the callback function 
	colNum： How many columns are there in this record 
	colContent: The specific content of each column 
	colName: The name of each column 
	*/

	eg:
int main(int argc, char *argv[])
{
	QocoreApplication a(argc,argv);
	sqlite3 * pdb;
	int res = sqlite3_open( "d : //test.db",&pdb);
	if(res != sQLITE_OK)
	{
		printf( "open db err\n");
		return -1;
	}
	printf ( "open db success\n" );
	char *errMsg;
	char sql[100]= {0} ;
	strcpy(sql,"CREATE TABLE user(id TEXT ,name TEXT,pwd TEXT); ");
	res = sqlite3_exec(pdb, sql,NULL,NULL,&errMsg);
	if(res != SQLITE_OK)
	{
		printf( "sqlite3_exec db err errMsg=[%s]\n", errMsg);
		return -1;
	}
	printf( "sqlite3_exec db success\n" );
	sqlite3_close;
	return a.exec();
}	

callback

	int callback(void* notused, int colNum, char** colContent, char**colName )
	~~~
	eg:
/ / callback Function is when querying SQL When the statement returns , Automatically called .
// Whenever there is a record , The function is called once 
// notused :sqlite3_exec Incoming parameter 
// colNum: The number of columns of this record 
// colContent: Array of strings , For the contents of each column 
// colName: The column name of each column of this record 
int firstFlg = 0;
int callback(void* notused,int colNum,char** colContent,char**colName )
{
	if( ! firstFlg)
	{
		{
			// Print column names 
			for ( int i = 0;i <colNo;i++){
			printf("%s ",colName[i]);
		}
		printf("\n");
		firstFlg++;
	for(int i = 0;i < colNum;i++)
	{
         printf("%s ", colContent[i]);
     }
}
	
	strcpy(sql,"SELECT * FROM user; ");
	firstFlg = 0;
	res = sqlite3_exec(pdb, sql,NULL,NULL,&errMsg);
	if(res != SQLITE_OK)
	{
		printf( "sqlite3_exec dr err errMsg=[%s]\n",errMsg);
		return -1;
	}
	printf("sqlite3_exec db success n" );

sqlite3_get_table

sqlite3_get_table, Execute a query SQL sentence , And return all the results of the query .

		sqlite3 *db,          /* An open database */
		  const char *zSql,     /* SQL to be evaluated */
		  char ***pazResult,    /* Results of the query */
		  int *pnRow,           /* Number of result rows written here */
		  int *pnColumn,        /* Number of result columns written here */
		  char **pzErrmsg       /* Error msg written here */
		 The first 1 No more parameters , Look at the previous example .
		 The first 2 The parameters are sql  sentence , Follow sqlite3_exec  Inside sql  It's the same . Is a very common with \0 At the end of the char* character string .
		 The first 3 The first parameter is the query result , It's still a one-dimensional array （ Don't think it's a two-dimensional array , Don't think it's a three-dimensional array ）.
				 Its memory layout is ： Field name , This is followed by the value of each field . Let's use an example .
		 The first 4 The first parameter is how many records are queried （ That is, find out how many lines , The line excluding the field name ）.
		 The first 5 How many fields are the parameters （ How many columns ）.
		 The first 6 The first parameter is the error message , It's the same as before , I won't say much more here .
	pazResult The result set is stored , The order of storage is each field in the header , Then each field of each record is stored 

	EG: The result set in the database is as follows ：
		Name        | Age
		-----------------------
		Alice       | 43
		Bob         | 28
		Cindy       | 21
		 Then the storage format is as follows 
		pazResult[0] = "Name";pazResult[1] = "Age";
		pazResult[2] = "Alice";pazResult[3] = "43";
		pazResult[4] = "Bob";pazResult[5] = "28";
		pazResult[6] = "Cindy";pazResult[7] = "21";

	sqlite3_close,  Close the database 

//lsqlite3 *db Open database handle 
//const char *zSql Of the query SQL sentence 
//char ***pazResult All result sets //
int *pnRow How many rows of data are there in the returned result 
//int *pncolumn How many columns of data are there in the returned result char **pazResult;
int row = 0;
int col = 0;
res =sqlite3_get_table(pdb,sql,&pazResult,&row,&col,&errMsg);
if(res != SQLITE_OK)
{
	printf( "sqlite3_exec db err errMsg=[%s]\n",errMsg);
	return -l;
}

for(int j = 0; j <= row;j++)
{
	for (int i = 0; i < col;i++)
	{
		printf("%-20s",pazResult[j*col+i]);
	}
		printf("\n");
}
sqlite3_free_table(pazResult);
sqlite3_close(pdb);

5.sqlite3 stay VS Environment configuration

• Take the header file （sqlite3.h） And libraries （sqlite3.lib） Put it in the source code folder of the project .
• Items in the menu -》 attribute -》 Configuration properties -》 The connector -》 routine -》 Additional Library Directory -》 Join in sqlite3.lib Path
• Items in the menu -》 attribute -》 Configuration properties -》 The connector -》 Input -》 Add dependency -》 Join in sqlite3.lib（ Tell the compiler the name of the library that needs to be loaded ）
• Items in the menu -》 attribute -》 Configuration properties -》 The connector -》 Input -> Ignore specific defaults *
• Join in LIBCD.lib
• Add... To the source file #include “sqlite3.h”, #pragma comment(lib,“sqlite3.lib”)

6.sqlite3 stay QT Environment configuration

• Take the header file （sqlite3.h） And libraries （libsqlite3.a） Put it in the source code folder of the project .
• stay QT Added to the project documents LIBS += -L D:\workspace\20220721\ -lsqlite3, hold libsqlite3.a Add to the project ()