【iOS开发-104】SQLite使用:注意查询时分步写while会出现死循环
注意点:
在SELECT查询时,不能分步写成如下形式,否则会出现死循环:
int stepResult=sqlite3_step(statement); while (stepResult==SQLITE_ROW) { }只能合并成一句写成:
while (sqlite3_step(statement)==SQLITE_ROW) { }
因为,sqlite3_step的执行方式是如下:
** ^If the SQL statement being executed returns any data, then [SQLITE_ROW]
** is returned each time a new row of data is ready for processing by the
** caller. The values may be accessed using the [column access functions].
** sqlite3_step() is called again to retrieve the next row of data.
#import "ViewController.h" #import <sqlite3.h> @interface ViewController () - (IBAction)insert:(id)sender; - (IBAction)delete:(id)sender; - (IBAction)update:(id)sender; - (IBAction)select:(id)sender; @end @implementation ViewController static sqlite3 *_db; - (void)viewDidLoad { [self setupDatabaseAndTable]; [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. } - (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Dispose of any resources that can be recreated. } -(void)setupDatabaseAndTable{ NSString *filename=[[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:@"iOS.sql"]; NSLog(@"%@",filename); int openDBResult=sqlite3_open(filename.UTF8String, &_db); if (openDBResult==SQLITE_OK) { //创建表 NSString *createTableSql=@"CREATE TABLE IF NOT EXISTS user (id INTEGER PRIMARY KEY AUTOINCREMENT,name TEXT,age INTEGER);"; char *error=nil; int createTableResult=sqlite3_exec(_db, createTableSql.UTF8String, NULL, 0, &error); if (createTableResult==SQLITE_OK) { NSLog(@"创建表成功"); }else{ NSLog(@"创建表失败"); } NSLog(@"打开数据库成功"); }else{ NSLog(@"打开数据库失败"); } } - (IBAction)insert:(id)sender { for (int i=0; i<50; i++) { NSString *name=[NSString stringWithFormat:@"jack%d",arc4random()%100]; int age=arc4random()%100; NSString *insertTableSql=[NSString stringWithFormat:@"INSERT INTO user(name,age) VALUES('%@',%d);",name,age]; char *error=nil; int insertResult=sqlite3_exec(_db, insertTableSql.UTF8String, NULL, 0, &error); if (insertResult==SQLITE_OK) { NSLog(@"%d %@ %d",i,name,age); }else{ NSLog(@"插入数据失败"); } } } - (IBAction)delete:(id)sender { NSString *deleteRowSql=@"DELETE FROM user WHERE id=1;"; char *error=nil; int deleteResult=sqlite3_exec(_db, deleteRowSql.UTF8String, NULL, 0, &error); if (deleteResult==SQLITE_OK) { NSLog(@"删除数据成功"); }else{ NSLog(@"删除数据失败"); } } - (IBAction)update:(id)sender { NSString *updateRowSql=@"UPDATE user SET age=130 WHERE id=3;"; char *error=nil; int updateResult=sqlite3_exec(_db, updateRowSql.UTF8String, NULL, 0, &error); if (updateResult==SQLITE_OK) { NSLog(@"修改数据库成功"); }else{ NSLog(@"修改数据库失败"); } } - (IBAction)select:(id)sender { NSString *selectRowSql=@"SELECT id,name,age FROM user WHERE id=5;"; sqlite3_stmt *statement; int stmtResult=sqlite3_prepare_v2(_db, selectRowSql.UTF8String, -1, &statement, nil); if (stmtResult==SQLITE_OK) { NSLog(@"stmtResult成功"); //以下语句分步写会出现死循环 while (sqlite3_step(statement)==SQLITE_ROW) { NSLog(@"stepResult成功"); int uid=sqlite3_column_int(statement, 0); const unsigned char *uname=sqlite3_column_text(statement, 1); int uage=sqlite3_column_int(statement, 2); NSLog(@"%d %s %d",uid,uname,uage); } }else{ NSLog(@"stmtResult失败"); } } @end
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