Question

所以我对iOS开发很新，我的select函数出现问题。我创建了一个函数，应该接受一个select查询和表名并返回一个结果数组，其中每个数组条目都是一个带有一行结果的字典。不知怎的，我对列名的查询是删除我的columnNames变量并返回疯狂的结果。我只想找出一种存储，访问，操作查询结果的简便方法

以下是将结果转换为数组的函数：

-(NSMutableArray *)selectQuery:(NSString*)query
                     table:(NSString*)table
{
NSMutableArray *returnArray = [NSMutableArray new];

if(sqlite3_open([databasePath UTF8String], &database) == SQLITE_OK)
{
    NSMutableArray *columnNames;
    NSString *tableQuery = [NSString stringWithFormat:@"PRAGMA table_info('%@')", table];

    if (sqlite3_prepare_v2(database, [tableQuery UTF8String], -1, &statement, nil) == SQLITE_OK)
    {
        while (sqlite3_step(statement) == SQLITE_ROW)
        {
            [columnNames addObject:[NSString stringWithUTF8String:(const char*)sqlite3_column_text(statement, 1)]];
        }
    }
    else
    {
        NSLog(@"Error preparing table query:");
        NSLog(tableQuery);
    }

    if (sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, nil) == SQLITE_OK)
    {
        while(sqlite3_step(statement)==SQLITE_ROW)
        {
            NSMutableDictionary *temp= [NSMutableDictionary new];

            for (int i = 0; i < [columnNames count]; i++)
            {
                [temp setObject:[NSString stringWithUTF8String:(const char*)sqlite3_column_text(statement, i)] forKey:columnNames[i]];
            }

            if (temp != nil)
            {
                [returnArray addObject:temp];

                temp = nil;
            }
        }
        sqlite3_reset(statement);
        sqlite3_close(database);
    }
    else
    {
        NSLog(@"Error preparing select statement with query:");
        NSLog(query);
    }
}
else
{
    NSLog(@"Could not open database");
}
return returnArray;
}

并继续致电

NSMutableArray *queryResults = [dbInstance selectQuery:[NSString stringWithFormat:@"SELECT gallons, mileage FROM fillups WHERE carId = \"%d\" ORDER BY date asc",
                                                   carId]
                                            table:@"fillups"];

Answer 1

您永远不会实例化columnNames。因此，您尝试将列名添加到该数组将不会成功。要解决这个问题，当你声明它时，你也希望实例化可变数组对象：

NSMutableArray *columnNames = [NSMutableArray array];

与此问题无关，当您完成检索列名称时，在准备第二个语句之前，请不要忘记释放与第一个预准备语句关联的内存：

sqlite3_finalize(statement);

最后，当您完成检索第二个准备好的SQL语句而不是调用sqlite3_reset时，您希望再次为该第二个预准备语句调用sqlite3_finalize。当您想要将新值绑定到语句中的sqlite3_reset占位符时，?用于重置语句，这在此处不适用，因此不需要sqlite3_reset。但是如果你不调用sqlite3_finalize，你就不会释放与准备好的语句相关的内存。

顺便说一下，如果你想动态检索列名和列类型（无需PRAGMA table_info），你可以这样做：

int rc;

if ((rc = sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, NULL)) != SQLITE_OK) {
    NSLog(@"select failed %d: %s", rc, sqlite3_errmsg(database));
}

NSMutableArray *returnArray = [NSMutableArray array];
NSInteger columnCount = sqlite3_column_count(statement);

id value;

while ((rc = sqlite3_step(statement)) == SQLITE_ROW) {
    NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];

    for (NSInteger i = 0; i < columnCount; i++) {
        NSString *columnName   = [NSString stringWithUTF8String:sqlite3_column_name(statement, i)];
        switch (sqlite3_column_type(statement, i)) {
            case SQLITE_NULL:
                value = [NSNull null];
                break;
            case SQLITE_TEXT:
                value = [NSString stringWithUTF8String:(const char *)sqlite3_column_text(statement, i)];
                break;
            case SQLITE_INTEGER:
                value = @(sqlite3_column_int64(statement, i));
                break;
            case SQLITE_FLOAT:
                value = @(sqlite3_column_double(statement, i));
                break;
            case SQLITE_BLOB:
            {
                NSInteger length  = sqlite3_column_bytes(statement, i);
                const void *bytes = sqlite3_column_blob(statement, i);
                value = [NSData dataWithBytes:bytes length:length];
                break;
            }
            default:
                NSLog(@"unknown column type");
                value = [NSNull null];
                break;
        }
        dictionary[columnName] = value;
    }

    [returnArray addObject:dictionary];
}

if (rc != SQLITE_DONE) {
    NSLog(@"error returning results %d %s", rc, sqlite3_errmsg(database));
}

sqlite3_finalize(statement);

从iOS中的选择查询填充数组

1 个答案: