给定一个关系模式R (A,B.CDEF)其中最小的函数依赖集 F=(AB->D,B->CE,D->F,F->A) <1>.写出该关系模式的候选码 <2>.该关系模式为什么不是2nfo <3>.将该关系模式分解为2nf并写出最小的函数依赖集 <4>.为什么分解后该关系模式是2nf?

1. 候选码为AB和BCD。

2. 因为存在函数依赖F->A，即非主属性A依赖于码F，违反了2NF的要求。

3. 将R分解为R1(ABD)和R2(BCEF)，其中R1和R2的属性集合为R的子集且满足以下函数依赖集：F1=(AB->D,F->A)和F2=(B->CE,D->F)。其中R1的候选码为AB，R2的候选码为BCD。

4. 分解后该关系模式是2NF，因为每个分解出的关系模式都只有一个候选码且不存在非主属性依赖于码的情况。

相关问题

下面程序段输出的结果是（） String s="ABCDEF"; s=s.subString(2); System.out.println(s); A BCDEF B CDEF C ABCDEF D AB 下面程序段输出的结果是（） String s="ABCDEF"; s=s.subString(2); System.out.println(s); A BCDEF B CDEF C ABCDEF D AB

A BCDEF。这段代码会输出字符串s从第2个字符开始到结尾的子串，即"ABCDEF"的第3个字符到最后一个字符，所以输出结果为"BCDEF"。需要注意的是，Java中的字符串下标从0开始，因此"ABCDEF"的第3个字符的下标为2。此外，需要注意字符串的`substring()`方法是左闭右开区间，即包含起始下标对应的字符，但不包含结束下标对应的字符。

Failed cleaning build dir for numpy Failed to build numpy Installing collected packages: numpy Running setup.py install for numpy ... error Complete output from command /usr/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-h5_vrlht/numpy/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-3koy23ws-record/install-record.txt --single-version-externally-managed --compile --user --prefix=: Running from numpy source directory. Note: if you need reliable uninstall behavior, then install with pip instead of using `setup.py install`: - `pip install .` (from a git repo or downloaded source release) - `pip install numpy` (last NumPy release on PyPi) Cythonizing sources Error compiling Cython file: ------------------------------------------------------------ ... cdef sfc64_state rng_state def __init__(self, seed=None): BitGenerator.__init__(self, seed) self._bitgen.state = <void *>&self.rng_state self._bitgen.next_uint64 = &sfc64_uint64 ^ ------------------------------------------------------------ _sfc64.pyx:90:35: Cannot assign type 'uint64_t (*)(void *) except? -1 nogil' to 'uint64_t (*)(void *) noexcept nogil' numpy/random/_bounded_integers.pxd.in has not changed Processing numpy/random/_sfc64.pyx Traceback (most recent call last): File "/tmp/pip-build-h5_vrlht/numpy/tools/cythonize.py", line 235, in <module> main() File "/tmp/pip-build-h5_vrlht/numpy/tools/cythonize.py", line 231, in main find_process_files(root_dir) File "/tmp/pip-build-h5_vrlht/numpy/tools/cythonize.py", line 222, in find_process_files process(root_dir, fromfile, tofile, function, hash_db) File "/tmp/pip-build-h5_vrlht/numpy/tools/cythonize.py", line 188, in process processor_function(fromfile, tofile) File "/tmp/pip-build-h5_vrlht/numpy/tools/cythonize.py", line 78, in process_pyx [sys.executable, '-m', 'cython'] + flags + ["-o", tofile, fromfile]) File "/usr/lib/python3.6/subprocess.py", line 311, in check_call raise CalledProcessError(retcode, cmd) subprocess.CalledProcessError: Command '['/usr/bin/python3', '-m', 'cython', '-3', '--fast-fail', '-o', '_sfc64.c', '_sfc64.pyx']' returned non-zero exit status 1. Traceback (most recent call last): File "<string>", line 1, in <module> File "/tmp/pip-build-h5_vrlht/numpy/setup.py", line 508, in <module> setup_package() File "/tmp/pip-build-h5_vrlht/numpy/setup.py", line 488, in setup_package generate_cython() File "/tmp/pip-build-h5_vrlht/numpy/setup.py", line 285, in generate_cython raise RuntimeError("Running cythonize failed!") RuntimeError: Running cythonize failed! ---------------------------------------- Command "/usr/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-h5_vrlht/numpy/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-3koy23ws-record/install-record.txt --single-version-externally-managed --compile --user --prefix=" failed with error code 1 in /tmp/pip-build-h5_vrlht/numpy/

您遇到了安装numpy时的错误。根据错误日志，问题出现在运行Cython编译器时。这可能是由于Cython版本不兼容或其他配置问题导致的。

您可以尝试以下解决方案：

1. 确保您使用的是最新版本的pip和setuptools。可以运行以下命令进行升级：

   pip install --upgrade pip setuptools

2. 确保您的系统已安装Cython。您可以尝试通过运行以下命令来安装Cython：

   pip install Cython

3. 如果您已经安装了Cython，并且问题仍然存在，请尝试降级Cython版本。运行以下命令：

   pip install Cython==0.29.21

然后再次尝试安装numpy。

希望这些解决方案可以帮助您解决问题。如果问题仍然存在，请提供更多详细信息，以便我可以更好地帮助您。