Question

很抱歉，如果这是重复的，但是当我使用诸如concat(column1, column2) as column3之类的解决方案时，我会得到奇怪的值。

我的数据如下：

Table1.Column1   Table2.Column2
A                NULL
B                NULL
NULL             C
NULL             D
E                NULL

注意：只有1列具有NULL值。

我想要以下内容：

 Column3
  A   
  B   
  C
  D
  E

两列之一具有NULL值的属性将成立。这只是糟糕的表/列设计，因为它本来应该是一个表，但实际上不是。我不允许更改架构，也不允许插入/更新值。我只允许进行明智的查询，以理智的方式检索数据。

Answer 1

您可以使用COALESCE处理NULL值：

SELECT CONCAT(COALESCE(Table1.Column1, ''), COALESCE(Table2.Column2, '') AS Column3
FROM ...

Answer 2

您可以使用if语句。

public class GameController : MonoBehaviour {

[SerializeField] private Text questionText;
[SerializeField] private Text scoreText;

private DataController dataController;
private RoundData currentRoundData;
private QuestionData[] questionPool;
private AnswerData answerData;
private AnswerButton answerButton;

public GameObject questionDisplay;
public GameObject roundEndDisplay;

private bool isRoundActive;
private float timeRemaining;
private int questionIndex;
private int playerScore;

private void Start()
{
    dataController = FindObjectOfType<DataController>();
    currentRoundData = dataController.GetCurrentRoundData();
    questionPool = currentRoundData.questions;
    timeRemaining = currentRoundData.timeLimitInSeconds;

    playerScore = 0;
    questionIndex = 0;

    ShowQuestion();
    isRoundActive = true;

}

private void ShowQuestion()
{
    QuestionData questionData = questionPool[questionIndex];
    questionText.text = questionData.questionText;
    answerButton.Setup(answerData);

}

public void AnswerButtonClicked(bool isCorrect)
{
    if (isCorrect)
    {
        playerScore += currentRoundData.pointsAddedForCorrectAnswer;
        scoreText.text = "Score: " + playerScore.ToString();
    }

    if (questionPool.Length > questionIndex + 1)
    {
        questionIndex++;
        ShowQuestion();
    } else
    {
        EndRound();
    }
}

public void EndRound()
{
    isRoundActive = false;

    questionDisplay.SetActive(false);
    roundEndDisplay.SetActive(true);
}
}

MySQL将两列合并为一列，其中一列具有NULL值（应删除）

2 个答案: