在mysql中是否可以编写连接取决于条件

Question

喜欢：

SELECT * FROM (`expenditures`) 
JOIN `categories`
ON `categories`.`cat_id` = `expenditures`.`cat_id` IF(`projects`.`project_id` ! = 0) 
JOIN `projects`  
ON `projects`.`project_id` = `expenditures`.`project_id` 
WHERE `expenditure_id` = $expenditure_id

Answer 1

你可以试试这个：

class BarcodeReader
{
    ArrayList barCode = new ArrayList();
    ArrayList barCodeTimes = new ArrayList();
    ArrayList barCodeDeltaTimes = new ArrayList();
    /// <summary>
    /// Input 1: delayTime (ms) - time for scanner to return values (threshold)[30 seems good],
    /// Input 2: KeyEventArgs - put in key [this.KeyDown += new KeyEventHandler(Form1_KeyDown)],
    /// Output 1: String of barcode read
    /// </summary>
    /// <param name="delayTime"></param>
    /// <param name="e"></param>
    /// <returns></returns>
    public string BarcodeValue(int delayTime, KeyEventArgs e)
    {
        string barCodeString = null;
        var isNumber = e.KeyCode >= Keys.D0 && e.KeyCode <= Keys.D9;
        var rtn = e.KeyCode == Keys.Enter;

        if (isNumber)
        {
            barCode.Add(Encoding.ASCII.GetString(new byte[] { (byte)e.KeyValue }));
            barCodeTimes.Add(DateTime.Now.TimeOfDay.TotalMilliseconds);
        }
        if (rtn)
        {
            barCodeString = ValuesToString(delayTime);
        }
        return barCodeString;
    }
    private string ValuesToString(int delayTime)
    {

        string barCodeString = null;

        foreach (double d in barCodeTimes)
        {
            double diff = 0;
            int index = barCodeTimes.IndexOf(d);
            if (index < barCodeTimes.Count - 1)
            {
                diff = (double)barCodeTimes[index + 1] - (double)barCodeTimes[index];
            }

            barCodeDeltaTimes.Add(diff);
        }
        foreach (double d in barCodeDeltaTimes)
        {
            if (d > delayTime)
            {
                barCode.RemoveAt(0);
            }
        }
        foreach (string s in barCode)
        {
            barCodeString += s;
        }

        barCode.Clear();
        barCodeTimes.Clear();
        barCodeDeltaTimes.Clear();

        return barCodeString;
    }

}

Answer 2

SELECT * FROM (`expenditures`) 
JOIN `categories`
ON `categories`.`cat_id` = `expenditures`.`cat_id`  
JOIN `projects`  
ON `projects`.`project_id` = `expenditures`.`project_id` and `projects`.`project_id` <>  0
WHERE `expenditure_id` = $expenditure_id