一尘不染

如何确定使用C#的SQL Server CE表中是否存在列?

sql

遗留代码以这种方式执行此操作:

public bool isValidField(string tableName, string fieldName)
{
    bool retVal;
    string tblQuery = string.Format("SELECT {0} FROM {1}", fieldName, tableName);
    checkConnection();
    try
    {
        SqlCeCommand cmd = objCon.CreateCommand();
        cmd.CommandText = tblQuery;
        object objvalid = cmd.ExecuteScalar();
        retVal = (null != objvalid);
    }
    catch
    {
        retVal = false; 
    }
    return retVal;
}

…但是我发现它并不总是有效。调用该方法并获取后false,一些代码会在表中更改以添加一些列,但是我得到了:

列ID在规范中出现了多次。

我在StackOverflow上发现了两个很有希望的SQL语句:

if exists(select * from sys.columns 
        where Name = N'columnName' and Object_ID = Object_ID(N'tableName'))

IF COL_LENGTH('table_name','column_name') IS NULL

…但是不确定如何在C#/ .NET 1.1中实现

我是否需要使用ExecuteScalar并将返回的值转换为布尔值?或者是其他东西?

更新

将其更改为此并不能解决问题:

public bool isValidField(string tableName, string columnName)
{
    bool retVal;
    string tblQuery = string.Format(
        "COL_LENGTH({0},{1}) IS NULL",
        tableName,
        columnName);

    checkConnection();
    try
    {
        SqlCeCommand cmd = objCon.CreateCommand();
        cmd.CommandText = tblQuery;
        object objvalid = cmd.ExecuteScalar();
        retVal = Convert.ToBoolean(objvalid);
    }
    catch
    {
        retVal = false; 
    }
    return retVal;
}

更新2

奇怪的是,我在新代码中看到了更多的错误消息。

更新3

当我以这种方式更改代码时,它没有什么不同:

string tblQuery = "SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS"
                  + " WHERE TABLE_NAME = @tableName AND COLUMN_NAME"
                  + " = @columnName";

checkConnection();
try
{
    SqlCeCommand cmd = objCon.CreateCommand();
    cmd.CommandText = tblQuery;
    SqlCeParameter tblNameParam = new SqlCeParameter(
        "@tableName",
        SqlDbType.NVarChar,
        128);

    tblNameParam.Value = tableName;
    cmd.Parameters.Add(tblNameParam);
    SqlCeParameter colNameParam = new SqlCeParameter(
        "@columnName",
        SqlDbType.NVarChar,
        128);

    colNameParam.Value = tableName;
    cmd.Parameters.Add(colNameParam);
    int i = (int)cmd.ExecuteScalar();
    retVal = i > 0;
}

…所以我不知道哪种方法更可取;对我来说,这似乎更直接了……任何想法,任何人?


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2021-03-10

共1个答案

一尘不染

您可以只查询信息模式表以获取所需的信息:

public bool isValidField(string tableName, string columnName)
{
    var tblQuery = "SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS"
                   + " WHERE TABLE_NAME = @tableName AND"
                   + " COLUMN_NAME = @columnName";

    SqlCeCommand cmd = objCon.CreateCommand();
    cmd.CommandText = tblQuery;
    var tblNameParam = new SqlCeParameter(
        "@tableName",
        SqlDbType.NVarChar,
        128);

    tblNameParam.Value = tableName
    cmd.Parameters.Add(tblNameParam);
    var colNameParam = new SqlCeParameter(
        "@columnName",
        SqlDbType.NVarChar,
        128);

    colNameParam.Value = columnName
    cmd.Parameters.Add(colNameParam);
    object objvalid = cmd.ExecuteScalar(); // will return 1 or null
    return objvalid != null;
}
2021-03-10