How can I determine whether a column exists in a SQL Server CE table with C#?

问题

The legacy code does it this way:

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;
}

...but I find it doesn't always work. After calling that method, and getting false, some code ALTERS the table to add some columns, but I'm getting:

A column ID occurred more than once in the specification.

I found here on StackOverflow a couple of promising SQL statements:

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

and

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

...but am not sure how to implement this in C# / .NET 1.1

Do I need to use ExecuteScalar and cast the returned value to a bool? Or something else?

UPDATE

Changing it to this didn't rectify matters:

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;
}

UPDATE 2

Oddly enough, I see even more of those error messages with the new code.

UPDATE 3

It made no difference when I altered the code this way:

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;
}

...so I don't know which way is preferable; this does seem more straightforward to me...any thoughts, anyone?

回答1:

You can just query the information schema tables to get the information you want:

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;
}

回答2:

    public bool IsValidField(SqlCeConnection objCon, string tableName, string columnName)
    {
        const string query = "SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS"
                                + " WHERE TABLE_NAME = @TABLENAME AND"
                                + " COLUMN_NAME = @COLUMNNAME";

        using (var cmd = new SqlCeCommand(query, objCon))
        {
            cmd.Parameters.Add("@TABLENAME", SqlDbType.NVarChar, 128).Value = tableName;
            cmd.Parameters.Add("@COLUMNNAME", SqlDbType.NVarChar, 128).Value = columnName;
            var objvalid = cmd.ExecuteScalar(); // will return 1 or null
            return objvalid != null;
        }
    }

来源：https://stackoverflow.com/questions/15693639/how-can-i-determine-whether-a-column-exists-in-a-sql-server-ce-table-with-c

标签

sql

compact-framework

sql-server-ce

windows-ce