Check if column exists in OleDb table

问题

I am trying to check if a column exists and if not, add it. I've tried a couple of solutions including this, but the syntax isn't correct for Access db.

This is what I have so far:

    public void Update(string task, string dbPath, string tableName = "Frames")
    {
        OleDbConnection db = new OleDbConnection("Provider=Microsoft.JET.OLEDB.4.0;data source=" + dbPath);
        db.Open();

        OleDbCommand command = db.CreateCommand();
        command.CommandText = "COL_LENGTH('Frames','SetNumber')";
        Debug.WriteLine(command.ExecuteReader());




        /*
        string[] restrictions = new string[] {null, null, tableName};

        DataTable dtColumns = db.GetOleDbSchemaTable(OleDbSchemaGuid.Columns, restrictions);

        foreach (DataColumn column in dtColumns.Columns)
        {
            Debug.WriteLine(column.ColumnName);
        }*/

    }

I also tried using GetOleDbSchemaTable but it isn't returning the right table or something. What am I missing?

回答1:

To check if a column exist in a datatable you could use the GetSchema method of the OleDbConnection

public void Update(string task, string dbPath, string colName, string tableName = "Frames") 
{ 
    using(OleDbConnection db = new OleDbConnection("........"))
    {
        db.Open(); 
        var schema = db.GetSchema("COLUMNS"); 
        var col = schema.Select("TABLE_NAME='" + tableName + 
                   " AND COLUMN_NAME='" + colName + "'" 

        if(col.Length > 0)
           // Column exist
        else
           // Column doesn't exist
} 

回答2:

Your approach with GetOleDbSchemaTable is the way to go. Each row of the schema table contains the information for a single column of your table and each column of the schema table represents one property of it. So, loop through the rows of the schema table, not the columns!

foreach (DataRow row in dtColumns.Rows) { // <== dtColumns.Rows
                                          //     (NOT dtColumns.Columns)
    string columnName = (string)row["COLUMN_NAME"];
    ....
}

Your approach yields the column names of the schema table itself.

来源：https://stackoverflow.com/questions/12148745/check-if-column-exists-in-oledb-table