Determining Primary Key columns via GetSchema

Question

Is there a way to determine whether a column is a primary key using the ADO.NET GetSchema method?

Here's what I've got so far:

public IEnumerable<DbColumnInfo> GetColumns(string providerName, string connectionString, string tableName)
{
    DbProviderFactory factory = DbProviderFactories.GetFactory(providerName);
    using (DbConnection connection = factory.CreateConnection())
    using (DbCommand command = factory.CreateCommand())
    {
        connection.ConnectionString = connectionString;
        connection.Open();
        command.Connection = connection;

        var columns = connection.GetSchema("Columns", tableName.Split('.'));
        foreach (DataRow row in columns.Rows)
        {
            yield return new DbColumnInfo()
            {
                Name = row.Field<string>(3),
                OrdinalPosition = row.Field<short>(4),
                DataType = this.FormatDataType(row),
                IsNullable = string.Equals(row.Field<string>(6), "yes", StringComparison.InvariantCultureIgnoreCase),
                IsPrimaryKey = // ... ?
            };
        }
    }
}

Answer 1

I'm afraid you can't determine with connection.GetSchema() ...

But as a workaround you can try the dataadapter, if it is suitable for you:

    var da = factory.CreateDataAdapter();
    command.CommandText = "select * from Employees";
    da.SelectCommand = command;
    da.MissingSchemaAction = MissingSchemaAction.AddWithKey;

    var dtab = new DataTable();
    da.FillSchema(dtab, SchemaType.Source);

    foreach (DataColumn col in dtab.Columns)
    {
        string name = col.ColumnName;
        bool isNull = col.AllowDBNull;
        bool isPrimary = dtab.PrimaryKey.Contains(col);
    }

Answer 2

Since qes asked, what I'm currently using (after several versions refactoring the code away since Laszlo provided his working solution) was not as elegant as I had hoped for, but it was more efficient and satisfied my needs. Basically I provide a DbInfoProvider class which is designed to produce DbTableInfo and DbColumnInfo for a given provider. The SqlDbInfoProvider uses this to get column information for SQL Server only:

public IEnumerable<DbColumnInfo> GetColumns(string connectionString, DbTableInfo table)
{
    DbProviderFactory factory = DbProviderFactories.GetFactory(this.providerName);
    using (DbConnection connection = factory.CreateConnection())
    using (DbCommand command = factory.CreateCommand())
    {
        connection.ConnectionString = connectionString;
        connection.Open();
        command.Connection = connection;
        command.CommandText = ColumnInfoQuery;
        command.CommandType = CommandType.Text;
        var tableSchema = factory.CreateParameter();
        tableSchema.ParameterName = "@tableSchema";
        tableSchema.DbType = DbType.String;
        tableSchema.Value = table.Schema;
        command.Parameters.Add(tableSchema);
        var tableName = factory.CreateParameter();
        tableName.ParameterName = "@tableName";
        tableName.DbType = DbType.String;
        tableName.Value = table.Name;
        command.Parameters.Add(tableName);

        var dataTable = new DataTable();
        using (var reader = command.ExecuteReader())
        {
            while (reader.Read())
            {
                yield return new DbColumnInfo()
                {
                    Name = reader.GetString(0),
                    OrdinalPosition = reader.GetInt32(1),
                    DataType = reader.GetString(2),
                    IsNullable = reader.GetBoolean(3),
                    IsPrimaryKey = reader.GetBoolean(4),
                    IsForeignKey = reader.GetBoolean(5),
                    IsUnique = reader.GetBoolean(6),
                };
            }
        }
    }
}

Where ColumnInfoQuery is a static string like this:

SELECT c.[column_name]
     , CAST(c.[ordinal_position] AS int) [ordinal_position]
     , CASE WHEN c.[data_type] IN ( 'bit', 'date', 'datetime', 'smalldatetime', 'int', 'bigint', 'smallint', 'tinyint', 'real', 'money', 'smallmoney', 'image', 'text', 'ntext' )  THEN c.[data_type]
            WHEN c.[character_maximum_length] IS NOT NULL
            THEN c.[data_type] + '(' + CAST(c.[character_maximum_length] AS VARCHAR(30)) + ')'
            WHEN c.[datetime_precision] IS NOT NULL 
            THEN c.[data_type] + '(' + CAST(c.[datetime_precision] AS VARCHAR(30)) + ')'
            WHEN c.[numeric_scale] IS NOT NULL 
            THEN c.[data_type] + '(' + CAST(c.[numeric_precision] AS VARCHAR(30)) + ',' + CAST(c.[numeric_scale] AS VARCHAR(30)) + ')'
            WHEN c.[numeric_precision] IS NOT NULL 
            THEN c.[data_type] + '(' + CAST(c.[numeric_precision] AS VARCHAR(30)) + ')'
            ELSE c.[data_type]
       END [data_type]
     , CAST(MAX(CASE c.[is_nullable] WHEN 'YES' THEN 1 ELSE 0 END) AS bit) [is_nullable]
     , CAST(MAX(CASE WHEN pk.[constraint_type] = 'PRIMARY KEY' THEN 1 ELSE 0 END) AS bit) [is_primary_key]
     , CAST(MAX(CASE WHEN pk.[constraint_type] = 'FOREIGN KEY' THEN 1 ELSE 0 END) AS bit) [is_foreign_key]
     , CAST(MAX(CASE WHEN pk.[constraint_type] = 'FOREIGN KEY' THEN 0 ELSE 1 END) AS bit) [is_unique]
FROM information_schema.columns c
LEFT JOIN information_schema.constraint_column_usage ccu 
        ON c.[column_name] = ccu.[column_name] 
        AND c.[table_name] = ccu.[table_name] 
        AND c.[table_schema] = ccu.[table_schema]
        AND c.[table_catalog] = ccu.[table_catalog]
LEFT JOIN information_schema.table_constraints pk 
        ON pk.[constraint_name] = ccu.[constraint_name]
        AND pk.[table_name] = ccu.[table_name] 
        AND pk.[constraint_schema] = ccu.[table_schema]
        AND pk.[constraint_catalog] = ccu.[table_catalog]
        AND pk.[constraint_type] IN ( 'PRIMARY KEY', 'FOREIGN KEY', 'UNIQUE' )
WHERE c.[table_schema] = @tableSchema
        AND c.[table_name] = @tableName
GROUP BY c.[table_schema], c.[table_name], c.[column_name], c.[ordinal_position]
       , c.[data_type], c.[character_maximum_length], c.[datetime_precision]
       , c.[numeric_precision], c.[numeric_scale], c.[is_nullable]

Answer 3

Yes, you can determine which columns are primary keys by requesting the schema for the Indexes. Then search the indexes for the specific colum / table

DataTable indexes = conn.GetSchema("Indexes");
List<string> PrimaryKeys = new List<string>();
foreach (DataRow row in indexes.Rows)
  if (Convert.ToBoolean(row["PRIMARY_KEY"]))
    PrimaryKeys.Add(row["TABLE_NAME"] + "." + row["COLUMN_NAME"]);

the PrimaryKeys will contain the list of primary keys in the database. simply check if your [table].[column] is in this list.