MYSQL获取数据库信息

$sql = "
    SELECT 
        COLUMNS.TABLE_NAME, COLUMNS.COLUMN_DEFAULT, COLUMNS.ORDINAL_POSITION, COLUMNS.COLUMN_NAME, COLUMNS.COLUMN_TYPE, COLUMNS.COLUMN_KEY, COLUMNS.EXTRA, COLUMNS.COLUMN_COMMENT, TABLES.TABLE_COMMENT       
    FROM 
        information_schema.COLUMNS 
        LEFT JOIN information_schema.TABLES ON TABLES.TABLE_NAME = COLUMNS.TABLE_NAME
    WHERE 
        COLUMNS.TABLE_SCHEMA = '{$dbname}'
";