Question

当我通过执行以下操作选择所有内容时，我可以选择查询返回的行数：

SELECT Count(*) FROM `foo`.`bar`;

但是，我现在想要选择返回的列数。有一个简单的方法吗？

Answer 1

使用information_schema数据库。

SELECT 
  COUNT(COLUMN_NAME) AS numcols
FROM COLUMNS
WHERE 
  TABLE_NAME = 'bar'
  AND TABLE_SCHEMA = 'foo'

要将两者组合在一起，请使用：

SELECT
  (SELECT COUNT(*) FROM foo.bar) AS numrows,
  (SELECT 
      COUNT(COLUMN_NAME) AS numcols
    FROM COLUMNS
    WHERE 
      TABLE_NAME = 'bar'
      AND TABLE_SCHEMA = 'foo'
  ) AS numcols

Answer 2

结果

DESCRIBE someTable然后mysql_count_rows()。

由于问题与C＃有关，我认为这应该有用 - 但我很久没有做过C＃。

MySqlConnection connection = new MySqlConnection(#...);
MySqlCommand command = connection.CreateCommand();
MySqlDataReader Reader;
command.CommandText = "DESCRIBE sometable";
connection.Open();
Reader = command.ExecuteReader();
Reader.FieldCount # this has the count of rows, and hence columns

Answer 3

如果您的表格mydb.mytable是MyISAM，则以下内容应运行良好：

SELECT
    row_count,column_count
FROM
    (
        SELECT table_rows row_count
        FROM information_schema.tables
        WHERE table_schema = 'mydb'
        AND table_name = 'mytable'
    ) rowcount,
    (
        SELECT MAX(ordinal_position) column_count
        FROM information_schema.columns
        WHERE table_schema = 'mydb'
        AND table_name = 'mytable'
    ) columncount
;

如何使用C＃计算MySQL表中的列数？

3 个答案: