Lambda函数

限制

• 不支持子查询，例如x->2+(SELECT3)。

• 不支持聚合，例如x -> max(y)。

语法

语法形式是：参数列表 -> 表达式或代码块

• 参数列表：是指Lambda函数接收的参数，可以为空或包含一个或多个参数。

• ->：分隔参数列表和表达式或代码块。

使用示例

• 示例1：获取数组列的平方值

  SELECT numbers,transform(numbers, n -> n * n) as squared_numbers
  FROM (
      VALUES (ARRAY(1, 2)),(ARRAY(3, 4)),(ARRAY(5, 6, 7))
  ) AS t(numbers);

  返回结果如下：

  +------------+-----------------+
  | numbers    | squared_numbers |
  +------------+-----------------+
  | [5,6,7]    | [25,36,49]      |
  | [3,4]      | [9,16]          |
  | [1,2]      | [1,4]           |
  +------------+-----------------+

• 示例2：将数组转换为字符串

  SELECT transform(prices, n -> CAST(n AS STRING ) || '$') as price_tags
  FROM (
      VALUES
          (ARRAY(100, 200)),(ARRAY(30, 4))
  ) AS t(prices);

  返回结果如下：

  +------------+
  | price_tags |
  +------------+
  | ["30$","4$"] |
  | ["100$","200$"] |
  +------------+

• 示例3：计算线性函数值

  SELECT xvalues, a, b,
         transform(xvalues, x -> a * x + b) as linear_function_values
  FROM (
      VALUES (ARRAY(1, 2), 10, 5),(ARRAY(3, 4), 4, 2)
  ) AS t(xvalues, a, b);

  返回结果如下：

  +------------+------------+------------+------------------------+
  | xvalues    | a          | b          | linear_function_values |
  +------------+------------+------------+------------------------+
  | [3,4]      | 4          | 2          | [14,18]                |
  | [1,2]      | 10         | 5          | [15,25]                |
  +------------+------------+------------+------------------------+