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Version: 2.5

Lambda expression

Lambda expressions are anonymous functions that can be passed as parameters into higher-order SQL functions. Lambda expressions allow you to develop code that is more concise, elegant, and extensible.

Lambda expressions are written with the -> operator, which reads "goes to". The left side of -> is the input parameters (if any) and the right side is an expression.

From v2.5 onwards, StarRocks supports using lambda expressions in the following higher-order SQL functions: array_map(), array_filter(), array_sum(), and array_sortby().

Syntax​

parameter -> expression

Parameters​

  • parameter: the input parameters for the lambda expression, which can accept zero, one, or more parameters. Two or more input parameters are enclosed in parentheses.

  • expression: a simple expression that references parameter. The expression must be valid for the input parameters.

Return value​

The type of the return value is determined by the result type of expression.

Usage notes​

Almost all scalar functions can be used in a lambda body. But there are a few exceptions:

  • Subqueries are not supported, for example, x -> 5 + (SELECT 3).
  • Aggregate functions are not supported, for example, x -> min(y).
  • Window functions are not supported.
  • Table functions are not supported.
  • Correlated columns cannot occur in lambda functions.

Examples​

Simple examples of lambda expressions:

-- Accepts no parameters and returns 5.
() -> 3    
-- Takes x and returns the value of (x + 2).
x -> x + 2 
-- Takes x and y, and returns their sum.
(x, y) -> x + y 
-- Takes x and applies a function to x.
x -> COALESCE(x, 0)
x -> day(x)
x -> split(x,",")
x -> if(x>0,"positive","negative")

An example of using lambda expressions in higher-order functions:

select array_map((x,y,z) -> x + y, [1], [2], [4]);
+----------------------------------------------+
| array_map((x, y, z) -> x + y, [1], [2], [4]) |
+----------------------------------------------+
| [3]                                          |
+----------------------------------------------+
1 row in set (0.01 sec)