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Lambda expressions are written with ->: 
    x -> x + 1
    (x, y) -> x + y
    x -> regexp_like(x, 'a+')
    x -> x[1] / x[2]
    x -> IF(x > 0, x, -x)
    x -> COALESCE(x, 0)
    x -> CAST(x AS JSON)
    x -> x + TRY(1 / 0)

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Limitations

Most SQL expressions can be used in a lambda body, with a few exceptions:
  • Subqueries are not supported: x -> 2 + (SELECT 3)
  • Aggregations are not supported: x -> max(y)

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Examples

Obtain the squared elements of an array column with transform: 
    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);
Results:
numberssquared_numbers
[1, 2][1, 4]
[3, 4][9, 16]
[5, 6, 7][25, 36, 49]
The function transform can be also employed to safely cast the elements of an array to strings: 
    SELECT transform(prices, n -> TRY_CAST(n AS VARCHAR) || '$') as price_tags
    FROM (
        VALUES
            (ARRAY[100, 200]),
            (ARRAY[30, 4])
    ) AS t(prices);
Results:
price_tags
[100,200, 200]
[30,4, 4]
Besides the array column being manipulated, other columns can be captured as well within the lambda expression. The following statement provides a showcase of this feature for calculating the value of the linear function f(x) = ax + b with transform: 
    SELECT xvalues,
           a,
           b,
           transform(xvalues, x -> a * x + b) as linear_function_values
    FROM (
        VALUES
            (ARRAY[1, 2], 10, 5),
            (ARRAY[3, 4], 4, 2)
    ) AS t(xvalues, a, b);
Results:
xvaluesablinear_function_values
[1, 2]105[15, 25]
[3, 4]42[14, 18]
Find the array elements containing at least one value greater than 100 with any_match: 
    SELECT numbers
    FROM (
        VALUES
            (ARRAY[1,NULL,3]),
            (ARRAY[10,20,30]),
            (ARRAY[100,200,300])
    ) AS t(numbers)
    WHERE any_match(numbers, n ->  COALESCE(n, 0) > 100);
    -- [100, 200, 300]
Capitalize the f word in a string via regexp_replace: 
    SELECT regexp_replace('once upon a time ...', '^(\w)(\w*)(\s+.*)$',x -> upper(x[1]) || x[2] || x[3]);
    -- Once upon a time ...
Lambda expressions can be also applied in aggregation functions. Following statement is a sample the overly complex calculation of the sum of all elements of a column by making use of reduce_agg: 
    SELECT reduce_agg(value, 0, (a, b) -> a + b, (a, b) -> a + b) sum_values
    FROM (
        VALUES (1), (2), (3), (4), (5)
    ) AS t(value);
    -- 15
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