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User-Defined Functions (UDFs)

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User-Defined Functions (UDFs) in BigQuery allow you to extend the capabilities of SQL by writing custom functions using JavaScript or SQL. UDFs can be used to encapsulate complex logic, perform transformations, and reuse code across multiple queries.

Key Concepts

  1. Types of UDFs:

    • SQL UDFs: Written in SQL, these functions allow you to encapsulate SQL logic.
    • JavaScript UDFs: Written in JavaScript, these functions provide more flexibility and can handle more complex logic.

  2. Creating UDFs:

    • SQL UDFs: Defined using the CREATE FUNCTION statement.
    • JavaScript UDFs: Defined using the CREATE FUNCTION statement with JavaScript code embedded.

  3. Using UDFs:

    • UDFs can be called in SQL queries just like built-in functions.
    • They can accept parameters and return values.

Creating SQL UDFs

Example: Simple SQL UDF

Let's create a simple SQL UDF that calculates the square of a number.

CREATE FUNCTION my_project.my_dataset.square(x INT64) 
RETURNS INT64 
AS (
  x * x
);

Explanation

  • CREATE FUNCTION my_project.my_dataset.square(x INT64): Defines a new function named square in the specified project and dataset. The function takes an integer parameter x.
  • RETURNS INT64: Specifies that the function returns an integer.
  • AS (x * x): The function body, which calculates the square of x.

Using the SQL UDF

SELECT my_project.my_dataset.square(4) AS result;

Output

result
16

Creating JavaScript UDFs

Example: JavaScript UDF

Let's create a JavaScript UDF that reverses a string.

CREATE FUNCTION my_project.my_dataset.reverse_string(input STRING) 
RETURNS STRING 
LANGUAGE js 
AS """
  return input.split('').reverse().join('');
""";

Explanation

  • CREATE FUNCTION my_project.my_dataset.reverse_string(input STRING): Defines a new function named reverse_string in the specified project and dataset. The function takes a string parameter input.
  • RETURNS STRING: Specifies that the function returns a string.
  • LANGUAGE js: Indicates that the function is written in JavaScript.
  • AS """ ... """: The function body, which contains JavaScript code to reverse the input string.

Using the JavaScript UDF

SELECT my_project.my_dataset.reverse_string('BigQuery') AS result;

Output

result
yreuQgiB

Practical Exercises

Exercise 1: Create a SQL UDF

Task: Create a SQL UDF named cube that calculates the cube of a number.

Solution:

CREATE FUNCTION my_project.my_dataset.cube(x INT64) 
RETURNS INT64 
AS (
  x * x * x
);

Test the UDF:

SELECT my_project.my_dataset.cube(3) AS result;

Expected Output:

result
27

Exercise 2: Create a JavaScript UDF

Task: Create a JavaScript UDF named to_uppercase that converts a string to uppercase.

Solution:

CREATE FUNCTION my_project.my_dataset.to_uppercase(input STRING) 
RETURNS STRING 
LANGUAGE js 
AS """
  return input.toUpperCase();
""";

Test the UDF:

SELECT my_project.my_dataset.to_uppercase('bigquery') AS result;

Expected Output:

result
BIGQUERY

Common Mistakes and Tips

  • Syntax Errors: Ensure that the SQL or JavaScript syntax is correct. Use the appropriate language keywords and structure.
  • Parameter Types: Make sure the parameter types match the expected input types.
  • Return Types: Ensure the return type of the UDF matches the expected output type.
  • Testing: Always test your UDFs with various inputs to ensure they work as expected.

Conclusion

User-Defined Functions (UDFs) in BigQuery provide a powerful way to extend SQL capabilities by encapsulating complex logic in reusable functions. By understanding how to create and use both SQL and JavaScript UDFs, you can enhance your data processing and analysis workflows in BigQuery. In the next section, we will explore partitioning and clustering to optimize query performance.

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