User-defined functions (UDFs) are user-defined reusable functions used to encapsulate common logic to enhance SQL query flexibility. 1. UDF can be used for data conversion, simplifying complex expressions, and improving code readability; 2. The creation method varies from database to database. For example, SQL Server uses CREATE FUNCTION, and PostgreSQL supports multiple languages; 3. When using it, avoid frequent access to tables and pay attention to performance impact; 4. Clear naming and add comments to facilitate maintenance and migration. The rational use of UDF can improve development efficiency, but it requires weighing performance and abstraction.

When writing SQL queries, built-in functions often cannot meet all requirements. At this time, you need to create user-defined functions (UDFs) yourself to make the query more flexible and the code is more concise. If you use it well, you can save a lot of repetitive operations, but you should also be careful not to abuse it.

What is a User-Defined Function (UDF)

Simply put, UDF is a function you wrote yourself , which can be called in SQL queries like built-in functions. It is usually used to encapsulate a piece of commonly used logic or calculations, such as formatting dates, calculating discount prices, extracting specific parts of a string, etc.

Unlike stored procedures, UDF can be directly embedded in SELECT, WHERE, JOIN and other statements , which makes it particularly suitable for data processing stages.

How to create a UDF

The syntax of different database systems is slightly different, but the basic structure is similar. Take SQL Server and PostgreSQL as examples:

The basic structure of SQL Server creating UDFs:

 CREATE FUNCTION dbo.CalculateDiscount (@price DECIMAL(10,2), @discountRate DECIMAL(5,2))
RETURNS DECIMAL(10,2)
AS
BEGIN
    RETURN @price * (1 - @discountRate / 100)
END

Then you can use it like this:

 SELECT ProductName, dbo.CalculateDiscount(Price, Discount) AS FinalPrice FROM Products;

PostgreSQL creates UDFs slightly differently. It generally uses CREATE OR REPLACE FUNCTION , which supports multiple languages ??such as PL/pgSQL.

After the creation is completed, remember to test whether there will be errors in input boundary values, such as whether negative numbers, NULL values, etc. have been processed.

Several practical scenarios for using UDF

• Unified data conversion logic : for example, converting timestamps into formats like "a few hours ago", or translating status codes into Chinese.
• Simplify complex expressions : Some calculations may require multiple conditional judgments, which is more convenient to maintain in functions.
• Improve readability : CalculateTax(@amount) is easier to understand than a bunch of tax rate formulas.

However, be aware that UDF may also cause performance problems . Especially when it contains complex logic or nested queries inside, it may slow down the entire query. In this case, consider rewriting to an inline expression or view.

Some precautions for using UDF

• Avoid frequent access to tables in UDFs, otherwise it will increase execution time.
• If the result returned by the function is deterministic (for example, relying only on input parameters), it can be marked as a "deterministic function", which helps the optimizer to optimize.
• Different databases have different levels of support for UDF, so please pay attention to compatibility when migrating.
• To give a clear name to UDF, it is best to add comments to explain the purpose and parameter meaning.

For example, in SQL Server, you can view existing functions in the following ways:

 SELECT name, type_desc FROM sys.objects WHERE type = 'FN';

Basically that's it. UDF is a good tool to improve SQL writing efficiency, but it also needs to be used according to actual conditions. Rationally encapsulating logic can make the code clearer, but excessive abstraction will affect performance and maintainability.

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