oracle function usage

Oracle function is one of the very important concepts in Oracle database. Simply put, functions are reusable pieces of code that receive input values ​​and return output values. Oracle database provides a very rich set of built-in functions, including functions for mathematics, strings, dates, conversions, etc. Functions can help developers write code more efficiently and improve program readability and maintainability. This article will introduce the usage of Oracle functions in detail.

1. Classification of Oracle functions

Oracle functions can be divided into the following two categories:

1. Built-in functions: function libraries provided by Oracle database, including mathematics and characters Functions for strings, dates, conversions, etc. These functions are already defined, usually without rewriting, and can be used directly in SQL statements or PL/SQL programs.
2. Custom function: Developers can write custom functions according to their own needs. Custom functions can encapsulate a piece of complex logic into a function for easy reuse.

2. Use of built-in functions

Oracle database provides a very rich set of built-in functions, which can help developers write code more efficiently. The following introduces some commonly used built-in functions and their usage.

1. Mathematical functions

Oracle database provides many mathematical functions, including absolute value functions, square root functions, exponential functions, logarithmic functions, etc. Usage is as follows:

ABS(x): Calculate the absolute value of x.

SQRT(x): Calculate the square root of x.

EXP(x): Calculate e raised to the x power.

LOG(x): Calculate the natural logarithm of x.

2. String functions

Oracle database provides many string functions, including string connection functions, string interception functions, string replacement functions, and string length functions etc. Usage is as follows:

CONCAT(s1, s2): Concatenate strings s1 and s2.

SUBSTR(s, start, length): Remove the substring starting from the start position and having a length of length from the string s.

REPLACE(s1, s2, s3): Replace s2 in string s1 with s3.

LENGTH(s): Calculate the length of string s.

3. Date functions

Oracle database provides many date functions, including date addition and subtraction functions, date formatting functions, date comparison functions, etc. The usage is as follows:

ADD_MONTHS(date, n): Add n months to date date.

TO_CHAR(date, format): Format date date into a string.

TO_DATE(str, format): Convert the string str into a date.

MONTHS_BETWEEN(date1, date2): Calculate the month difference between dates date1 and date2.

4. Conversion functions

Oracle database provides many conversion functions, including type conversion functions, character set conversion functions, and so on. The usage is as follows:

TO_NUMBER(str, format): Convert the string str into a number.

TO_CHAR(val, format): Format the number val into a string.

CAST(val AS type): Convert the data type val to type type.

NLS_CHARSET_DECL_LEN(str): Calculate the number of bytes occupied by the string str.

3. Use of custom functions

Developers can write custom functions according to their own needs. The following describes how to create and use custom functions.

1. Create a custom function

To create a custom function, you need to use the CREATE FUNCTION statement. For example:

CREATE FUNCTION my_function(p1 IN NUMBER, p2 IN NUMBER)
RETURN NUMBER
IS
BEGIN

-- 在这里编写函数体
RETURN p1 + p2;

END;

This is A simple custom function that takes two numeric arguments and returns their sum.

2. Call a custom function

To use a custom function in a SQL statement or PL/SQL program, you need to use the following syntax:

SELECT my_function (1, 2) FROM dual;

This will call the my_function function and pass in parameters 1 and 2. The function will return 3.

4. Summary

Oracle functions are one of the very important concepts in the Oracle database. They can help developers write code more efficiently and improve the readability and maintainability of the program. Oracle database provides a very rich set of built-in functions, including functions for mathematics, strings, dates, conversions, etc. Developers can also write custom functions according to their needs. When using functions, you need to pay attention to the types of parameters and the type of return values ​​to avoid type conversion errors.

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