什麼是 MySQL 儲存函數以及如何建立它們？

MySQL stored function is basically a special kind of stored program that returns a single value. We can use stored functions in MySQL to encapsulate mainly simple formulas orbus encapsulates . among SQL statements or stored programs. Other than that stored functions are used in SQL statements whenever an expression is used.

This feature of stored functions is different from store procedures .alt. IN parameter of the stored procedure as the functions use RETURN keyword to determine what is passed back. Its syntax can be as follows −

Syntax

CREATE
   [DEFINER = { user | CURRENT_USER }]
   FUNCTION sp_name ([func_parameter[,...]])
   RETURNS type
   [characteristic ...] routine_body

func_parameter:
   param_name type

type:
   Any valid MySQL data type

characteristic:
   COMMENT 'string'
 | LANGUAGE SQL
 | [NOT] DETERMINISTIC
 | { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA }
 | SQL SECURITY { DEFINER | INVOKER }

routine_body:
   Valid SQL routine statement

函數只有輸入參數和傳回值，因此在輸入參數和傳回值函數定義中必須有一個RETURNS子句來指示傳回值的資料型態。此外，在函數體內必須至少有一個RETURN語句來將值傳回給呼叫者。

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