=> 是 Oracle 中调用存储过程的时候, 指定参数名进行调用。一般是， 某些参数有默认值的时候，你需要跳过某些参数来进行调用。
下面是具体的例子。
参数的默认值
SQL> CREATE OR REPLACE PROCEDURE HelloWorld3 (
2 p_user_name VARCHAR2,
3 p_val1 VARCHAR2 DEFAULT ' Good Moning,',
4 p_val2 VARCHAR2 DEFAULT ' Nice to Meet you'
5 ) AS
6 BEGIN
7 dbms_output.put_line('Hello ' || p_user_name || p_val1 || p_val2 || '!'
);
8 END HelloWorld3;
9 /
Procedure created.
SQL> BEGIN
2 HelloWorld3('Edward');
3 HelloWorld3('Edward', ' Good Night,');
4 HelloWorld3('Edward', ' Good Night,', 'Bye');
5 END;
6 /
输出：
Hello Edward Good Moning, Nice to Meet you!
Hello Edward Good Night, Nice to Meet you!
Hello Edward Good Night,Bye!
PL/SQL procedure successfully completed.
指定参数名称调用。此部分使用 “参数默认值”那一小节的存储过程。用于说明当最后2个参数是有默认的时候，如何跳过中间那个。
SQL> BEGIN
2 HelloWorld3('Edward');
3 HelloWorld3('Edward', p_val1 => ' Good Night,');
4 HelloWorld3('Edward', p_val1 => ' Good Night,', p_val2 => 'Bye');
5 HelloWorld3('Edward', p_val2 => ' HeiHei ');
6 END;
7 /
输出：
Hello Edward Good Moning, Nice to Meet you!
Hello Edward Good Night, Nice to Meet you!
Hello Edward Good Night,Bye!
Hello Edward Good Moning, HeiHei !