table NISHAN.TBL_ADMIN is mutating, trigger/function may not see it - oracle

Question

I have a trigger named tr_admin_user_role that automatically insert values into tbl_user_role table when we perform a insert in another table called tbl_admin. There is no error at compile time but whenever I insert a value into tbl_admin table it shows me an error and error is like
This is my tbl_admin table
CREATE TABLE tbl_admin(
admin_id INTEGER,
username VARCHAR2(50) NOT NULL UNIQUE,
passwords VARCHAR2(50) NOT NULL,
email VARCHAR2(100) UNIQUE,
enabled CHAR(1) DEFAULT 1 NOT NULL,
created_at DATE DEFAULT SYSDATE NOT NULL,
CONSTRAINT pk_admin_id PRIMARY KEY(admin_id)
);
tbl_user_role table
CREATE TABLE tbl_user_role(
user_role_id INTEGER,
username VARCHAR2(50) NOT NULL,
user_role VARCHAR2(50) DEFAULT 'ROLE_ADMIN' NOT NULL,
CONSTRAINT pk_user_role_id PRIMARY KEY(user_role_id)
);
Trigger that i have created
CREATE OR REPLACE TRIGGER tr_admin_user_role
AFTER INSERT ON tbl_admin
FOR EACH ROW
DECLARE
new_username TBL_ADMIN.username%TYPE;
BEGIN
SELECT username INTO new_username FROM (
SELECT username FROM tbl_admin ORDER BY username DESC
) WHERE ROWNUM = 1;
INSERT INTO tbl_user_role(username, user_role) VALUES(new_username, 'ROLE_ADMIN');
END;
Insert statement
INSERT INTO tbl_admin(username, passwords) VALUES('nisha', 'nisha');

Answer 1

That's not how you fetch the newly inserted / updated / previous value of a column in a Trigger. You should use the :OLD.column_name and :NEW.column_name to refer the old and new column values.Read the documentation to understand more.
So, your Trigger could be rewritten as
CREATE OR REPLACE TRIGGER tr_admin_user_role AFTER
INSERT ON tbl_admin
FOR EACH ROW
BEGIN
INSERT INTO tbl_user_role (
username,
user_role
) VALUES (
:NEW.username,
'ROLE_ADMIN'
);
END;
/
I assume you are using another trigger to generate
admin_id and user_role_id since they are declared as PRIMARY KEYs
and you are not including them in your inserts.
Db fiddle demo
Here I've used dummy values for those columns.