How to add a column with a default value to an existing table in SQL

CREATE TABLE student (
    student_id INT ,
    student_name VARCHAR(50),
    major VARCHAR(50),
    batch INT
);

INSERT INTO student(student_id, student_name, major, batch)
VALUES (2, 'Dave', 'Medicine', 2017);
INSERT INTO student(student_id, student_name, major, batch)
VALUES (100, 'Jack', 'Arts', 2010);
INSERT INTO student(student_id, student_name, major, batch)
VALUES (12, 'Rose', 'Computer', 2012);

-- Altered 'student' table to add a new column 'country' with default value 'USA'.
-- Running this query will add the column along with defaulting its value as 'USA' in all previously existing rows.

-- For SQL SERVER
ALTER TABLE student
    ADD country VARCHAR(50) NOT NULL
CONSTRAINT cons_student_country
    DEFAULT ('USA');

SELECT * FROM student

-- Output
-- New column country is added and the value is defaulted to 'USA'

student_id  student_name   major      batch      country
--------------------------------------------------------------
2           Dave           Medicine   2017        USA
100         Jack           Arts       2010        USA
12          Rose           Computer   2012        USA

-- If we don't give any value in 'country' column in any newly add row 'USA' will be taken as its default value
INSERT INTO student(student_id, student_name, major, batch)
VALUES (55, 'Joe', 'History', 2016);

-- If we give a value in 'country' column in newly add row, the given data will be add instead of default value
INSERT INTO student(student_id, student_name, major, batch, country)
VALUES (55, 'Logan', 'History', 2016, 'Canada');

SELECT * FROM student

-- Output

student_id student_name    major        batch      country
--------------------------------------------------------------
2          Dave            Medicine     2017        USA
100        Jack            Arts         2010        USA
12         Rose            Computer     2012        USA
55         Joe             History      2016        USA
56         Logan           History      2016        Canada

-- Altered 'student' table to add a new column 'country' with default value 'USA'.
-- Running this query will add the column along with defaulting its value as 'USA' in all previously existing rows.
 
-- For MySQL
ALTER TABLE student
ADD (country VARCHAR(50) NOT NULL DEFAULT 'USA');

SELECT * FROM student

-- Output
-- New column country is added and the value has defaulted to 'USA'

student_id  student_name    major      batch      country
--------------------------------------------------------------
2            Dave           Medicine   2017        USA
100          Jack           Arts       2010        USA
12           Rose           Computer   2012        USA

-- If we don't give any value in 'country' column in any newly add row 'USA' will be taken as its default value
INSERT INTO student(student_id, student_name, major, batch)
VALUES (55, 'Joe', 'History', 2016);

-- If we give a value in 'country' column in newly add row, the given data will be add instead of default value
INSERT INTO student(student_id, student_name, major, batch, country)
VALUES (55, 'Logan', 'History', 2016, 'Canada');

SELECT * FROM student

-- Output

student_id student_name     major      batch      country
--------------------------------------------------------------
2           Dave            Medicine    2017        USA
100         Jack            Arts        2010        USA
12          Rose            Computer    2012        USA
55          Joe             History     2016        USA
56          Logan           History     2016        Canada