SQL

adding a default constraint to an existing column in sql

--Altering an existing column to add a default constraint:
ALTER TABLE {TABLE NAME}
ADD CONSTRAINT {CONSTRAINT NAME}
DEFAULT {DEFAULT VALUE} FOR {EXISTING COLUMN NAME}

--Adding a new column, with default value, to an existing table:
ALTER TABLE {TABLE NAME}
ADD {COLUMN NAME} {DATA TYPE} {NULL | NOT NULL}
CONSTRAINT {CONSTRAINT NAME} DEFAULT {DEFAULT VALUE}

--Dropping a contraint:
ALTER TABLE {TABLE NAME}
DROP CONSTRAINT {CONSTRAINT NAME}

SQL DEFAULT Constraint With Alter Table

#SQL Server

ALTER TABLE College
ADD CONSTRAINT country_default
DEFAULT 'US' FOR college_country;

#PostgreSQL

ALTER TABLE College
ALTER COLUMN college_code SET DEFAULT 'US';

#MySQL

ALTER TABLE College
ALTER college_country SET DEFAULT 'US';

#Oracle

ALTER TABLE College
MODIFY college_country DEFAULT 'US';

SQL DEFAULT Constraint

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255) DEFAULT 'Sandnes'
);