SQL

alter table

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

alter table

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

sql alter column

Changes the data type of a table’s column.
Example: In the ‘users’ table, make the column ‘incept_date’ into a
‘datetime’ type.
ALTER TABLE users
ALTER COLUMN incept_date datetime;

sql alter table

-- With check if field already exists

IF NOT EXISTS(SELECT 1 FROM sys.columns 
          WHERE Name = N'Email'
          AND Object_ID = Object_ID(N'dbo.Customers'))
BEGIN    
	ALTER TABLE Customers
    ADD Email varchar(255);
END

alter table

ALTER TABLE table_name 
DROP PRIMARY KEY;

alter table

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

alter table

ALTER TABLE table_name 
DROP CONSTRAINT MyUniqueConstraint;

alter table

ALTER TABLE table_name 
ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

alter table

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint UNIQUE(column1, column2...);

alter table

ALTER TABLE table_name 
DROP INDEX MyUniqueConstraint;