ALTER TABLE

1. To ALTER a TABLE by adding NewColumn to a TABLE

ALTER TABLE TableName
ADD NewColumn nvarchar(20)

2. To ALTER a TABLE by changing the datatype of a Column

ALTER TABLE TableName
ALTER COLUMN Column1 int
#where int is the new datatype of Column1

3. To ALTER a TABLE by dropping/truncating a single Column

ALTER TABLE TableName
DROP/TRUNCATE Column Column1

Similarly, the ALTER KeyWord can be use in many different ways as shown below:

ALTER TABLE TableName
ADD unique(Column1)
#This makes Column1 as a unique column

ALTER TABLE TableName
ALTER COLUMN Column1 int not null
#This makes Column1 as an integer datatype in which NULL value is not allowed

ALTER TABLE TableName
ADD constraint pk_Tablename_Column1 primary key(Column1)
#This makes Column1 as the primary key of TableName

ALTER TABLE TableName
DROP constraint pk_Tablename_Column1
#This drops the primary key status

ALTER TABLE TableName1
ADD foreign key(Column1) references TableName2(Column1)
#This adds foreign key in TableName1 to reference TableName2