The ALTER TABLE statement in SQL is used to modify an existing table structure—you can add, delete, or modify columns, change data types, and manage constraints.

For adding new column to existing table 

• ALTER TABLE table_name ADD column_name datatype;

For dropping a column from existing table

• ALTER TABLE table_name DROP COLUMN column_name;

For Changing datatype of existing table column

• ALTER TABLE table_name ALTER COLUMN column_name datatype;

For Adding primary key

• ALTER TABLE table_name ADD PRIMARY KEY (column_name);

For Dropping a constraint from existing table

• ALTER TABLE table_name DROP CONSTRAINT constraint_name;

For Renaming the column name

• EXEC sp_rename 'table_name .old_name', 'new_name', 'COLUMN';