Alter Table

Add Field

ALTER TABLE SchemaName.TableName
ADD ColumnName DECIMAL(8,2) NULL DEFAULT(0)

Note: you cannot add a NOT NULL without a default, since all existing rows need a value

Add Constraint

ALTER TABLE SchemaName.TableName
ADD CONSTRAINT DF_TableName_ConstraintName 
DEFAULT('') FOR ColumnName

Drop Constraint

ALTER TABLE SchemaName.TableName
DROP CONSTRAINT DFxxxxxxx

Drop Field

Note: you must drop constraints on a column before dropping a column

ALTER TABLE SchemaName.TableName
DROP ColumnName

Alter Field

Note: this removes a column and makes a new column

Note: when changing to NOT NULL, you will have to ensure that all existing values are NOT NULL with an update

ALTER TABLE SchemaName.TableName
ALTER ColumnName DECIMAL(9,2) NOT NULL