unique method
Set column as UNIQUE
Implementation
ColumnDefinition unique([String? constraintName]) {
if (constraintName != null) {
_unique = false;
_uniqueConstraintName = constraintName;
_schema.addCompositeUniqueConstraint(constraintName, [_name]);
} else {
_unique = true;
_uniqueConstraintName = null;
}
return this;
}