You need to add a constraint.

Referring to SQL SERVER – Create Default Constraint Over Table Column

ALTER TABLE Employee 
  ADD CONSTRAINT DF_Employee _JobTitle
  DEFAULT '' FOR sJobTitle

try this query. this query does not change current datas to default.

ALTER TABLE Employee
ADD DEFAULT ('') FOR sJobTitle

maybe it work.

ALTER TABLE Employee 
ADD sJobTitle varchar(200) NOT NULL DEFAULT ''

simplest answer.

You can try this, single and multiple column

Single column:

ALTER TABLE `DBname`.`table_name` 
 MODIFY COLUMN `foo_name` VARCHAR(255) NOT NULL DEFAULT '';

multiple column:

ALTER TABLE `DBname`.`table_name`
 MODIFY COLUMN `foo_name` VARCHAR(255) NOT NULL DEFAULT '',
 MODIFY COLUMN `foo_roll` VARCHAR(65) NOT NULL DEFAULT '';