Write the data definition statements of SQL that modify the structures of a database listed on a page $4$ of the

examination paper in the way described below. Note, that some of the modification may require more than

one data definition statements of SQL statement.

$(1)$ Add a foreign key constraint in the database that a department manager is an employee.

$(2$ marks$)$

$(2)$ Add a check constraint in the database that the value of the attribute Sex is either M or F$.$

$(2$ marks$)$

$(3)$ It is possible that the salary of an employee can be up to $$999999\mathrm{.}99.$

$(2$ marks$)$

$(4)$ In the future information about a department manager will be optional.

$(2$ marks$)$

$(5)$ We would like to store information in the database about the percentage of an employee works on a

project. The value of a percentage is between $0$ and $1.$ For example, $0\mathrm{.}25$ means $25\%,$ and $1\mathrm{.}0$ means

$100\%.$