I already create tables but I had difficulties with typing the constraint

( You need to have the following constraints in your system: The employees salary should be between the grade range (Example: Grade A - Salary between 30000-50000). Employees phone number, Hiredate, Date of Birth should not be empty. The age should between 18 and 60. Vehicle next maintenance is within 3 month of current date. Each position should not exceed the predefined allowances. The System will need a constraint for Gender (M,F). The Username must be Unique in Table Login..)

how can i apply these constraint?

like what are the correct syntax to type those constraint?

I only did for the gender and it was like this

constraint something check (gender='m' or gender='f')