How to write an alter table statement to add a constraint - sql

Question

I'm, new to SQL. I have created few tables:
CREATE TABLE MAINTINANCE
(Maint_mname char(10),
Maint_date date,
Maint_duedate date NOT NULL,
Maint_mdesc char (15));
CREATE TABLE DESIGNERR
(Dez_emp_number varchar(11),
Dez_field char(12),
Dez_qualification char(10) NOT NULL,
Dez_experience smallint);
For the first table I am adding the following constraint:
ALTER TABLE MAINTINANCE ADD CONSTRAINT CHK_maintdate CHECK(Maint_date<MAint_duedate);
but I am getting the error invalid ALTER TABLE option. Could you please let me know why this is appearing? The same is working for a friend but not for me.
For the second table I have to write a SQL command for the business rule:
If the Qualification of a Designer is BS then a Minimum of 4 years
Experience is required. But, if the Qualification of the Designer is
MS then a Minimum of 2 years Experience is sufficient.
How can we define this business rule in SQL?

Answer 1

The code you posted works
SQL> CREATE TABLE MAINTINANCE
2 (Maint_mname char(10),
3 Maint_date date,
4 Maint_duedate date NOT NULL,
5 Maint_mdesc char (15));
Table created.
SQL> ALTER TABLE MAINTINANCE ADD CONSTRAINT CHK_maintdate CHECK(Maint_date<MAint_duedate);
Table altered.
If you're getting an error,
Cut and paste from a SQL*Plus session that shows exactly what statements you are executing
Post the full error stack
Although it does not affect your code, the word "MAINTINANCE" is misspelled. It should be "Maintenance". Future human developers will be grateful if your table names are spelled correctly.