read the schema of each table in the university database from ddl.sql or using .schema before working on this problem.) write the following queries in sql, using the university schema. (you should actually check the queries you write on the database.) (a) to insert into instructors those students who have earned more than 140 credits with salary 90000 to their respective departments. (b) to raise salary of instructors by 10% unless the salary becomes greater than 90,000; in such cases, give only a 3% raise. (c) to delete all instructors whose salary is less than half of the average salary of all instructors. (d) suppose we have three tables t1(a, b), t2(b, c), and t3(b, d), with all attributes declared as not null. consider the expressions (1) t1 natural left outer join (t2 natural left outer join t3) and (2) (t1 natural left outer join t2) natural left outer join t3 2 give an instance of t1, t2 and t3 such that in the result of the second expression, attribute c has a null value but attribute d has a non-null value. (e) in the above question, is c null and d not null possible in the result of the first expression? explain why or why not.