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APPENDIX D ANSWERS TO THE EXERCISES
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Solution 5-8b. Second Solution, Using Character Classes SQL> select comments 2 from history 3 where regexp_like(comments, '([[:alnum:]+[:punct:]]+[[:space:]]+){8,}'); COMMENTS -----------------------------------------------------------Not a great trainer; let's try the sales department! Sales also turns out to be not a success... Hired as the new manager for the accounting department Junior sales rep -- has lots to learn... :-) SQL>
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Note See Appendix C or Oracle SQL Reference for more details about character classes.
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7 Exercises
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1. Listing 7-5 defines the constraint E_SALES_CHK in a rather cryptic way. Formulate the same constraint without using DECODE and NVL2. Solution 7-1a. Solution 1 check ((job = 'SALESREP' and comm is not null) or (job <>'SALESREP' and comm is null) ) Solution 7-1b. Solution 2 check ((job = 'SALESREP' or comm is null) and not (job = 'SALESREP' and comm is null) ) 2. Why do you think the constraint E_DEPT_FK (in Listing 7-7) is created with a separate ALTER TABLE command Answer: You must define this constraint with an ALTER TABLE command because you have a chicken/egg problem. A foreign key constraint can refer to only an existing table, and you have two tables (EMPLOYEES and DEPARTMENTS) referring to each other. 3. Although this is not covered in this chapter, try to come up with an explanation of the following phenomenon: when using sequences, you cannot use the pseudo column CURRVAL in your session without first calling the pseudo column NEXTVAL.
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APPENDIX D ANSWERS TO THE EXERCISES
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Answer: In a multiuser environment, multiple database users can use the same sequence generator at the same time. Therefore, they will be using different CURRVAL values at the same time; that is, there is no database-wide current CURRVAL value. On the other hand, NEXTVAL is always defined as the next available sequence value. 4. Why is it better to use sequences in a multiuser environment, as opposed to maintaining a secondary table with the last/current sequence values Answer: A secondary table will become a performance bottleneck. Each update to a sequence value will lock the corresponding row. The next update can take place only after the first transaction has committed. In other words, all transactions needing a sequence value will be serialized. Sequences are better because they don t have this problem. With sequences, multiple transactions can be served simultaneously and independently. 5. How is it possible that the EVALUATION column of the REGISTRATIONS table accepts null values, in spite of the constraint R_EVAL_CHK (see Listing 7-11) Answer: This is caused by three-valued logic. A CHECK constraint condition can result in TRUE, FALSE, or UNKNOWN. Moreover, a CHECK constraint reports a violation only if its corresponding condition returns FALSE.
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Note This implies that you always need an explicit NOT NULL constraint if you want your
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columns to be mandatory; a CHECK constraint as shown in Listing 7-11 is not enough.
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6. If you define a PRIMARY KEY or UNIQUE constraint, the Oracle DBMS normally creates a unique index under the covers (if none of the existing indexes can be used) to check the constraint. Investigate and explain what happens if you define such a constraint as DEFERRABLE. Answer: If you define PRIMARY KEY or UNIQUE constraints as DEFERRABLE, the Oracle DBMS creates nonunique indexes. This is because indexes must be maintained immediately. Therefore, indexes for deferrable constraints must allow for temporary duplicate values until the end of your transactions. 7. You can use function-based indexes to implement conditional uniqueness constraints. Create a unique function-based index on the REGISTRATIONS table to check the following constraint: employees are allowed to attend the OAU course only once. They may attend other courses as many times as they like. Test your solution with the following command (it should fail): SQL> insert into registrations values (7900,'OAU',trunc(sysdate),null); Hint: You can use a CASE expression in the index expression.
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APPENDIX D ANSWERS TO THE EXERCISES
Solution 7-7. SQL> create unique index oau_reg on registrations 2 ( case course when 'OAU' then attendee else null end 3 , case course when 'OAU' then course else null end ); Index created. SQL> The trick is to create a function-based index on (ATTENDEE, COURSE) combinations, while ignoring all non-OAU course registrations. Here s the test: SQL> insert into registrations values (7900,'OAU',sysdate,null); insert into registrations values (7900,'OAU',sysdate,null) * ERROR at line 1: ORA-00001: unique constraint (BOOK.OAU_REG) violated SQL>
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