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Figure 7-20. A COMMENT command syntax diagram Listing 7-19 shows how you can use the COMMENT command to add comments to the data dictionary for a table (SALGRADES) and a column (EMPLOYEES.COMM), and how you can retrieve that information from the data dictionary. Listing 7-19. Adding Comments to Columns and Tables SQL> comment on table salgrades 2 is 'Salary grades and net bonuses'; Comment created. SQL> comment on column employees.comm 2 is 'For sales reps only'; Comment created. SQL> select comments 2 from user_tab_comments 3 where table_name = 'SALGRADES'; COMMENTS ------------------------------------------Salary grades and net bonuses SQL> 2 3 4 select from where and comments user_col_comments table_name = 'EMPLOYEES' column_name = 'COMM';
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COMMENTS ------------------------------------------For sales reps only SQL> Think of adding comments like documentation. The little extra effort to document your columns and tables will help define and clarify your objects if questions arise in the future.
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DATA DEFINITION, PART II
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7.13 Exercises
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The following exercises will help you to better understand the concepts described in this chapter. The answers are presented in Appendix D. 1. 2. 3. Listing 7-5 defines the constraint E_SALES_CHK in a rather cryptic way. Formulate the same constraint without using DECODE and NVL2. Why do you think the constraint E_DEPT_FK (in Listing 7-7) is created with a separate ALTER TABLE command 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: SQL> select deptno_seq.currval from dual; select deptno_seq.currval from dual * ERROR at line 1: ORA-08002: sequence DEPTNO_SEQ.CURRVAL is not yet defined in this session SQL> 4. 5. 6. Why is it better to use sequences in a multiuser environment, as opposed to maintaining a secondary table with the last/current sequence values 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) 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. 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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Retrieval: Multiple Tables and Aggregation
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This chapter resumes the discussion of the retrieval possibilities of the SQL language. It is a logical continuation of s 4 and 5. The first section introduces the concept of row or tuple variables. We did not discuss them so far, because we haven t needed them up to now. By the way, most SQL textbooks don t mention tuple variables at all at least not the way this book does. When you start specifying multiple tables in the FROM clause of your SELECT statements, it is a good idea to start using tuple variables (also referred to as table aliases in Oracle) in a consistent way. Section 8.2 explains joins, which specify a comma-separated list of table names in the FROM clause and filter the desired row combinations with the WHERE clause. Section 8.3 shows the ANSI/ISO standard syntax to produce joins (supported since Oracle9i), and Section 8.4 goes into more details about outer joins. In large information systems (containing huge amounts of detailed information), it is quite common to be interested in aggregated (condensed) information. For example, you may want to get a course overview for a specific year, showing the number of attendees per course, with the average evaluation scores. You can formulate the underlying queries you need for such reports by using the GROUP BY clause of the SELECT command. Group functions (such as COUNT, AVG, MIN, and MAX) play an important role in such queries. If you have aggregated your data with a GROUP BY clause, you can optionally use the HAVING clause to filter query results at the group level. Topics surrounding basic aggregation are covered in Sections 8.5, 8.6, and 8.7. Section 8.8 continues the discussion of aggregation to introduce some more advanced features of the GROUP BY clause, such as CUBE and ROLLUP. Section 8.9 introduces the concept of partitioned outer joins. Section 8.10 finishes with the three set operators of the SQL language: UNION, MINUS, and INTERSECT.
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