Interview Questions

Mainframe, QA

Wednesday, August 02, 2006

DB2 Qs and As

1.How would you find out the total number of rows in a table?

A. Use SELECT COUNT(*) ... Allstate uses DB2v6. and QMF V3.3.

2. How do you eliminate duplicate values in SELECT?

A. Use SELECT DISTINCT ...

3. How do you select a row using indexes?

A. Specify the indexed columns in the WHERE clause.

4. Can you use MAX on a CHAR column?

A. YES.

5. My SQL statement SELECT AVG(SALARY) FROM EMP yields inaccurate results. Why?

A. Because SALARY is not declared to have NULLs and the employees for whom the salary is not known are also counted.

6. What is the use of VALUE function?

A. 1. Avoid -ve SQLCODEs by handling nulls and zeroes in computations

2. Substitute a numeric value for any nulls used in computation

7. How do you concatenate the FIRSTNAME and LASTNAME from EMP table to give a complete name?

A. SELECT FIRSTNAME ' ' LASTNAME FROM EMP;

8. What is the use of VALUE function?

A. Avoid -ve SQLCODE s by handling nulls and zeroes in computations by substituting a numeric value for any nulls used in computation

9. What is the restriction on using UNION in embedded SQL?

A. It has to be in a CURSOR.

10. In the WHERE clause what is BETWEEN and IN?

A. BETWEEN supplies a range of values while IN supplies a list of values.

11. Is BETWEEN inclusive of the range values specified?

A. Yes.

12. What is 'LIKE' used for in WHERE clause? What are the wildcard characters?

A. LIKE is used for partial string matches. '%' ( for a string of any character ) and '_' (for any single character ) are the two wild card characters.

13. When do you use a LIKE statement?

A. To do partial search e.g. to search employee by name, you need not specify the complete name; using LIKE, you can search for partial string matches.

14. What is the meaning of underscore ( '_' ) in the LIKE statement?

A. Match for any single character.

15. What do you accomplish by GROUP BY ... HAVING clause?

A. GROUP BY partitions the selected rows on the distinct values of the column on which you group by. HAVING selects GROUP s, which match the criteria specified

16. Apart from cursor, what other ways are available to you to retrieve a row from a table in embedded SQL?

Single row SELECTs.

17. What are correlated Sub-queries?

A. Sub-query in which the inner ( nested ) query refers back to the table in the outer query. Correlated. Sub-queries must be evaluated for each qualified row of the outer query that is referred to.

18. Consider the employee table with column PROJECT nullable. How can you get a list of employees who are not assigned to any project?

A. SELECT EMPNO

FROM EMP

WHERE PROJECT IS NULL;

19. How and where do you specify and use a cursor in a COBOL program?

Use DECLARE CURSOR statement either in working storage or in procedure division(before open cursor), to specify the SELECT statement. Then use OPEN, FETCH rows in a loop and finally CLOSE.

20. What is the result of this query if no rows are selected?

SELECT SUM(SALARY)

FROM EMP

WHERE QUAL='MSC';

A. NULL

21. Why SELECT * is not preferred in embedded SQL programs?

A. For three reasons:


If the table structure is changed ( a field is added ), the program will have to be modified
Program might retrieve the columns which it might not use, leading on I/O over head.
The chance of an index only scan is lost.

22. How would you retrieve rows from a DB2 table in embedded SQL?

A. Either by using the single row SELECT statements, or by using the CURSOR.

23. What happens when you say OPEN CURSOR?

A. If there is an ORDER BY clause, rows are fetched, sorted and made available for the FETCH statement. Other wise simply the cursor is placed on the first row.

24. Is DECLARE CURSOR executable?

A. No.

25. Can you have more than one cursor open at any one time in a program ?

A. Yes.

26.. When you COMMIT, is the cursor closed?

A. Yes.

27. How do you leave the cursor open after issuing a COMMIT? ( for DB2 2.3 or above only )

Use WITH HOLD option in DECLARE CURSOR statement. But, it has not effect in psuedo-conversational CICS programs.

28. Give the COBOL definition of a VARCHAR field.

A. A VARCHAR column REMARKS would be defined as follows:
...

10 REMARKS.

49 REMARKS-LEN PIC S9(4) USAGE COMP.

49 REMARKS-TEXT PIC X(1920).

29. What is the COBOL picture clause for a DB2 column defined as DECIMAL(11,2)?

A. PIC S9(9)V99 COMP-3. Note: In DECIMAL(11,2), 11 indicates the size of the data type and 2 indicates the precision.

30. What is DCLGEN ?

A. It's an acronym for DeCLarations GENerator: used to create the host language copy books for the table definitions. Also creates the DECLARE table.

31. What are the contents of a DCLGEN?

A. 1. EXEC SQL DECLARE TABLE statement which gives the layout of the table/view in terms of DB2 datatypes. 2. A host language copy book that gives the host variable definitions for the column names.

32. Is it mandatory to use DCLGEN? If not, why would you use it at all?

A. It is not mandatory to use DCLGEN. Using DCLGEN, helps detect wrongly spelt column names etc. during the pre-compile stage itself ( because of the DECLARE TABLE ). DCLGEN being a tool, would generate accurate host variable definitions for the table reducing chances of error.

33. Is DECLARE TABLE in DCLGEN necessary? Why it used?

A. It not necessary to have DECLARE TABLE statement in DCLGEN. This is used by the pre-compiler to validate the table-name, view-name, column name etc., during pre-compile.

34. Will precompile of an DB2-COBOL program bomb, if DB2 is down?

A. No. Because the precompiler does not refer to the DB2 catalogue tables.

35. Name some fields from SQLCA.


SQLCODE, SQLERRM, SQLERRD.

36. How is a typical DB2 batch program executed ?

A. 1. Use DSN utility to run a DB2 batch program from native TSO. An example is shown:

DSN SYSTEM(DSP3)

RUN PROGRAM(EDD470BD) PLAN(EDD470BD) LIB('EDGS01T.OBJ.LOADLIB')

END

2. Use IKJEFT01 utility program to run the above DSN command in a JCL.

37. Assuming that a site's standard is that pgm name = plan name, what is the easiest way to find out which programs are affected by change in a table's structure ?

A. Query the catalogue tables SYSPLANDEP and SYSPACKDEP.

38. How can you quickly find out the # of rows updated after an update statement?

A. Check the value stored in SQLERRD(3).

39. What do you need to do before you do EXPLAIN?

A. Make sure that the PLAN_TABLE is created under the AUTHID.

40. Where is the output of EXPLAIN stored?

A. In userid. PLAN_TABLE

41. EXPLAIN has output with MATCHCOLS = 0. What does it mean?


A non-matching index scan if ACCESSTYPE = I.

42. How do you do the EXPLAIN of a dynamic SQL statement?

A. 1. Use SPUFI or QMF to EXPLAIN the dynamic SQL statement

2. Include EXPLAIN command in the embedded dynamic SQL statements

43. How do you simulate the EXPLAIN of an embedded SQL statement in SPUFI/QMF? Give an example with a host variable in WHERE clause.)

A. Use a question mark in place of a host variable ( or an unknown value ). e.g.

SELECT EMP_NAME

FROM EMP

WHERE EMP_SALARY > ?

44. What are the isolation levels possible? Where do you specify them?

A. CS: Cursor Stability - Releases the lock on a page after use. RR: Repeatable Read - Retains all locks acquired till end of transaction. By using the ISOLATION LEVEL parameter during the bind process.

45. When do you specify the isolation level? How?

A. During the BIND process. ISOLATION ( CS/RR )...

46. I use CS and update a page. Will the lock be released after I am done with that page?

A. No.

47. What are the various locking levels available?

PAGE, TABLE, TABLESPACE

48. How does DB2 determine what lock-size to use?

A. 1. Based on the lock-size given while creating the tablespace. 2. Programmer can direct the DB2 what lock-size to use. 3. If lock-size ANY is specified, DB2 usually chooses a lock-size of PAGE.

49. What are the disadvantages of PAGE level lock?

A. High resource utilization if large updates are to be done

50. What is lock escalation?

A. Promoting a PAGE lock-size to table or tablespace lock-size when a transaction has acquired more locks than specified in NUMLKTS. Locks should be taken on objects in single tablespace for escalation.

51. What are the various locks available?

A. SHARE, EXCLUSIVE, UPDATE

52. Can I use LOCK TABLE on a view?

A. No. To lock a view, take lock on the underlying tables.

53. What is ALTER ?

A. SQL command used to change the definition of DB2 objects.

54. What is a DBRM, PLAN ?

A. DBRM: DataBase Request Module, has the SQL statements extracted from the host language program by the pre-compiler. PLAN: A result of the BIND process. It has the executable code for the SQL statements in DBRM.

55. What is ACQUIRE/RELEASE in BIND?

A. Determine the point at which DB2 acquires or releases locks against table and tablespaces, including intent locks.

56. What else is there in the PLAN apart from the access path?

A. PLAN has the executable code for the SQL statements in the host program

57. What happens to the PLAN if index used by it is dropped?

A. Plan is marked as invalid. The next time the plan is accessed, it is rebound.

58. What are PACKAGES ?

A. They contain executable code for SQL statements for one DBRM.

59. What are the advantages of using a PACKAGE?

A. 1. Avoid having to bind a large number of DBRM members into a plan. 2. Avoid cost of a large bind. 3. Avoid the entire transaction being unavailable during bind and automatic rebind of a plan. 4. Minmize fallback complexities if changes result in an error.

60. What is a collection?


A user defined name that is the anchor for packages. It has not physical existence. Main usage is to group packages.


SPUFI suppose you want to select max. of 1000 rows , but the select returns only 200 rows. What are the 2 sqlcodes that are returned?

A. 100 (for successful completion of the query ), 0 (for successful COMMIT if AUTOCOMMIT is set to Yes).

62. How would you print the output of an SQL statement from SPUFI?

A. Print the output dataset.

63. How do you pull up a query which was previously saved in QMF ?

A. By using the 'DISPLAY' command. LIST QUERIES (OWNER=ALL) is used to list the queries saved by using the SAVE QUERY 'dataset name' by various users with

64. Lot of updates have been done on a table due to which indexes have gone haywire. What do you do?

A. Looks like index page split has occurred. DO a REORG of the indexes.

65. What is dynamic SQL?

A. Dynamic SQL is a SQL statement created at program execution time.

66. When is the access path determined for dynamic SQL?

A. At run time, when the PREPARE statement is issued.

67. Suppose I have a program which uses a dynamic SQL and it has been performing well till now. Off late, I find that the performance has deteriorated. What happened?

A. Probably RUN STATS is not done and the program is using a wrong index due to incorrect statistics. Probably RUNSTATS is done and optimizer has chosen a wrong access path based on the latest stats.

68. How does DB2 store NULL physically?

A. As an extra-byte prefix to the column value. physically, the nul prefix is Hex '00' if the value is present and Hex 'FF' if it is not.

69. How do you retrieve the data from a nullable column?

A. Use null indicators. Syntax ... INTO :HOSTVAR:NULLIND

70. What is the picture clause of the null indicator variable?

A. S9(4) COMP.

71. What does it mean if the null indicator has -1, 0, -2?

A. -1 : the field is null. 0 : the field is not null. -2 : the field value is truncated

72. How do you insert a record with a nullable column?

A. To insert a NULL, move -1 to the null indicator. To insert a valid value, move 0 to the null indicator.

73. Give some example of statistics collected during RUNSTATS?

A. # of rows in the table, Percent of rows in clustering sequence, # of distinct values of indexed column. # of rows moved to a nearby/farway page due to row length increase

74. What is REORG? When is it used?


REORG reorganizes data on physical storage to reclutser rows, positioning oveflowed rows in their proper sequence, to reclaim space, to restore free space. It is used after heavy updates, inserts and delete activity and after segments of a segmented tablespace have become fragemented.

75. What is IMAGECOPY ?

A. It is full backup of a DB2 table which can be used in recovery.

76. When do you use the IMAGECOPY?

A. To take routine backup of tables After a LOAD with LOG NO

77. What is COPY PENDING status?

A. State in which, an image copy on a table needs to be taken, In this status, the table is available only for queries. You cannot update this table. To remove the COPY PENDING status, you take an image copy or use REPAIR utility.

78. What is CHECK PENDING ?

A. When a table is LOADed with ENFORCE NO option, then the table is left in CHECK PENDING status. It means that the LOAD utility did not perform constraint checking.

79. What is QUIESCE?

A QUIESCE flushes all DB2 buffers on to the disk. This gives a correct snapshot of the database and should be used before and after any IMAGECOPY to maintain consistency.

80. What is a clustering index ?

A. Causes the data rows to be stored in the order specified in the index. A mandatory index defined on a partitioned table space.

81. How many clustering indexes can be defined for a table?

A. Only one.

82. What is the difference between primary key & unique index ?

A. Primary : a relational database constraint. Primary key consists of one or more columns that uniquely identify a row in the table. For a normalized relation, there is one designated primary key.

Unique index: a physical object that stores only unique values. There can be one or more unique indexes on a table.

83. What is sqlcode -922 ?

A. Authorization failure.

84. What is sqlcode -811?

A. SELECT statement has resulted in retrieval of more than one row.

85. What does the sqlcode of -818 pertain to?

A. This is generated when the consistency tokens in the DBRM and the load module are different.

86. Are views updateable?

A. Not all of them. Some views are updateable e.g. single table view with all the fields or mandatory fields. Examples of non-updateable views are views which are joins, views that contain aggregate functions(such as MIN), and views that have GROUP BY clause.

87. If I have a view, which is a join of two or more tables, can this view be updateable?

A. No.

88. What are the 4 environments which can access DB2?

A. TSO, CICS, IMS and BATCH

89. What is filter factor?

A. One divided by the number of distinct values of a column.

90. What is an inner join, and an outer join ?

A. Inner Join: combine information from two or more tables by comparing all values that meet the search criteria in the designated column or columns on table with all the calues in corresponding columns of the other table or tables. This kind of join which involve a match in both columns are called inner joins. Outer join is one in which you want both matching and non matching rows to be returned. DB2 has no specific operator for outer joins it can be simulated by combining a join and a correlated sub query with a UNION.

91. What is FREEPAGE and PCTFREE in TABLESPACE creation?


PCTFREE: percentage of each page to be left free. FREEPAGE: Number of pages to be loaded with data between each free page.

92. What are simple, segmented and partitioned table spaces ?

A. Simple Tablespace: Can contain one or more tables. Rows from multiple tables can be interleaved on a page under the DBA 's control and maintenance

Segmented Tablespace: Can contain one or more tables. Tablespace is divided into segments of 4 to 64 pages in increments of 4 pages. Each segment is dedicated to single table. A table can occupy multiple segments

Partitioned Tablespace: Can contain one table. Tablespace is divided into parts and each part is put in a separate VSAM dataset.

93. What is the difference between SYNONYM and ALIAS?

A. SYNONYM: is dropped when the table or tablespace is dropped. Synonym is available only to the creator.

ALIAS: is retained even if table or tablespace is dropped. ALIAS can be created even if the table does not exist. It is used mainly in distributed environment to hide the location info from programs. Alias is a global object & is available to all.

94. What do you mean by NOT NULL WITH DEFAULT? When will you use it?

A. This column cannot have nulls and while insertion, if no value is supplied then it wil have zeroes, spaces or date/time depending on whether it is numeric, character or date/time.

Use it when you do not want to have nulls but at the same time cannot give values all the time you insert this row.

95. What do you mean by NOT NULL? When will you use it?

A. The column cannot have nulls. Use it for key fields.

96. When would you prefer to use VARCHAR?

A. When a column which contains long text, e.g. remarks, notes, may have in most cases less than 50% of the maximum length.

97. What are the disadvantages of using VARCHAR?

A. 1. Can lead to high space utilization if most of the values are close to maximum length.

2. Positioning of VARCHAR column has to be done carefully as it has performance implications.

3. Relocation of rows to different pages can lead to more I/Os on retrieval.

98. How do I create a table MANAGER ( EMP#, MANAGER) where MANAGER is a foreign key which references to EMP# in the same table? Give the exact DDL.

A. First CREATE MANAGER table with EMP# as the primary key. Then ALTER it to define the foreign key.

99. When is the authorization check on DB2 objects done - at BIND time or run time?

A. At run time.

100.What is auditing?

A. Recording SQL statements that access a table. Specified at table creation time or through alter.

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