1) How would you find out the total number of rows in a table? –
Use SELECT COUNT(*)
...
2) How do you eliminate duplicate values in SELECT? –
Use SELECT DISTINCT
...
3) How do you select a row using indexes? –
Specify the indexed
columns in the WHERE clause.
4) What are aggregate functions?
Bulit-in mathematical
functions for use in SELECT clause.
5) How do you find the maximum value in a column? –
Use SELECT MAX(...
6) Can you use MAX on a CHAR column?
YES.
7) My SQL statement SELECT AVG(SALARY) FROM EMP yields inaccurate results. Why?
Because SALARY is not
declared to have NULLs and the employees for whom the salary is not
known are also counted.
8) How do you retrieve the first 5 characters of FIRSTNAME column of EMP table?
SELECT
SUBSTR(FIRSTNAME,1,5) FROM EMP;
9) How do you concatenate the FIRSTNAME and
LASTNAME from EMP table to give a complete name?
SELECT FIRSTNAME || ‘
‘ || LASTNAME FROM EMP;
10) What is the use of VALUE function?
1. Avoid -ve SQLCODEs
by handling nulls and zeroes in computations
2. Substitute a
numeric value for any nulls used in computation
11) What is UNION,UNION ALL? –
UNION : eliminates
duplicates
UNION ALL: retains
duplicates
Both these are used to
combine the results of different SELECT statements.
Suppose I have five
SQL SELECT statements connected by UNION/UNION ALL, how many times
should I specify UNION to eliminate the duplicate rows? -
Once.
12) What is the restriction on using UNION in embedded SQL?
It has to be in a
CURSOR.
13) In the WHERE clause what is BETWEEN and IN? –
BETWEEN supplies a
range of values while IN supplies a list of values.
14) Is BETWEEN inclusive of the range values specified? –
Yes.
15) What is 'LIKE' used for in WHERE clause? What are the wildcard characters? –
LIKE is used for
partial string matches. ‘%’ ( for a string of any character ) and
‘_’ (for any single character ) are the two wild card characters.
16) When do you use a LIKE statement?
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.
17) What is the meaning of underscore ( ‘_’ ) in the LIKE statement? –
Match for any single
character.
18) What do you accomplish by GROUP BY ... HAVING clause? –
GROUP BY partitions
the selected rows on the distinct values of the column on which you
group by.
HAVING selects GROUPs
which match the criteria specified
19) Consider the employee table with column
PROJECT nullable. How can you get a list of employees who are not
assigned to any project?
SELECT EMPNO
FROM EMP
WHERE PROJECT IS NULL;
20) What is the result of this query if no rows
are selected:
SELECT SUM(SALARY)
FROM EMP
WHERE QUAL=‘MSC’;
NULL
21) Why SELECT * is not preferred in embedded SQL programs?
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.
What are correlated
subqueries? -
A subquery in which
the inner ( nested ) query refers back to the table in the outer
query. Correlated subqueries must be evaluated for each qualified row
of the outer query that is referred to.
22) What are the issues related with correlated subqueries? –
???
23) What is a cursor? why should it be used? –
Cursor is a
programming device that allows the SELECT to find a set of rows but
return them one at a time.
Cursor should be used
because the host language can deal with only one row at a time.
24) How would you retrieve rows from a DB2 table in embedded SQL? –
Either by using the
single row SELECT statements, or by using the CURSOR.
Apart from cursor,
what other ways are available to you to retrieve a row from a table
in embedded SQL? -
Single row SELECTs.
25) Where would you specify the DECLARE CURSOR statement? –
See answer to next
question.
26) How 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.
27) What happens when you say OPEN CURSOR?
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.
28) Is DECLARE CURSOR executable?
No.
29) Can you have more than one cursor open at any one time in a program ? –
Yes.
30) When you COMMIT, is the cursor closed?
Yes.
31) 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.
32) Give the COBOL definition of a VARCHAR
field.
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).
33) What is the physical storage length of each
of the following DB2 data types:
DATE, TIME, TIMESTAMP?
DATE: 4bytes
TIME: 3bytes
TIMESTAMP: 10bytes
34) What is the COBOL picture clause of the
following DB2 data types:
DATE, TIME, TIMESTAMP?
DATE: PIC X(10)
TIME : PIC X(08)
TIMESTAMP: PIC X(26)
35) What is the COBOL picture clause for a DB2 column defined as DECIMAL(11,2)? -
PIC S9(9)V99 COMP-3.
Note: In
DECIMAL(11,2), 11 indicates the size of the data type and 2 indicates
the precision.
36) What is DCLGEN ? -
DeCLarations
GENerator: used to create the host language copy books for the table
definitions. Also creates the DECLARE table.
37) What are the contents of a DCLGEN? -
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.
38) Is it mandatory to use DCLGEN? If not, why would you use it at all? -
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.
39) Is DECLARE TABLE in DCLGEN necessary? Why it used?
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.
40) Will precompile of an DB2-COBOL program bomb, if DB2 is down?
No. Because the
precompiler does not refer to the DB2 catalogue tables.
41) How is a typical DB2 batch pgm executed ?
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('ED 01T.OBJ.LOADLIB')
END
2. Use IKJEFT01
utility program to run the above DSN command in a JCL.
Assuming that a site’s
standard is that pgm name = plan name, what is the easiest way to
find out which pgms are affected by change in a table’s structure ?
Query the catalogue
tables SYSPLANDEP and SYSPACKDEP.
42) Name some fields from SQLCA.
SQLCODE, SQLERRM,
SQLERRD
43) How can you quickly find out the # of rows updated after an update statement?
Check the value stored
in SQLERRD(3).
44) What is EXPLAIN? –
EXPLAIN is used to
display the access path as determined by the optimizer for a SQL
statement. It can be used in SPUFI (for single SQL statement ) or in
BIND step (for embedded SQL ).
45) What do you need to do before you do EXPLAIN?
Make sure that the
PLAN_TABLE is created under the AUTHID.
46) Where is the output of EXPLAIN stored? –
In userid.PLAN_TABLE
47) EXPLAIN has output with MATCHCOLS = 0. What does it mean? –
a nonmatching index
scan if ACCESSTYPE = I.
48) How do you do the EXPLAIN of a dynamic SQL statement?
1. Use SPUFI or QMF to
EXPLAIN the dynamic SQL statement
2. Include EXPLAIN
command in the embedded dynamic SQL statements
49) How do you simulate the EXPLAIN of an embedded
SQL statement in SPUFI/QMF? Give an example with a host variable in
WHERE clause.)
Use a question mark in
place of a host variable ( or an unknown value ). e.g.
SELECT EMP_NAME
FROM EMP
WHERE EMP_SALARY >
?
50) What are the isolation levels possible ? –
CS: Cursor Stability
RR: Repeatable Read
51) What is the difference between CS and RR isolation levels?
CS: Releases the lock
on a page after use
RR: Retains all locks
acquired till end of transaction
52) Where do you specify them ?
ISOLATION LEVEL is a
parameter for the bind process.
53) When do you specify the isolation level? How?
During the BIND
process. ISOLATION ( CS/RR )...
I use CS and update a
page. Will the lock be released after I am done with that page?
No.
54) What are the various locking levels available?
PAGE, TABLE,
TABLESPACE
55) How does DB2 determine what lock-size to use?
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
56) What are the disadvantages of PAGE level lock?
High resource
utilization if large updates are to be done
57) What is lock escalation?
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 to occur.
58) What are the various locks available?
SHARE, EXCLUSIVE,
UPDATE
59) Can I use LOCK TABLE on a view?
No. To lock a view,
take lock on the underlying tables.
60) What is ALTER ? –
SQL command used to
change the definition of DB2 objects.
61) What is a DBRM, PLAN ?
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
the DBRM.
62) What is ACQUIRE/RELEASE in BIND?
Determine the point at
which DB2 acquires or releases locks against table and tablespaces,
including intent locks.
63) What else is there in the PLAN apart from the access path? –
PLAN has the
executable code for the SQL statements in the host program
64) What happens to the PLAN if index used by it is dropped?
Plan is marked as
invalid. The next time the plan is accessed, it is rebound.
65) What are PACKAGES ? –
They contain
executable code for SQL statements for one DBRM.
66) What are the advantages of using a PACKAGE?
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. Minimize fallback
complexities if changes result in an error.
67) 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.
In SPUFI suppose you
want to select max. of 1000 rows , but the select returns only 200
rows.
68) What are the 2 sqlcodes that are returned? –
100 ( for successful
completion of the query ), 0 (for successful COMMIT if AUTOCOMMIT is
set to Yes).
69) How would you print the output of an SQL statement from SPUFI? –
Print the output
dataset.
70) How do you pull up a query which was previously saved in QMF ? –
??
71) Lot of updates have been done on a table due to which indexes have gone haywire. What do you do? –
Looks like index page
split has occurred. DO a REORG of the indexes.
72) What is dynamic SQL? –
Dynamic SQL is a SQL
statement created at program execution time.
73) When is the access path determined for dynamic SQL? –
At run time, when the
PREPARE statement is issued.
74) 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? –
Probably RUN STATS is
not done and the program is using a wrong index due to incorrect
stats.
Probably RUNSTATS is
done and optimizer has chosen a wrong access path based on the latest
statistics.
75) How does DB2 store NULL physically?
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.
76) How do you retrieve the data from a nullable column? –
Use null indicators.
Syntax ... INTO :HOSTVAR:NULLIND
77) What is the picture clause of the null indicator variable? –
S9(4) COMP.
78) What does it mean if the null indicator has -1, 0, -2? –
-1 : the field is null
0 : the field is not
null
-2 : the field value
is truncated
79) How do you insert a record with a nullable column?
To insert a NULL, move
-1 to the null indicator
To insert a valid
value, move 0 to the null indicator
80) What is RUNSTATS? –
A DB2 utility used to
collect statistics about the data values in tables which can be used
by the optimizer to decide the access path. It also collects
statistics used for space management. These statistics are stored in
DB2 catalog tables.
81) When will you chose to run RUNSTATS?
After a load, or after
mass updates, inserts, deletes, or after REORG.
82) Give some example of statistics collected during RUNSTATS?
# 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
83) What is REORG? When is it used?
REORG reorganizes data
on physical storage to reclutser rows, positioning overflowed 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 fragmented.
84) What is IMAGECOPY ? –
It is full backup of a
DB2 table which can be used in recovery.
85) When do you use the IMAGECOPY? –
To take routine backup
of tables
After a LOAD with LOG
NO
After REORG with LOG
NO
86) 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.
87) What is CHECK PENDING ?
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.
88) 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.
89) What is a clustering index ? –
Causes the data rows
to be stored in the order specified in the index. A mandatory index
defined on a partitioned table space.
90) How many clustering indexes can be defined for a table?
Only one.
91) What is the difference between primary key & unique index ?
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.
92) What is sqlcode -922 ?
Authorization failure
93) What is sqlcode -811?
SELECT statement has
resulted in retrieval of more than one row.
94) What does the sqlcode of -818 pertain to? –
This is generated when
the consistency tokens in the DBRM and the load module are different.
95) Are views updateable ?
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.
96) If I have a view which is a join of two or more tables, can this view be updateable? –
No.
97) What are the 4 environments which can access DB2 ?
TSO, CICS, IMS and
BATCH
98) What is an inner join, and an outer join ?
Inner Join: combine
information from two or more tables by comparing all values that meet
the search criteria in the designated column or columns of on e table
with all the clause 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.
99) 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
100) What are simple, segmented and partitioned table spaces ?
Simple Tablespace:
Can contain one or
more tables
Rows from multiple
tables can be interleaved on a page under the DBAs 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.
101) What is filter factor?
one divided by the
number of distinct values of a column.
102) What is index cardinality? –
The number of distinct
values a column or columns contain.
103) What is a synonym ?
Synonym is an
alternate name for a table or view used mainly to hide the leading
qualifier of a table or view.. A synonym is accessible only by the
creator.
104) What is the difference between SYNONYM and ALIAS?
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.
105) What do you mean by NOT NULL WITH DEFAULT? When will you use it?
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.
106) What do you mean by NOT NULL? When will you use it?
The column cannot have
nulls. Use it for key fields.
107) When would you prefer to use VARCHAR?
When a column which
contains long text, e.g. remarks, notes, may have in most cases less
than 50% of the maximum length.
108) What are the disadvantages of using VARCHAR?
1. Can lead to high
space utilization if most of the values are close to maximum.
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.
109) 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.
First CREATE MANAGER
table with EMP# as the primary key. Then ALTER it to define the
foreign key.
When is the
authorization check on DB2 objects done - at BIND time or run time?
At run time.
110) What is auditing?
Recording SQL
statements that access a table. Specified at table creation time or
through alter.
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