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.

What is ACQUIRE/RELEASE in BIND?

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

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

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.

What are PACKAGES ?

They contain executable code for SQL statements for one DBRM.

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.

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.

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).

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 > ?

 What are the isolation levels possible ?

CS: Cursor Stability

RR: Repeatable Read

 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

 Where do you specify them ?

ISOLATION LEVEL is a parameter for the bind process.

 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.

What are the various locking levels available?

PAGE, TABLE, TABLESPACE

 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

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

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

 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.

 Name some fields from SQLCA.

SQLCODE, SQLERRM, SQLERRD

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

Check the value stored in SQLERRD(3).

 What is EXPLAIN? – drona questions

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 ).

 What do you need to do before you do EXPLAIN?

Make sure that the PLAN_TABLE is created under the AUTHID.

 Where is the output of EXPLAIN stored? –

In userid.PLAN_TABLE

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

a nonmatching index scan if ACCESSTYPE = I.

 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

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).

 What is the physical storage length of each of the following DB2 data types:

DATE, TIME, TIMESTAMP?

DATE: 4bytes

TIME: 3bytes

TIMESTAMP: 10bytes

 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)

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

PIC S9(9)V99 COMP-3.

Note: In DECIMAL(11,2), 11 indicates the size of the data type and 2 indicates the precision.

 What is DCLGEN ? –

DeCLarations GENerator: used to create the host language copy books for the table definitions. Also creates the DECLARE table.

 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.

 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.

 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.

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.

 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.

 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.

 Where would you specify the DECLARE CURSOR statement?

See answer to next question.

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.

 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.

 Is DECLARE CURSOR executable?

No.

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

Yes.

 When you COMMIT, is the cursor closed? – drona questions

Yes.

 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.

 What is the meaning of underscore ( ‘_’ ) in the LIKE statement? 

Match for any single character.

 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

 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;

What is the result of this query if no rows are selected:

SELECT SUM(SALARY)

FROM EMP

WHERE QUAL=‘MSC’;

NULL

 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 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

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.

 What is the restriction on using UNION in embedded SQL?

It has to be in a CURSOR.

 In the WHERE clause what is BETWEEN and IN? –

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

 Is BETWEEN inclusive of the range values specified? –

Yes.

 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.

 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.

How do you retrieve the first 5 characters of FIRSTNAME column of DB2 table EMP ?

SQL Query : SELECT SUBSTR(FIRSTNAME,1,5) FROM EMP;

 What are aggregate functions?

Bulit-in mathematical functions for use in SELECT clause.

 Can you use MAX on a CHAR column?

YES.

 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.

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

SELECT FIRSTNAME || ‘ ‘ || LASTNAME FROM EMP;

 How would you find out the total number of rows in a DB2 table?

Use SELECT COUNT(*) … in db2 query

 How do you eliminate duplicate values in DB2 SELECT ?

Use SELECT DISTINCT … in db2 query

 How do you select a row using indexes in DB2?

Specify the indexed columns in the WHERE clause of db2 query.

 How do you find the maximum value in a column in db2?

Use SELECT MAX(…) .. in db2 query