The OPTIMIZE FOR N ROWS is a DB2 clause which we can add in the query to give priority for the retrieval of the first few rows only. This clause will enable the optimizer to choose the access path that minimizes the response time for fetching first few rows.The OPTIMIZE FOR N ROWS clause is not effective on SELECT DISTINCT and COUNT function because DB2 will need the entire qualifying rows in order to fetch the DISTINCT rows or COUNT the number of rows. The OPTIMIZE FOR N ROWS clause gives DB2 a better opportunity to establish the access path.The ... Read More

The TRIGGERS are the event driven database programs which are triggered automatically by the database. The TRIGGERS are created using the CREATE TRIGGER statement.For example, we want to create a TRIGGER which will update ORDER_COMMISION column of the ORDERS table to 5% of the ORDER_TOTAL value after every new record insertion in ORDERS table.ExampleCREATE TRIGGER ORDERCOMMUPD AFTER INSERT ON ORDERS FOR EACH ROW MODE DB2SQL BEGIN ATOMIC UPDATE ORDERS SET ORDER_COMMISION=(5*ORDER_TOTAL)/100;Using the above statement, we have created an AFTER trigger which will be triggered automatically after any new row is inserted in the ORDERS table. Similarly, we can have BEFORE ... Read More

The TRIGGERS are database programs which are triggered automatically by DBMS in response to any modification done on the specified table. A TRIGGER can be associated with only a single table and they cannot be skipped if the desired event occurs.The TRIGGERS are like STORED PROCEDURES in the sense, both are pieces of code which are directly managed by DB2. However, TRIGGERS are event driven and executed automatically once the desired event occurs (INSERT/UPDATE/DELETE) on the specified table.The STORED PROCEDURES on the other hand needs to be called explicitly in the application program. Another difference between them is, TRIGGERS are ... Read More

A STORED PROCEDURE generally contains the SQLs which are often used in one or more programs. The main advantage of STORED PROCEDURE is that it reduces the data traffic between the COBOL and DB2 as the STORED PROCEDURES resides in DB2.A COBOL-DB2 program can call a STORED PROCEDURE using a CALL statement and we can have nested STORED PROCEDURE upto 16 levels. For example, if we have STORED PROCEDURE with a name ORDERSTAT, then we can call it in our COBOL-DB2 program using the below command:ExampleEXEC SQL    CALL ORDERSTAT (:WS-ORDER-ID, :WS-ORDER-STATUS) END-EXECIn order to create a DB2 procedure, we ... Read More

Problem: What will be the result if a COBOL-DB2 program tries to query a DB2 table, but the database in which table is residing is down?SolutionWhen we try to access any table using a COBOL-DB2 program and the DB2 database in which that table is residing is down then the COBOL-DB2 program will abend. In this case, the SQLCODE of SQLCA field will have the value as -904.As per the IBM documentation SQLCODE -904 states.“Unavailable resource. The database or tablespace is not available for use”There are several methods which we can use in order to find the current status of ... Read More

We can find the duplicate TRANSACTION_ID in the ORDERS DB2 table using the below query:ExampleSELECT TRANSACTION_ID, COUNT(*) AS TRANSACTION_COUNT FROM ORDER GROUP BY TRANSACTION_ID HAVING COUNT(*) > 1The purpose of COUNT(*) is to count the number of rows. We will group the result based on the TRANSACTION_ID using GROUP BY function and to display the duplicate transaction ids, we will place a predicate using HAVING statement for COUNT(*) greater than one.For example, consider the below TRANSACTIONS DB2 table:TRANSACTION_IDTRANSACTION_STATUSIRN22345PAIDIRN22345PAIDIRN22345PAIDIRN56902PAIDIRN99781UNPAIDIRN56902PAIDThe query will give the below result:TRANSACTION_IDTRANSACTION_COUNTIRN223453IRN569022IRN997811Read More

Problem: Write a SQL query to count the number of orders which were placed today from the ORDERS DB2 table. (The date should not be hardcoded)SolutionWe can find the count of orders which are placed today using the below DB2 query:ExampleSELECT COUNT(ORDER_ID) AS ORDER_COUNT FROM ORDERS WHERE ORDER_DATE = CURRENT DATEIn this query, we have used the COUNT COLUMN function which will count the total number of ORDER_ID (primary key). In the WHERE clause, we will use the predicate for the ORDER_DATE column. The CURRENT DATE is a DB2 inbuilt function which will return the current system date.For example, if ... Read More

Problem: Write a SQL query to display 2 columns. First column should have ORDER_ID, the second column should give the value as YES/NO for free shipping based on ORDER_TOTAL > 500.SolutionThe query to display ORDER_ID and free shipping result based on the ORDER_TOTAL criteria can be written as below.ExampleSELECT ORDER_ID,    CASE WHEN ORDER_TOTAL > 500 THEN ‘YES’       ELSE ‘NO’ AS FREE_SHIPPING    END FROM ORDERSWe will use CASE expressions through which we can implement a logic to check the ORDER_TOTAL. If the ORDER_TOTAL is greater than 500 then we will get ‘YES’ for the free shipping ... Read More

Problem: A COBOL-DB2 program failed due to deadlock. How will you find the resource due to which the program failed?SolutionA DEADLOCK condition occurs when two or more applications are stuck, waiting for each other to release the locks on the resources needed by them. A detailed information and logs can be found in the DB2 system job DSNZMSTR job. The DSNZ is the name of the installed DB2 sub-system and it can vary from installation to installation. The SYSOUT of this job continues to display the DB2 level system logs. The logs related to the deadlock are also available in ... Read More

A DB2 LOCK PROMOTION is defined as the process of acquiring more restrictive locks on a particular resource. DB2 uses LOCK PROMOTION for the concurrent processes which are trying to access the same DB2 resource. Basically, there are three types of locks.Shared lock(S)The concurrent processes can place a shared lock on a resource (DB2 table, row, page, etc) but cannot update the data. In order to update the data, concurrent processes have to promote their lock to UPDATE.Update lock(U)The concurrent process can read the data but cannot update it. Update lock indicates that the process is ready to update the ... Read More