The direct index look-up is chosen by the DB2 optimizer when all the columns used in the predicate of the WHERE clause is part of the index.For example, if we have ORDERS DB2 table as below.ORDER_IDORDER_DATEORDER_TOTALZ2234530-10-2020342Z3341214-08-2020543Z5699019-10-2020431Z5690221-09-20206743Z9978104-11-2020443Z5611229-08-2020889In this table, there is one index which is built having columns named ORDER_ID and ORDER_DATE. For the below query, DB2 optimizer will choose direct index look-up because the columns used in the SELECT statement are also part of the index.ExampleSELECT ORDER_ID, ORDER_DATE, INVOICE_ID FROM ORDERS    WHERE ORDER_ID = ‘Z33412’ AND ORDER_DATE = ‘14-08-2020’The result of the above query will be as follows.ORDER_IDORDER_DATEZ3341214-08-2020In the ... Read More

We can find the highest ORDER_TOTAL datewise from the ORDERS DB2 table using below query.ExampleSELECT ORDER_DATE, MAX(ORDER_TOTAL) FROM ORDERS GROUP BY ORDER_DATEWe will use ‘GROUP BY’ on ORDER_DATE to group the result date wise and MAX aggregate function will help us to get the maximum ORDER_TOTAL placed at that particular date.For example, if we have below ORDERS DB2 table.ORDER_IDORDER_TOTALORDER_DATEZ2234534229-07-2020Z6299854330-07-2020Z5699043128-07-2020Z56902674329-07-2020Z9978144328-07-2020Z5611288930-07-2020 Then the SQL query - SELECT ORDER_DATE, MAX(ORDER_TOTAL) FROM ORDERS GROUP BY ORDER_DATE will return the result below.ORDER_DATEORDER_TOTAL28-07-202044329-07-2020674330-07-2020889Read More

We can find the third highest ORDER_TOTAL in the ORDERS DB2 table using the below query.ExampleSELECT ORDER_ID, MIN(ORDER_TOTAL) FROM ORDERS    ORDER BY ORDER_TOTAL DESC    FETCH FIRST 3 ROWS ONLYThe ‘FETCH FIRST 3 ROWS ONLY’ clause will give only 3 rows in the output and these 3 rows will be in descending order. The first row will have the highest ORDER_TOTAL in the entire ORDERS table, second row will have the second highest ORDER_TOTAL in the entire ORDERS table and so on.The MIN aggregate function will give the least value of the ORDER_TOTAL among those 3 rows and this ... Read More

Problem: Write the DB2 SQL query to give the sum of ORDER_TOTAL for the orders placed on 29th July and 30th July individually. The result should come in a single table.SolutionWe can find the sum of ORDER_TOTAL for the orders placed on 29th and 30th July individually using aggregate function SUM, GROUP BY and HAVING.For example, if we have an ORDER table as below.ORDER_IDORDER_TOTALORDER_DATEZ2234534229-07-2020Z6299854330-07-2020Z5699043112-07-2020Z56902674329-07-2020Z9978144310-07-2020Z5611288930-07-2020 Below is the query which will give the desired result.ExampleSELECT ORDER_DATE, SUM(ORDER_TOTAL) FROM ORDERS GROUP BY ORDER_DATE HAVING ORDER_DATE IN (‘29-07-2020’, ‘30-07-2020’)In this query, we have selected ORDER_DATE and ORDER_TOTAL with aggregate function SUM.The GROUP BY will ... Read More

Problem: How will you find the ORDER_ID, TRANSACTION_ID and TRANSACTION_STATUS from ORDERS and TRANSACTIONS DB2 table using joins?SolutionWe can find ORDER_ID, TRANSACTION_ID and TRANSACTION_STATUS from ORDERS and TRANSACTIONS table using the INNER JOIN query.For example, if we have below 2 ORDERS table.ORDER_IDTRANSACTION_IDORDER_DATEZ22345ITX448923-10-2020Z62998ITX431121-10-2020Z56902ITX312026-10-2020 TRANSACTION_IDTRANS_AMTTRANSACTION_STATUSITX44891128PAIDITX43112318PAIDITX312088956UNPAIDITX21675612FAILEDWe can use an inner join query as below.ExampleSELECT ORDER_ID, TRANSACTION_ID, TRANSACTION_STATUS FROM ORDERS INNER JOIN TRANSACTIONS ON ORDERS.TRANSACTION_ID = TRANSACTIONS.TRANSACTION_IDThe above query will return the result below.ORDER_IDTRANSACTION_IDTRANSACTION_STATUSZ22345ITX4489PAIDZ62998ITX4311PAIDZ56902ITX3120UNPAIDRead More

We can find all the ORDER_ID which are having ORDER_TOTAL greater than the average value of all the ORDER_TOTAL present in the ORDERS table using the sub-query.For example, if we have below ORDERS table.ORDER_IDORDER_TOTALA223451867A629985634A569027615A5691187960A56915132A5691880363Below is the subquery to find out the desired data.ExampleSELECT ORDER_ID, ORDER_TOTAL FROM ORDERS    WHERE ORDER_TOTAL > (SELECT AVG(ORDER_TOTAL) FROM ORDERS)The result of the above query will be as below.ORDER_IDORDER_TOTALA2234587960A6299880363

Problem: How will the COBOL-DB2 program behave once the number of locks placed on the table space exceeds the defined limit?SolutionThe number of locks which an application can place on a DB2 resource such as page, table row, etc., is defined in DSNZPARM. Once the number of page and row level locks in any table exceeds the permissible limit, then the lock escalation takes place.In lock escalation, DB2 releases the page or row level lock which it has held and attempts to acquire a tablespace level or higher lock. In this case, the application now has wider access/scope to DB2 ... Read More

DB2 optimizer plays an important role in the overall performance of the database. The optimizer selects the optimal access path for each query through which data can be fetched from the database. It identifies the indexes to follow, query predicates, etc.The optimizer selects the access path automatically and we can easily find the access path using EXPLAIN DB2 command. We have to SET the query number first and then give our SQL query to find out its access path in three simple steps.For example, We have a DB2 ORDERS table and we want to examine the SELECT query which has ... Read More

Problem: How can you implement a logic to automatically generate a unique value in a DB2 column for every new row inserted?SolutionWe can implement a logic in a DB2 table through which we can have one column which will have an automatically generated value for every new row inserted. This column can serve as a primary key and hence it is very useful for the random access of the DB2 table. This logic can be implemented via ROW-ID and SEQUENCE.Any one column of the DB2 table can be defined as type ROW-ID following which DB2 will automatically assign a new ... Read More

Problem: What are DB2 subqueries and what is the use of subqueries? Explain subqueries with the help of an example on the ORDERS table.SolutionA subquery in DB2 is a query within a query, i.e., nested query. A subquery is executed first followed by its parent query. We can have a subquery up to 15 levels.For example, if we have below ORDERS table in DB2.ORDER_IDORDER_TOTALZ223451267Z629986734Z569028815Z5691178990Z56915432Z5691877453If we want to extract all the orders from the ORDERS table which have ORDER_TOTAL more than overall average, then we can use the below sub query.ExampleSELECT ORDER_ID FROM ORDERS WHERE ORDER_TOTAL > (SELECT AVG(ORDER_TOTAL) FROM ORDERS)The ... Read More