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Q: What is ROW_NUMBER()?
Answer:
ROW_NUMBER() assigns sequential numbers. ROW_NUMBER() OVER (ORDER BY col). Numbers restart per PARTITION BY group. Use for paging, deduplication (keep first), ranking. Result is numeric.
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Q: When should I use COMMIT in DB2?
Answer:
COMMIT saves all changes since the last COMMIT and releases locks.
When to COMMIT:
- After processing a logical unit of work
- Periodically in long-running batch (every N records)
- Before ending the program successfully
Frequency Guidelines:
- Too frequent: Performance overhead
- Too rare: Lock contention, large log
- Typical: Every 100-1000 records in batch
Best Practice:
MOVE 0 TO WS-COMMIT-COUNT.
PERFORM PROCESS-RECORD.
ADD 1 TO WS-COMMIT-COUNT.
IF WS-COMMIT-COUNT >= 500
EXEC SQL COMMIT END-EXEC
MOVE 0 TO WS-COMMIT-COUNT
END-IF.
Note: In CICS, syncpoint (COMMIT) happens automatically at task end. Explicit SYNCPOINT is rarely needed.
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Q: What is CTE (Common Table Expression)?
Answer:
CTE defines temporary result set. WITH cte_name AS (SELECT...) SELECT * FROM cte_name. Improves readability. Can be recursive for hierarchies. Multiple CTEs comma-separated. Referenced in main query. Scope is single statement.
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Q: What is UDF (User Defined Function)?
Answer:
UDF extends SQL with custom functions. CREATE FUNCTION name(params) RETURNS type AS BEGIN logic END. Scalar returns single value. Table function returns rows. Use in SELECT, WHERE like built-in functions.
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Q: How to handle SQLCODE -805?
Answer:
-805 is DBRM or package not found. Plan bound but package missing or invalidated. REBIND PACKAGE. Check collection, package name. Verify BIND completed successfully. May need BIND PLAN to add package.
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Q: What is SQLCODE and what are common values?
Answer:
SQLCODE is a return code in SQLCA indicating the result of SQL execution.
Common SQLCODE values:
| 0 | Successful execution |
| 100 | No data found / End of cursor |
| -803 | Duplicate key on insert |
| -811 | Multiple rows returned for singleton SELECT |
| -904 | Unavailable resource |
| -911 | Deadlock/timeout |
| -922 | Authorization failure |
| -927 | DB2 not available |
Negative = Error, 0 = Success, 100 = No data
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Q: What is RECOVER utility?
Answer:
RECOVER restores data from backup/logs. RECOVER TABLESPACE db.ts. Options: TOCOPY, TORBA, TOLOGPOINT. Image copy is backup. RECOVER applies logs to copy. Essential for disaster recovery.
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Q: Explain the difference between INNER JOIN and LEFT OUTER JOIN.
Answer:
INNER JOIN:
- Returns only matching rows from both tables
- If no match, row is excluded
LEFT OUTER JOIN:
- Returns all rows from left table
- Matching rows from right table
- NULL for right table if no match
-- INNER JOIN SELECT E.NAME, D.DEPT_NAME FROM EMPLOYEE E INNER JOIN DEPARTMENT D ON E.DEPT_ID = D.DEPT_ID; -- LEFT OUTER JOIN SELECT E.NAME, D.DEPT_NAME FROM EMPLOYEE E LEFT OUTER JOIN DEPARTMENT D ON E.DEPT_ID = D.DEPT_ID;
Use LEFT JOIN when you want all employees even those without department.
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Q: How to create image copy?
Answer:
COPY utility creates backup. COPY TABLESPACE db.ts. FULL or INCREMENTAL. SHRLEVEL REFERENCE or CHANGE. Store in GDG for versions. Regular copies essential for recovery. COPYTOCOPY duplicates copies.
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Q: What is UNLOAD utility?
Answer:
UNLOAD extracts data to sequential file. UNLOAD FROM TABLE name. Output format matches LOAD input. Used for data migration, backup, extract. Can include WHERE clause for filtering.
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Q: How to handle restrictive states?
Answer:
COPY PENDING needs image copy. CHECK PENDING needs CHECK DATA. REBUILD PENDING needs REBUILD INDEX. REORG PENDING needs REORG. RECOVER PENDING needs RECOVER. Use -DISPLAY DATABASE to see status.
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Q: Explain predicate types
Answer:
Stage 1 predicates evaluated by storage engine. Stage 2 evaluated by DB2 (more expensive). Indexable predicates can use index. Non-indexable force scan. Design for stage 1/indexable predicates. EXPLAIN shows predicate staging.
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Q: What is index-only access?
Answer:
Index-only access retrieves data from index without table access. All needed columns in index (including by include clause). Best performance. EXPLAIN shows INDEXONLY=Y. Design indexes for common queries.
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Q: How to tune DB2 queries?
Answer:
Check EXPLAIN output. Look at: access type (index vs scan), join method, sort operations. Add indexes for predicates. Rewrite with hints. Update statistics. Check for full tablespace scans. Monitor actual execution.
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Q: What is identity column?
Answer:
Identity column auto-generates values. col INTEGER GENERATED ALWAYS AS IDENTITY. Or GENERATED BY DEFAULT (allows override). Start, increment configurable. Alternative to sequence for single-table keys.
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Q: How to handle large objects (LOB)?
Answer:
BLOB/CLOB/DBCLOB for large data. Stored in auxiliary tablespace. Use LOB locators for efficiency. FETCH with INTO :lobvar. INSERT with CLOB(text). LOG NO for LOB tablespace optional.
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Q: What is temporal table?
Answer:
Temporal tables track historical data. SYSTEM_TIME versioning automatic. APPLICATION_TIME for business time. Query AS OF for point-in-time. History table stores old versions. Built-in time travel.
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Q: What is dynamic SQL?
Answer:
Dynamic SQL constructed at runtime. PREPARE creates executable. EXECUTE runs it. EXECUTE IMMEDIATE for one-time. DECLARE CURSOR for queries. More flexible but less efficient than static. Security concerns (injection).
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Q: Explain VIEW creation
Answer:
VIEW is saved query. CREATE VIEW name AS SELECT... WITH CHECK OPTION enforces WHERE on updates. Can be updatable if simple. Views for security, simplification. Materialized views (MQT) store data.
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Q: What is MQT?
Answer:
MQT (Materialized Query Table) stores computed results. CREATE TABLE name AS (SELECT...) DATA INITIALLY DEFERRED REFRESH DEFERRED. Improves query performance. REFRESH TABLE updates. Use for aggregations, joins.
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Q: How to handle -551 SQLCODE?
Answer:
-551 is authorization failure. No privilege for operation. Check GRANT statements. May need SELECT, UPDATE, DELETE, INSERT authority. GRANT privilege ON object TO user/role. Check SYSIBM.SYSTABAUTH.
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Q: What is GRANT and REVOKE?
Answer:
GRANT gives privileges: GRANT SELECT ON table TO user. REVOKE removes: REVOKE SELECT ON table FROM user. Privileges: SELECT, INSERT, UPDATE, DELETE, EXECUTE. WITH GRANT OPTION allows re-granting. Essential for security.
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Q: Explain database design normalization
Answer:
1NF: atomic values, no repeating groups. 2NF: 1NF + no partial dependencies. 3NF: 2NF + no transitive dependencies. BCNF: stricter 3NF. Higher forms reduce redundancy. Denormalize for performance. Balance is key.
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Q: What is referential integrity?
Answer:
RI ensures foreign key values exist in parent. CREATE TABLE child ... REFERENCES parent(key). ON DELETE CASCADE/SET NULL/RESTRICT. DB2 enforces automatically. Constraint violations return -530/-531. Design carefully.
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Q: How to alter table structure?
Answer:
ALTER TABLE name ADD COLUMN col type. ALTER TABLE name DROP COLUMN col. ALTER TABLE name ALTER COLUMN col SET DATA TYPE. Some changes need REORG. Adding columns easy; changes may need unload/reload.
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Q: What is tablespace?
Answer:
Tablespace is storage container for tables. CREATE TABLESPACE name IN database USING STOGROUP. Contains one or more tables. Segmented/universal tablespace types. Management at tablespace level (REORG, COPY, RECOVER).
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Q: What is encoding scheme?
Answer:
Encoding defines character representation. EBCDIC for mainframe. ASCII for open systems. Unicode for international. CCSID specifies exact encoding. Mixed encoding needs careful handling. TRANSLATE for conversion.
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Q: How to handle CLOB in COBOL?
Answer:
Declare: 01 CLOB-VAR SQL TYPE IS CLOB(1M). Or use LOB locator: 01 CLOB-LOC SQL TYPE IS CLOB_LOCATOR. FREE LOCATOR releases. DBMS_LOB procedures for manipulation. Large CLOBs need special handling.
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Q: What is XML support in DB2?
Answer:
DB2 stores XML natively. XMLTYPE column. XMLPARSE, XMLSERIALIZE for conversion. XQuery for querying. XMLTABLE extracts relational from XML. XML indexes for performance. Powerful for document storage.
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Q: How to use EXPLAIN tables?
Answer:
PLAN_TABLE primary output. DSN_STATEMNT_TABLE for cost. DSN_FUNCTION_TABLE for functions. INSERT EXPLAIN before statement. Query tables after. PLANNO, ACCESSTYPE, MATCHCOLS important columns.
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Q: What is Distributed Data Facility (DDF)?
Answer:
DDF enables remote DB2 access. TCP/IP and SNA connectivity. DRDA protocol. Location name identifies target. Three-part names: location.schema.table. Network security considerations.
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Q: How to handle SQLCODE 100?
Answer:
SQLCODE 100 means not found or end of data. For SELECT INTO: no matching row. For FETCH: no more rows. For UPDATE/DELETE: no rows affected. Check context - may be normal condition, not error.
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Q: What is OPTIMIZE FOR n ROWS?
Answer:
OPTIMIZE FOR n ROWS hints expected rows. SELECT * FROM t ORDER BY c OPTIMIZE FOR 1 ROW. Influences access path. Low n favors index. High n may favor scan. Use when you know actual row count.
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Q: What is the difference between INNER JOIN and LEFT OUTER JOIN?
Answer:
INNER JOIN returns only matching rows from both tables. LEFT OUTER JOIN returns all rows from left table plus matching rows from right (NULL if no match). LEFT preserves all left table rows regardless of match. INNER excludes non-matching rows from both sides.
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Q: What is RUNSTATS and when to use it?
Answer:
RUNSTATS collects table/index statistics for optimizer. Run after significant data changes (loads, deletes). Updates catalog tables (SYSTABLES, SYSINDEXES). Optimizer uses for access path selection. RUNSTATS TABLESPACE db.ts INDEX(ALL).
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Q: How does REORG work?
Answer:
REORG physically reorganizes tablespace or index. Eliminates fragmentation, reclaims space, restores clustering. REORG TABLESPACE db.ts. Options: SHRLEVEL (REFERENCE/CHANGE), LOG (YES/NO). Schedule during low activity. Run RUNSTATS after.
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Q: What is a DB2 plan vs package?
Answer:
Plan is executable form of program's SQL, bound from DBRM. Package is independent unit, can be shared. Plans contain packages or direct SQL. Packages allow separate rebind. Modern approach: package collections with small plans.
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Q: What is DB2 catalog?
Answer:
Catalog tables describe DB2 objects. SYSIBM.SYSTABLES for tables, SYSIBM.SYSCOLUMNS for columns, SYSIBM.SYSINDEXES for indexes. Query catalog for metadata. Read-only except through DDL. Essential for documentation and analysis.
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Q: Explain NULL handling in DB2
Answer:
NULL means unknown/missing value. NULL != NULL returns unknown. Use IS NULL, IS NOT NULL. COALESCE(col, default) substitutes. NULL in arithmetic yields NULL. Indicator variables detect NULL in COBOL. NVL function alternative.
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Q: What is indicator variable?
Answer:
Indicator variable detects NULL values. Declare: 01 col-var. 01 col-ind PIC S9(4) COMP. Use: INTO :col-var:col-ind. If col-ind < 0, value is NULL. Set indicator negative to insert NULL. Required for nullable columns.
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Q: Explain DB2 locking
Answer:
DB2 locks at row, page, table, tablespace level. X (Exclusive) for writes, S (Share) for reads. IX/IS for intent. Lock escalation moves to higher level. LOCK TABLE statement forces mode. Timeout if wait too long.
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Q: How to handle -904 SQLCODE?
Answer:
-904 is resource unavailable. Tablespace/index in restricted state, stopped, or unavailable. Check display status. May need START DATABASE command. Check for REORG, RECOVER, LOAD running. Wait and retry.
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Q: What is DB2 subsystem?
Answer:
Subsystem is DB2 instance. Has unique name (4 chars). Multiple subsystems on same LPAR. Each has own catalog, logs, data. SSID in application connection. Data sharing allows multi-subsystem access.
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Q: Explain COMMIT and ROLLBACK
Answer:
COMMIT makes changes permanent, releases locks. ROLLBACK undoes changes since last COMMIT. Implicit COMMIT at program end (normal). Implicit ROLLBACK on abend. Frequent COMMIT reduces lock duration and log usage.
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Q: How to use UNION?
Answer:
UNION combines SELECT results. UNION removes duplicates, UNION ALL keeps all. Column count and types must match. ORDER BY at end only. UNION expensive due to sort. Use UNION ALL if duplicates OK or impossible.
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Q: What is DISTINCT keyword?
Answer:
DISTINCT eliminates duplicate rows. SELECT DISTINCT col FROM table. Applies to entire row, not single column. Causes sort for duplicate elimination. Performance impact. Avoid if possible. Sometimes indicates bad design.
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Q: What is CURRENT DATE function?
Answer:
CURRENT DATE returns today's date. CURRENT TIME for time. CURRENT TIMESTAMP for both. No parentheses needed. Used in SELECT, WHERE, INSERT. Can compare: WHERE hire_date > CURRENT DATE - 30 DAYS.
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Q: How to handle date arithmetic?
Answer:
DATE + n DAYS adds days. DATE - n MONTHS subtracts months. DAYS(date2) - DAYS(date1) gives day count. DATEDIFF function available. TIMESTAMPDIFF for time differences. DATE/TIME functions for extraction.
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Q: Explain COALESCE function
Answer:
COALESCE returns first non-NULL argument. COALESCE(col1, col2, 'default'). Useful for NULL handling. VALUE is synonym. Common: COALESCE(nullable_col, 0) for calculations. Can chain multiple expressions.
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Q: Explain CREATE INDEX
Answer:
CREATE INDEX creates index on table. CREATE INDEX idx ON table(col1, col2). UNIQUE prevents duplicates. CLUSTER determines physical order. Include columns for index-only access. Drop unused indexes for insert performance.
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Q: What is clustered index?
Answer:
Clustered index determines physical row order. Only one per table. Other indexes are secondary. CLUSTER keyword on CREATE INDEX. Good for range queries on cluster key. REORG restores clustering after updates.
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Q: What is BETWEEN operator?
Answer:
BETWEEN tests range inclusively. WHERE col BETWEEN 1 AND 100. Equivalent to col >= 1 AND col <= 100. Works with dates, times. Can use NOT BETWEEN for exclusion. Index can be used for BETWEEN.
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Q: Explain UPDATE with JOIN
Answer:
UPDATE table SET col = value FROM table t1 INNER JOIN table t2 ON... WHERE condition. Or: UPDATE t1 SET col = (SELECT col FROM t2 WHERE t2.key = t1.key). Merge statement is alternative.
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Q: How to delete with JOIN?
Answer:
DELETE FROM table WHERE key IN (SELECT key FROM other WHERE condition). Or: DELETE FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.key = t2.key). Correlated subquery common for conditional delete.
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Q: What is INSERT...SELECT?
Answer:
INSERT INTO table SELECT cols FROM other_table WHERE condition. Bulk insert from query. Column count/types must match. Faster than row-by-row. Can transform data during insert. Good for data migration.
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Q: What is FETCH FIRST?
Answer:
FETCH FIRST n ROWS ONLY limits result set. SELECT * FROM t ORDER BY col FETCH FIRST 10 ROWS ONLY. Optimizes query - doesn't retrieve all. OFFSET for paging. WITH TIES includes equal values.
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Q: How to use window functions?
Answer:
ROW_NUMBER() OVER (PARTITION BY col ORDER BY col2). RANK, DENSE_RANK for rankings. SUM/AVG/COUNT OVER for running totals. PARTITION BY groups within result. ORDER BY within partition. Powerful analytics.
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Q: Explain RANK vs DENSE_RANK
Answer:
RANK() creates gaps for ties: 1,2,2,4. DENSE_RANK() no gaps: 1,2,2,3. ROW_NUMBER() no ties: 1,2,3,4. RANK useful for competition ranking. DENSE_RANK for level assignment. Choose based on need.
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Q: How to write recursive CTE?
Answer:
WITH RECURSIVE cte AS (base-case UNION ALL recursive-case referencing cte) SELECT * FROM cte. For hierarchies: start with root, join to find children. DEPTH limit prevents infinite recursion.
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Q: What is stored procedure?
Answer:
Stored procedure is saved SQL code. CREATE PROCEDURE name(params) BEGIN SQL statements END. CALL name(values) executes. Can have IN/OUT/INOUT parameters. Reduces network traffic. Logic in database. Security benefits.
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Q: Explain DB2 triggers
Answer:
Trigger fires automatically on INSERT/UPDATE/DELETE. CREATE TRIGGER name AFTER/BEFORE/INSTEAD OF event ON table FOR EACH ROW/STATEMENT. Use for audit, validation, derived columns. Can impact performance.
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Q: What is DCLGEN?
Answer:
DCLGEN generates COBOL declarations from table. DCLGEN TABLE(name) LIBRARY(dataset) STRUCTURE(host-struct-name). Creates copybook with field definitions. Essential for maintaining table-program consistency.
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Q: Explain DB2 logging
Answer:
DB2 logs all changes for recovery. Active log (circular), archive log (offloaded). LOG YES/NO on DDL. COMMIT writes to log. Recovery uses logs. Log full causes issues - monitor usage.
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Q: What is CHECK utility?
Answer:
CHECK validates data integrity. CHECK DATA TABLESPACE db.ts. Finds constraint violations, orphan rows. CHECK INDEX validates index structure. CHECK LOB for LOB issues. Run periodically or after issues.
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Q: Explain LOAD utility
Answer:
LOAD inserts large data volumes. LOAD DATA INTO TABLE name. Options: REPLACE, RESUME, LOG NO. Input is SYSREC dataset. LOAD faster than INSERT. Puts tablespace in COPY PENDING after LOG NO.
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Q: What is data sharing?
Answer:
Data sharing allows multiple DB2s to access same data. Group of subsystems share data. Uses Coupling Facility for coordination. Provides availability, scalability. Complex but powerful for high availability.
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Q: What is VOLATILE table?
Answer:
VOLATILE TABLE hints optimizer to expect different row counts. Useful for varying cardinality. CREATE TABLE ... VOLATILE CARDINALITY. Or ALTER TABLE ... VOLATILE. Helps when statistics mislead optimizer.
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Q: Explain sequence objects
Answer:
SEQUENCE generates unique numbers. CREATE SEQUENCE name START WITH 1 INCREMENT BY 1. NEXT VALUE FOR sequence-name gets next. Used for keys. Cached for performance. No gaps guaranteed.
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Q: Explain partitioning strategies
Answer:
Range partitioning by value ranges (date, key ranges). Hash partitioning distributes evenly. Partition independence allows parallel operations. Partition pruning improves queries. Rotate partitions for time-series.
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Q: What is table compression?
Answer:
Compression reduces storage. Huffman encoding in dictionary. CREATE TABLE ... COMPRESS YES. REORG to compress existing. CPU trade-off for I/O savings. Good for read-heavy, large tables.
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Q: How to monitor DB2 performance?
Answer:
Use OMEGAMON, DB2PM, or built-in monitor. Check: buffer pool hit ratios, lock waits, CPU time, elapsed time. DISPLAY commands show real-time. Statistics trace for analysis. EXPLAIN for query level.
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Q: What is REBIND and when needed?
Answer:
REBIND updates access path without precompile. REBIND PACKAGE after RUNSTATS, index changes. REBIND PLAN for plan-level changes. Can improve or degrade performance - test first. EXPLAIN before/after.
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Q: Explain host variable rules
Answer:
Host variables prefixed with : in SQL. Declare in WORKING-STORAGE. Must be compatible types. Use indicator for NULL. Cannot use in dynamic object names. VARCHAR needs two-part structure in COBOL.
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Q: How to prevent SQL injection?
Answer:
Use parameter markers (?), not concatenation. PREPARE stmt FROM 'SELECT * FROM t WHERE c = ?'. EXECUTE stmt USING :hostvar. Never build SQL with user input directly. Validate input. Use static SQL when possible.
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Q: What is ALIAS?
Answer:
ALIAS is alternate name for table. CREATE ALIAS alias FOR table. Useful for cross-subsystem access. Synonym is similar. PUBLIC alias visible to all. ALIAS can point to table in different schema.
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Q: Explain storage group
Answer:
Storage group defines volumes for data. CREATE STOGROUP name VOLUMES(vol1, vol2). Tablespaces assigned to stogroups. DB2 manages space within. PRIQTY/SECQTY control allocation. Foundation of storage management.
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Q: Explain access path selection
Answer:
Optimizer chooses access path based on statistics, predicates, indexes. Index scan vs tablespace scan. Join methods: nested loop, merge scan, hybrid. Sort operations. EXPLAIN reveals choice. Tuning influences path.
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Q: What is profile table?
Answer:
Profile tables store user-specific optimizer settings. DSN_PROFILE_TABLE, DSN_PROFILE_ATTRIBUTES. Override defaults for specific statements. Use QUERYNO correlation. Advanced tuning technique.
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Q: What is zparm?
Answer:
ZPARM (DSNZPARMs) are DB2 installation parameters. Control system behavior, limits, defaults. DSNZPARM module loaded at startup. Changes need restart usually. Critical for performance and security tuning.
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Q: Explain thread management
Answer:
Thread is DB2 connection. Allied thread for TSO/batch. DBAT (database access thread) for DDF. Pool thread for efficient reuse. Max threads controlled by zparm. Monitor active threads.
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Q: How to handle -818 SQLCODE?
Answer:
-818 is timestamp mismatch between plan and DBRM. DBRM precompiled after last BIND. Solutions: REBIND plan/package, ensure DBRM library current, check promotion procedures. Timestamp in DBRM must match bound plan.
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Q: What is EXPLAIN and how to use it?
Answer:
EXPLAIN analyzes query access path. EXPLAIN PLAN SET QUERYNO=1 FOR SELECT... Results in PLAN_TABLE. Shows index usage, join method, sort operations. Use for performance tuning. Review ACCESSTYPE, MATCHCOLS, PREFETCH columns.
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Q: What causes -911 SQLCODE?
Answer:
-911 is deadlock or timeout. Two processes waiting for each other's resources. Solutions: consistent lock order, shorter transactions, COMMIT frequently, appropriate isolation level. -913 is similar (deadlock victim). Retry transaction.
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Q: Explain DB2 isolation levels
Answer:
UR (Uncommitted Read) reads dirty data. CS (Cursor Stability) locks current row. RS (Read Stability) locks all accessed rows. RR (Repeatable Read) locks range, prevents phantom. Higher isolation = more consistency but more locking. CS most common.
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Q: Explain DECLARE CURSOR syntax
Answer:
DECLARE cursor-name CURSOR FOR SELECT... WITH HOLD keeps open after COMMIT. WITH RETURN returns to caller. FOR UPDATE OF allows positioned update. FOR READ ONLY optimizes read. ORDER BY for sorting. Static or dynamic declaration.
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Q: What is SQLCA structure?
Answer:
SQLCA (SQL Communication Area) contains execution results. SQLCODE for return code. SQLERRM for message. SQLERRD(3) for rows affected. SQLWARN for warnings. INCLUDE SQLCA in WORKING-STORAGE. Check after each SQL statement.
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Q: How to handle -803 SQLCODE?
Answer:
-803 is duplicate key violation. Primary key or unique index constraint. Check SQLERRD(3) for index ID. Solutions: check data, use IGNORE_DUPLICATE, handle in program logic. May indicate data quality issue.
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Q: How to do paging with DB2?
Answer:
Use FETCH FIRST n ROWS ONLY with OFFSET or ROW_NUMBER(). Modern: SELECT * FROM (SELECT ROW_NUMBER() OVER(ORDER BY col) AS rn, t.* FROM table t) WHERE rn BETWEEN start AND end. Or cursor with FETCH FIRST.
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Q: What is SPUFI?
Answer:
SPUFI (SQL Processing Using File Input) is TSO tool for interactive SQL. Input dataset with SQL statements. Output shows results. Good for testing queries, DDL, quick data checks. Not for production processing.
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Q: What is BIND command?
Answer:
BIND creates plan/package from DBRM. BIND PLAN(name) MEMBER(dbrm). Options: ISOLATION, VALIDATE, EXPLAIN. REBIND updates existing. BIND PACKAGE for packages. Precompile creates DBRM, BIND makes executable.
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Q: What is DB2 buffer pool?
Answer:
Buffer pool caches data/index pages in memory. BP0, BP1, etc. Hit ratio critical for performance. GETPAGE vs SYNCIO shows effectiveness. Size appropriately. Virtual buffer pool for each pool. Monitor with statistics.
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Q: Explain EXISTS vs IN
Answer:
EXISTS checks for row existence: WHERE EXISTS (SELECT 1 FROM...). IN checks value in list: WHERE col IN (SELECT...). EXISTS usually faster with correlated subquery. IN better for small static lists. EXISTS stops at first match.
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Q: What is GROUP BY clause?
Answer:
GROUP BY aggregates rows. SELECT col, COUNT(*) FROM t GROUP BY col. All non-aggregate SELECT columns must be in GROUP BY. HAVING filters groups (vs WHERE filters rows). GROUP BY ROLLUP/CUBE for subtotals.
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Q: What is CASE expression?
Answer:
CASE provides if-then logic. CASE WHEN cond1 THEN val1 WHEN cond2 THEN val2 ELSE default END. Simple CASE: CASE col WHEN 'A' THEN 'Active' END. Use in SELECT, WHERE, ORDER BY. Powerful for transformations.
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Q: How to concatenate strings?
Answer:
Use CONCAT(str1, str2) or || operator. CONCAT('Hello', ' ', 'World'). || works same: col1 || col2. Handles VARCHAR properly. NULL concatenation yields NULL (use COALESCE). RTRIM to remove trailing spaces.
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Q: What is SUBSTR function?
Answer:
SUBSTR extracts substring. SUBSTR(string, start, length). SUBSTR(name, 1, 3) gets first 3 chars. Position starts at 1. Length optional (to end). Can use in WHERE for pattern matching.
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Q: How to use LIKE for patterns?
Answer:
LIKE matches patterns. % matches zero or more chars. _ matches single char. WHERE name LIKE 'SM%' finds Smith, Smart. ESCAPE clause for literal % or _. Not index friendly unless prefix match (e.g., 'ABC%').
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Q: What is MERGE statement?
Answer:
MERGE updates or inserts conditionally. MERGE INTO target USING source ON condition WHEN MATCHED THEN UPDATE WHEN NOT MATCHED THEN INSERT. Combines INSERT/UPDATE logic. Efficient for sync operations.
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Q: What is FETCH SENSITIVE cursor?
Answer:
SENSITIVE cursor reflects concurrent changes by other processes. EXEC SQL DECLARE cursor SENSITIVE STATIC/DYNAMIC SCROLL CURSOR. INSENSITIVE does not see changes. Affects isolation behavior. Consider performance impact.
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Q: Explain ORDER BY options
Answer:
ORDER BY sorts results. ORDER BY col1 ASC, col2 DESC. ASC default. NULLS FIRST/LAST controls NULL position. Ordinal: ORDER BY 1, 2 uses column positions. ORDER BY CASE for custom order. Affects performance.
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Q: Explain NOFOR option in SELECT?
Answer:
NOFOR prevents FOR UPDATE locking. SELECT ... NOFOR. Read-only access without lock overhead. For display-only queries. Cannot UPDATE WHERE CURRENT OF cursor with NOFOR.