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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: 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: 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 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 HURBA?
Answer:
HURBA (High Used RBA) marks end of data. Records beyond HURBA are unused space. LISTCAT shows HURBA. Grows as records added. REORG resets to eliminate gaps in ESDS.
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Q: How to do file BROWSE?
Answer:
BROWSE reads sequentially. EXEC CICS STARTBR FILE(name) RIDFLD(key). READNEXT/READPREV in loop. ENDBR ends browse. TOKEN for concurrent browses. Can use generic key to position.
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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 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 maximum VSAM key length?
Answer:
Maximum key length is 255 bytes. KEYS(255 offset). Practical limits lower for performance. Index size grows with key. Keep keys reasonably sized.
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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: 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: How to access AIX?
Answer:
Open PATH to AIX, not base cluster. READ via alternate key. Can browse by alternate key sequence. Updates depend on PATH UPDATE option. Non-unique AIX returns first, then READ NEXT for others.
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Q: How to access VSAM via CICS?
Answer:
Define FILE in CICS FCT (File Control Table). Or RDO DEFINE FILE. EXEC CICS READ FILE. VSAM must be closed to batch when CICS owns. NSRV or LSR buffering. BROWSE for sequential.
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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: 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 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 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.