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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: 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: Explain SYMBOLIC CHARACTERS
Answer:
SPECIAL-NAMES. SYMBOLIC CHARACTERS NULL-CHAR IS 1. Assigns name to ordinal position in collating sequence. Position 1 is X'00' in EBCDIC. Use: MOVE NULL-CHAR TO field. Define non-printable characters readably.
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Q: Explain NULLFILE keyword?
Answer:
NULLFILE is alternate for DUMMY. //DD DD NULLFILE. Discards output, provides EOF for input. DSN=NULLFILE equivalent. No actual dataset. Useful for testing without actual files.
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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: 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 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 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: Explain DD DUMMY statement
Answer:
DD DUMMY discards output/provides EOF for input. No I/O actually performed. DUMMY, DSN=NULLFILE equivalent. Use to skip optional outputs or provide empty input. Program sees immediate EOF on read. Writes succeed but discarded.
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Q: What is IEFBR14?
Answer:
IEFBR14 is null program (Branch to Register 14 = return). Returns immediately. Used for dataset management: create, delete, catalog datasets. //DELETE EXEC PGM=IEFBR14 //DD DD DSN=name,DISP=(MOD,DELETE). Common for housekeeping.
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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: 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.