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Q: Explain SECTION vs PARAGRAPH
Answer:
SECTION groups related paragraphs, terminated by next SECTION or end of program. PARAGRAPH is a named block of code, terminated by next paragraph/section. PERFORM SECTION executes all paragraphs within. SECTION needed for segmentation, PARAGRAPH for simple procedures.
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Q: Explain database design normalization
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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 MOVE CORRESPONDING?
Answer:
MOVE CORRESPONDING moves all fields with matching names between groups. MOVE CORR INPUT-REC TO OUTPUT-REC. Only moves if names match exactly. Moves all matches, not just first. Useful for record transformations. Debug carefully-silent partial moves.
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Q: Explain generation data groups
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GDG maintains versions. Base defined with IDCAMS: DEFINE GDG NAME(base) LIMIT(10) SCRATCH. Reference: base(+1) new, base(0) current, base(-1) previous. LIMIT controls kept generations. NOEMPTY/EMPTY for empty condition.
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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 storage group
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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 SPECIAL-NAMES paragraph
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SPECIAL-NAMES maps system names to COBOL names. DECIMAL-POINT IS COMMA for European notation. CURRENCY SIGN IS '$'. SYMBOLIC CHARACTERS name = value. ALPHABET for custom collating. CLASS for character groups.
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Q: What is CNTL statement?
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CNTL groups control statements. //name CNTL establishes scope. //name ENDCNTL ends it. Used with OUTPUT or other complex statements. Provides modularity. Can be referenced by name from multiple DDs.
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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.