DBMS, SQL Queries & Indexing

Database questions test relational algebra, transaction isolation levels, and query execution order (FROM -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY -> LIMIT).

Exam Frequency: 3 to 5 questions in technical MCQ rounds

Essential Formulas & Shortcut Rules

Query Order: FROM -> JOIN -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY

Logical processing phases of an SQL query.

ACID = Atomicity, Consistency, Isolation, Durability

Core transaction reliability guarantees.

Worked Step-by-Step Examples

Example 1: Which SQL clause is used to filter records that satisfy aggregate conditions such as "HAVING COUNT(*) > 5"?

Step-by-Step Mathematical Derivation:

•WHERE operates on individual tuples before grouping.

•HAVING operates on aggregated grouped records generated by GROUP BY.

•Therefore, aggregate conditions must always reside in the HAVING clause.

Answer: HAVING clause

💡 Speed Shortcut: Remember: WHERE filters records; HAVING filters groups.

Common Exam Traps to Avoid

✕Using aggregate functions like SUM() or COUNT() inside a WHERE clause.
✕Assuming primary keys can permit NULL values (Primary Key = Unique + NOT NULL).

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Consider the following SQL query: SELECT department_id, COUNT(*) FROM employees WHERE salary > 50000 GROUP BY department_id HAVING COUNT(*) > 5; In what order are the clauses logically evaluated by the database engine?

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