Q51. What is true about Cartesian Products?
A Cartesian product occurs when tables are joined **without a condition**. Every row in Table A pairs with every row in Table B: `m × n` rows total. It's usually a bug, b...
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Q52. INTERSECTION in relational algebra?
In relational algebra and set theory, the **INTERSECTION** of two sets $A \cap B$ yields only the tuples that appear in **both** relation sets simultaneously. ### SQL Imp...
Q53. UNION in relational algebra?
In relational algebra, the **UNION** of two sets $A \cup B$ combines all distinct tuples from both relations into a single unified relation. ### `UNION` vs `UNION ALL` in...
Q54. UNION vs UNION ALL?
| UNION | UNION ALL | |-------|-----------| | Removes duplicates | Keeps duplicates | | Slower (sort + dedup) | Faster | | Use when uniqueness matters | Use when sets are...
Q55. Which set operator for "searched but didn't buy"?
`MINUS` (Oracle) or `EXCEPT` (PostgreSQL/SQL Server): ```sql SELECT name FROM searchers WHERE product = 'X' EXCEPT SELECT name FROM buyers WHERE product = 'X'; ``` ---
Q56. A SELECT statement whose results are used in filtering the main query is called?
A `SELECT` statement nested inside another SQL statement to supply values for filtering or computation is called a **subquery** (also known as an **inner query** or **nes...
Q57. Subqueries can be nested in...
Subqueries can be nested within virtually every major SQL clause and statement: 1. **`WHERE` Clause**: Filtering candidate rows (`WHERE id IN (SELECT ...)`) 2. **`FROM` C...
Q58. Row comparison operators with subqueries?
`IN`, `ANY`, `ALL`, `EXISTS`, `NOT EXISTS`, and standard comparison operators (`=`, `>`, `=`, ` ALL (SELECT salary FROM employees WHERE dept = 'Sales') WHERE salary > ANY...
Q59. A subquery inside a subquery is called?
A subquery placed inside another subquery is called a **nested subquery** (or multi-level subquery): ```sql SELECT customer_name FROM customers WHERE customer_id IN ( SEL...
Q60. A subquery that references the outer query?
A **correlated subquery**. It executes once for each row of the outer query. ```sql SELECT * FROM employees e WHERE salary > (SELECT AVG(salary) FROM employees WHERE dept...