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DBMS → Introduction to DBMS 29

ROW_NUMBER() vs RANK(): key difference
ROW_NUMBER gives unique numbers even for ties; RANK skips numbers after ties
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DENSE_RANK() vs RANK(): key difference
RANK skips numbers after ties; DENSE_RANK does NOT skip
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PERCENT_RANK() of lowest-ranked row (rank N of N total) is
1.0
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CUME_DIST() of top row is
1.0/N
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Window function can appear in which SQL clause
SELECT and ORDER BY only
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INTERSECT returns rows common to
BOTH result sets
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EXCEPT (MINUS) returns
Rows in FIRST result NOT in second
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A correlated subquery references
Columns from the outer query and executes once per outer row
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A derived table is a subquery in
FROM clause
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EXISTS vs IN: which is preferred for large subquery results
EXISTS is often better for large subquery results (stops at first match)
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ALL operator: salary > ALL(subquery) means salary is
Greater than EVERY value in subquery (greater than MAX)
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ANY/SOME operator: salary > ANY(subquery) means salary is
Greater than AT LEAST ONE value in subquery (greater than MIN)
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Recursive CTE ANCHOR member is
The base case (initial result set)
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COALESCE vs ISNULL: key difference
ISNULL takes only 2 arguments; COALESCE takes multiple arguments
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CASE WHEN grade>=90 THEN 'A' WHEN grade>=80 THEN 'B' ELSE 'C' END assigns
Grade categories using IF-THEN-ELSE logic
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PIVOT operation converts
Rows into columns (rotates table)
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CTE advantages over subqueries include
Improved readability and can be referenced multiple times in same query
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Which prevents SQL injection in parameterized queries
Using ? or named parameters so user input treated as data not SQL code
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GRANT SELECT,INSERT ON questions TO user1 grants
SELECT and INSERT privileges on questions table to user1
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REVOKE ALL PRIVILEGES ON questions FROM user1
Removes all privileges on questions table from user1
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IDENTITY/AUTO_INCREMENT column value
Is automatically generated as next sequential integer
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Which SQL returns only the top 5 highest paid employees
SELECT * FROM emp ORDER BY salary DESC LIMIT 5
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Which SQL data type is best for storing money/currency values
DECIMAL(15,2)
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VARCHAR vs CHAR: VARCHAR stores
Variable-length strings using only needed space
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TEXT vs VARCHAR: TEXT stores
Large text (up to 65,535 chars) without specifying max length
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DATETIME vs TIMESTAMP in MySQL
TIMESTAMP converts to UTC; DATETIME stores as-is
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Which SQL command creates a backup of table structure only
CREATE TABLE backup LIKE source
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Index on column with 1 million rows and 2 distinct values (gender) is
Useless since selectivity is extremely low (50% match) - optimizer may skip it
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Covering index for SELECT id,name FROM emp WHERE dept='IT' should include
dept, id, name columns
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DBMS → SQL Basics 4

FIRST_VALUE(salary) OVER(PARTITION BY dept ORDER BY hire_date) returns
First hire's salary in each department
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LAST_VALUE(salary) OVER(PARTITION BY dept ORDER BY hire_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) returns
Last hire's salary in each department
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Which SQL aggregates strings across rows
GROUP_CONCAT() or STRING_AGG()
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Which SQL creates a view that prevents DML violating its WHERE clause
CREATE VIEW v AS SELECT...WITH CHECK OPTION
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DBMS → Transactions 2

BEGIN TRANSACTION / START TRANSACTION
Explicitly starts a new transaction
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SAVEPOINT sp1 followed by ROLLBACK TO sp1
Undoes ONLY work done AFTER savepoint sp1 was created
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DBMS → Relational Model 1

ON DELETE SET NULL foreign key option
Sets child FK column to NULL when parent row is deleted
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DBMS → Views 1

Updatable view conditions: a view is NOT updatable when it contains
DISTINCT, GROUP BY, aggregate functions, UNION, or joins to multiple tables
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DBMS → PL/SQL 3

BEFORE trigger can be used to
Validate or modify NEW values before they are saved to the database
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AFTER trigger is best for
Audit logging - recording what changed after the operation succeeded
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Stored procedure IN OUT parameter difference
IN: pass value to procedure. OUT: return value from procedure. INOUT: both
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