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DBMS → SQL Basics 4

ROLLUP in GROUP BY
Generates subtotals and grand total across hierarchy levels
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CUBE in GROUP BY
Generates subtotals for ALL possible combinations of specified grouping columns
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NTILE(4) OVER(ORDER BY salary) divides
Ordered rows into 4 approximately equal-sized buckets
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Which SQL clause is mandatory in a SELECT statement?
FROM
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DBMS → Introduction to DBMS 32

PERCENT_RANK() of highest-ranked row is
0.0
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SUM(salary) OVER(ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) computes
Running cumulative total of salary
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CUME_DIST() computes
Cumulative distribution: fraction of rows ≤ current row value
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ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING includes
Previous row + current row + next row (3-row window)
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PAXOS consensus protocol is used in
Distributed systems for fault-tolerant consensus among multiple nodes
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Base property of NoSQL - Eventually consistent means
Given enough time (no updates) all replicas will converge to same value
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Vertical fragmentation in distributed DB divides relation by
Columns (attributes) keeping PK in each fragment
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Horizontal fragmentation in distributed DB divides relation by
Rows (subsets of tuples) based on selection predicates
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Location transparency in distributed DB means
User can query data without knowing its physical location
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Impedance mismatch in databases refers to
Mismatch between OO programming model and relational data model
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ORM (Object-Relational Mapper) bridges
Object-oriented programming model and relational database (impedance mismatch)
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Laravel's Eloquent uses which ORM pattern?
Active Record pattern
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Which SQL function returns current date only (no time)?
CURDATE() or CURRENT_DATE
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FLOOR(4.9) returns
4
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CEIL(4.1) returns
5
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ROUND(4.567,2) returns
4.57
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ABS(-15) returns
15
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MOD(17,5) returns
2
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UPPER('dbms') returns
DBMS
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LOWER('DBMS') returns
dbms
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LENGTH('RMSSSB') returns
6
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SUBSTRING('Database',1,4) returns
Data
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TRIM(' hello ') returns
hello
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REPLACE('SQL is hard','hard','easy') returns
SQL is easy
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CHARINDEX('S','DBMS') returns
4
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REVERSE('SQL') returns
LQS
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CONCAT('Hello',' ','World') returns
Hello World
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SELECT * FROM questions LIMIT 10 OFFSET 20 returns
Rows 21 through 30
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Which SQL adds a new column to existing table?
ALTER TABLE questions ADD COLUMN new_col INT
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Which SQL renames a table?
ALTER TABLE old RENAME TO new OR RENAME TABLE old TO new
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CREATE INDEX idx ON questions(q_level) creates
A data structure to speed up queries on q_level column
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Which prevents phantom reads?
SERIALIZABLE
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DBMS → Transactions 2

Two-Phase Commit (2PC) is used for
Distributed database transactions across multiple nodes
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Which isolation level allows dirty reads?
READ UNCOMMITTED
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DBMS → Joins 1

Semi-join optimization in distributed queries sends
Only join-attribute projection to remote site to reduce data transfer
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DBMS → DDL Commands 1

DROP TABLE removes
Table structure, all data, indexes, and constraints permanently
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