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GK One-Line Question & Answer

15541+ short questions with short answers, covering every category and sub-category on the site — no long articles to scroll through. Good for a fast recap before an exam, or a few minutes of daily practice.

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DBMS → Introduction to DBMS 26

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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Table-valued function returns
A result set (table) usable in FROM clause
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Which constraint is validated at statement execution time by default
NOT DEFERRABLE (default)
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DEFERRABLE INITIALLY DEFERRED constraint
Is checked at end of transaction (COMMIT time) by default
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Which SQL creates a partial/filtered index
CREATE INDEX idx ON emp(salary) WHERE dept='IT'
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Composite index (A,B,C): which query CAN use this index
WHERE A=1 AND B=5 (leading columns)
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Index fragmentation above 30% should be addressed by
REBUILD INDEX (drops and recreates, eliminates all fragmentation)
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Which SQL checks index fragmentation in SQL Server
sys.dm_db_index_physical_stats
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UPDATE STATISTICS command purpose
Refresh the statistical metadata the optimizer uses for query planning
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Which SQL best finds all tables in current database
SHOW TABLES or SELECT * FROM information_schema.TABLES
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INFORMATION_SCHEMA is
A system database providing metadata about all database objects
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Which SQL lists all columns of the questions table
SHOW COLUMNS FROM questions OR DESCRIBE questions
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EXPLAIN SELECT * FROM questions WHERE q_level=3 shows
The execution plan: whether index is used, rows examined, join type
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Which helps identify slow queries in MySQL
EXPLAIN and slow_query_log
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Which lock type is compatible with another shared lock
Shared (S) lock
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X (exclusive) lock is compatible with
No other lock type
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Which CC protocol allows both readers and writers without blocking each other
MVCC (Multi-Version Concurrency Control)
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Optimistic CC three phases are
Read, Validate, Write (commit if valid; else restart)
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DBMS → SQL Basics 1

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 → 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 6

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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Which SQL creates a stored procedure with output parameter
CREATE PROCEDURE p(OUT result INT) BEGIN...END
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Recursive stored procedure/function requires
A termination condition to prevent infinite recursion
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Which SQL creates trigger to log inserts on questions table
CREATE TRIGGER trg AFTER INSERT ON questions FOR EACH ROW INSERT INTO audit_log...
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DBMS → Transactions 3

Which isolation level is the SQL standard default in most databases
READ COMMITTED
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MySQL InnoDB default isolation level is
REPEATABLE READ
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Database transaction log is used for
Recovery: undoing incomplete transactions and redoing committed ones after crashes
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DBMS → DDL Commands 1

Which DDL command modifies column data type
ALTER TABLE t MODIFY/ALTER COLUMN col newtype
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DBMS → Relational Model 1

PRIMARY KEY vs UNIQUE: key difference
UNIQUE allows one NULL; PRIMARY KEY does not allow any NULL
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DBMS → Deadlock 1

Deadlock detection frequency tradeoff
Higher frequency: detects deadlocks sooner but adds overhead; lower: less overhead but longer wait
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