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DBMS → Introduction to DBMS 24

R−S returns
Tuples in R that do NOT exist in S
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Cartesian product R×S produces
All possible combinations of tuples from R and S
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Renaming operator in relational algebra is
ρ
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Division R÷S is useful for
Universal quantification queries (for all/every conditions)
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Five basic RA operators are
σ,π,∪,−,× (selection,projection,union,difference,cross-product)
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If |R|=5 rows and |S|=4 rows, R×S has
20 rows
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R∩S expressed using basic operators equals
R−(R−S)
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SQL stands for
Structured Query Language
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SELECT retrieves
Data from one or more tables
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SELECT DISTINCT removes
Duplicate rows from result set
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COUNT(*) counts
ALL rows including those with NULLs
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EXISTS subquery returns TRUE when
Subquery returns at least one row
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UNION combines results and by default
Removes duplicate rows
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UNION ALL keeps
ALL rows including duplicates
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MAX() returns
Highest value
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SUM() returns
Total sum of numeric values
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AVG() returns
Average (arithmetic mean) of values
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A scalar subquery returns
Exactly one row and one column (single value)
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WITH clause (CTE) creates
Named temporary result set valid within one SQL statement
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ROW_NUMBER() assigns
Unique sequential integer to each row
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RANK() gives 1,2,2,4 for ties because
Skips rank numbers AFTER ties (gap after tied ranks)
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DENSE_RANK() gives 1,2,2,3 for ties because
It never skips rank numbers after ties
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GRANT command gives
Specific privileges to a user on a database object
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REVOKE command
Removes privileges
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DBMS → Joins 7

Theta join R⊲⊳θS equals
σθ(R×S)
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Equijoin uses which condition operator
= (equality)
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Natural join joins on
All attributes with same name in both relations
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Anti-join R▷S returns
Tuples in R that have NO matching tuple in S
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INNER JOIN returns
Only rows where join condition satisfied in BOTH tables
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LEFT JOIN returns
All left table rows plus matching right rows (NULLs for no match)
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FULL OUTER JOIN returns
All rows from both; NULLs for non-matches
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DBMS → SQL Basics 6

WHERE clause filters
Individual rows BEFORE grouping based on condition
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HAVING clause filters
Groups AFTER GROUP BY aggregation
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ORDER BY clause
Sorts result set in ASC (default) or DESC order
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GROUP BY groups rows that
Share same values in specified columns
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LAG(salary,1) OVER(ORDER BY date) returns
Previous row salary value in ordered partition
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LEAD(salary,1) OVER(ORDER BY date) returns
Next row salary value in ordered partition
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DBMS → Views 1

A view in SQL is
Virtual table defined by a SELECT query
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DBMS → PL/SQL 1

A trigger automatically executes when
Specified DML event occurs on a table
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DBMS → Transactions 1

COMMIT permanently
Saves all transaction changes to database
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