!" #Chris Irwin Davis, Ph.D. Email: [email protected] Phone: (972) 883-3574 Office: ECSS 4.705Chapter 5: More SQLCS-6360 Database Design!" #Chapter 5 Outline§More Complex SQL Retrieval Queries§Specifying Constraints as Assertions and Actions as Triggers§Views (Virtual Tables) in SQL§Schema Change Statements in SQL2!" #More Complex SQL Retrieval Queries§Additional features allow users to specify more complex retrievals from database:§Derived values§Nested queries§Joined tables§Outer joins§Aggregate functions and grouping3!" #Selection Criteria in WHERE Clause•FROM clause (logically) generates Cartesian Product of tables°e.g. FROM T1, T2 ⇔ T1 ╳ T2 •WHERE clause filters based on selection criteria°Each “wide” tuple of FROM clause is considered individually, in sequence4!" #Comparisons Involving NULL"and Three-Valued Logic§Meanings of NULL §Unknown value §Unavailable or withheld value §Not applicable attribute §Each individual NULL value considered to be different and distinct from every other NULL value§SQL uses a three-valued logic:§TRUE, FALSE, and UNKNOWN §Why significant?5!" #Comparisons Involving NULL"and Three-Valued Logic (cont’d.)6!" #Comparisons Involving NULL"and Three-Valued Logic (cont’d.)7!" #Comparisons Involving NULL"and Three-Valued Logic (cont’d.)§SQL allows queries that check whether an attribute value is NULL §IS NULL or IS NOT NULL8!" #Comparisons Involving NULL"and Three-Valued Logic (cont’d.)§SQL allows queries that check whether an attribute value is NULL §IS NULL or IS NOT NULL9Nested Queries!" #Nested Queries•Nested in WHERE clause°WHERE [NOT] attribute ⟨comp_op⟩ (subquery)°WHERE [NOT] attribute ⟨comp_op⟩ ALL (subquery)°WHERE [NOT] attribute ⟨comp_op⟩ ANY (subquery)°WHERE attribute [NOT] IN (subquery)•Nested in FROM clause (creating a View on-the-fly)°FROM (subquery) AS alias11!" #IN Clause§Comparison set operator IN §Compares value v with a set (or multiset) of values V §Evaluates to TRUE if v is one of the elements in V12!" #Nested Queries (cont’d.)13UNION!" #Nested Queries (cont’d.)§Use tuples of values in comparisons §Place them within parentheses14!" #Nested Queries (cont’d.)§Use tuples of values in comparisons §Place them within parentheses15SELECT DISTINCT Essn FROM Works_on WHERE (Pno, Hours) IN (SELECT Pno, Hours FROM Works_on WHERE Essn = '123456789');!" #IN Operator with Explicit Set• SELECT *!FROM Department!WHERE Dnumber IN (1,7,8);16!" #Explicit Sets and Renaming of Attributes in SQL§Can use explicit set of values in WHERE clause17!" #Explicit Sets and Renaming of Attributes in SQL§Can use explicit set of values in WHERE clause18!" #§Use other comparison operators to compare a single value v §= ANY (or = SOME) operator •Returns TRUE if the value v is equal to some value in the set V and is hence equivalent to IN §Other operators that can be combined with ANY (or SOME): >, >=, <, <=, and <>Nested Queries (cont’d.)19!" #Nested Queries (cont’d.)§Avoid potential errors and ambiguities§Create tuple variables (aliases) for all tables referenced in SQL query20!" #Correlated Nested Queries§Correlated nested query §Whenever a condition in the WHERE clause of a nested query references some attribute of a relation declared in the outer query, the two queries are said to be correlated.§We can understand a correlated query better by considering that the nested query is evaluated once for each tuple (or combination of tuples) in the outer query.§For example, we can think of Q16 as follows: For each EMPLOYEE tuple, evaluate the nested query, which retrieves the Essn values for all DEPENDENT tuples with the same sex and name as that EMPLOYEE tuple; if the Ssn value of the EMPLOYEE tuple is in the result of the nested query, then select that EMPLOYEE tuple.21!" #The EXISTS Functions in SQL§EXISTS function §Check whether the result of a nested query is empty or not§No comparison with result set!!§EXISTS and NOT EXISTS §Typically used in conjunction with a correlated nested query22!" #Alternate Query 16 + Query 623!" #Explicit Sets and Renaming of Attributes in SQL§Renaming of Attributes§Use qualifier AS followed by desired new name§Rename any attribute that appears in the result of a query24Joins!" #Joined Tables in SQL and Outer Joins§Joined table §Permits users to specify a table resulting from a join operation in the FROM clause of a query§The FROM clause in Q1A §Contains a single joined table26!" #Joined Tables in SQL and Outer Joins (cont’d.)§Specify different types of join§NATURAL JOIN §Various types of OUTER JOIN§NATURAL JOIN on two relations R and S§No join condition specified§Implicit EQUIJOIN condition for each pair of attributes with same name from R and S27!" #Joined Tables in SQL and Outer Joins (cont’d.)§Inner join§Default type of join in a joined table§Tuple is included in the result only if a matching tuple exists in the other relation28!" #Joined Tables in SQL and Outer Joins (cont’d.)§Outer joins§LEFT JOIN §Every tuple in left table must appear in result§If no matching tuple, padded with NULL values for attributes of right table§RIGHT JOIN§Not supported by SQLite§FULL JOIN§Not supported by MySQL, SQLite§Can be simulated with Left Join / Right Join UNION29Aggregate Functions, GROUP BY, and HAVING!" #Aggregate Functions in SQL§Used to summarize information from multiple tuples into a single-tuple summary§Built-in Aggregate Functions §COUNT, SUM, MAX, MIN, and AVG31!" #GROUP BY and HAVING in SQL§Used to summarize information from multiple tuples into a single-tuple summary§Grouping (GROUP BY) §Create subgroups of tuples before summarizing §Aggregate functions can be used in the SELECT clause or in a HAVING clause, but not WHERE clause.32!" #Aggregate Functions in SQL (cont’d.)§NULL values discarded when aggregate functions are applied to a particular column33!" #Grouping: The GROUP BY and HAVING Clauses§Partition relation into subsets of tuples§Based on grouping attribute(s) §Apply function to each such group independently§GROUP BY clause §Specifies grouping attributes§If NULLs exist in grouping attribute §Separate group created
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