Comprehensive Guide to Simple Queries in SQL for Beginners

2. 6.1 Simple Queries in SQL • The portion of SQL that supports queries has capabilities very close to that of relational algebra. • ANSI SQL, SQL2, SQL-99, SQL:2003 • SQL-99 extended with object-relational features and a number of other new capabilities • SQL:2003 a collection of extensions to SQL-99 • SELECT, FROM, WHERE • Example (See Fig. 6.1) Database Systems

3. 6.1 Simple Queries in SQL Database Systems

4. 6.1 Simple Queries in SQL • “All movies produced by Disney Studios in 1990” • SELECT * • FROM Movies • WHERE studioName=’Disney’ AND year=1990; • 6.1.1 Projection in SQL • Example • SELECT title, length • Alias • SELECT title AS name, length AS duration • An expression in place of an attribute • SELECT title AS name, length*0.016667 AS lengthInHours Database Systems

5. 6.1 Simple Queries in SQL • A constant as an expression • SELECT title AS name, length*0.016667 AS length, • ‘hrs.’ AS inHours • 6.1.2 Selection in SQL • Example • SELECT title • FROM Movies • WHERE (year>1970 OR length<90) AND • studioName=’MGM’; Database Systems

6. 6.1 Simple Queries in SQL • 6.1.3 Comparison of Strings • Lexicographic order • 6.1.4 Pattern Matching in SQL • s LIKE p where s is a string and p is a pattern • % and _ • Example • SELECT title • FROM Movies • WHERE title LIKE ’Star ____’; Database Systems

7. 6.1 Simple Queries in SQL • Example • SELECT title • FROM Movies • WHERE title LIKE ’%’’s%’; • 6.1.5 Dates and Times • Example • DATE ’1948-05-14’ • TIME ’15:00:02.5’ • TIMESTAMP ’1948-05-14 12:00:00’ Database Systems

8. 6.1 Simple Queries in SQL • 6.1.6 Null Values and Comparisons Involving NULL • Different interpretations: • 1) Value unknown • 2) Value inapplicable • 3) Value withheld • Two important rules: • 1) When we operate on a NULL and any value, including • another NULL, using an arithmetic operator like * or • +, the result is NULL. • 2) When we compare a NULL value and any value, • including another NULL, using a comparison operator • like = or >, the result is UNKNOWN. Database Systems

9. 6.1 Simple Queries in SQL • We cannot use NULL explicitly as an operand. • Example • NULL+3, NULL=3: not legal • The correct way to ask if x has the value NULL is with the expression x IS NULL. • 6.1.7 The Truth-Value UNKNOWN • The rule: TRUE as 1, FALSE as 0, UNKNOWN as 1/2 • 1) The AND of two truth-values is the minimum of those values. • 2) The OR of two truth-values is the maximum of those values. • 3) The negation of true-value v is 1-v. • (See Fig. 6.2) Database Systems

10. 6.1 Simple Queries in SQL Database Systems

11. 6.1 Simple Queries in SQL • Example • SELECT * • FROM Movies • WHERE length<=120 OR length>120 • The true meaning of the query is “find all the Movies tuples with non-NULL lengths.” • 6.1.8 Ordering the Output • ORDER BY <list of attributes> • DESC and ASC Database Systems

12. 6.1 Simple Queries in SQL • Example • SELECT * • FROM Movies • WHERE studioName=’Disney’ AND year=1990 • ORDER BY length, title; Database Systems

13. 6.2 Queries Involving More than One Relation • Joins, products, unions, intersections, and differences • 6.2.1 Products and Joins in SQL • Example • “The name of the producer of Star Wars” • SELECT name • FROM Movies, MovieExec • WHERE title=’Star Wars’ AND producerC#=cert#; • (See Fig. 6.3) Database Systems

14. 6.2 Queries Involving More than One Relation Database Systems

15. 6.2 Queries Involving More than One Relation • 6.2.2 Disambiguating Attributes • R.A refers to the attribute A of relation R. • Example • SELECT MovieStar.name, MovieExec.name • FROM MovieStar, MovieExec • WHERE MovieStar.address=MovieExec.address; • SELECT MovieExec.name • FROM Movies, MovieExec • WHERE Movies.title=’Star Wars’ • AND Movies.producerC#=MovieExec.cert#; Database Systems

16. 6.2 Queries Involving More than One Relation • 6.2.3 Tuple Variables • Tuple variable • Example • SELECT Star1.name, Star2.name • FROM MovieStar Star1, MovieStar Star2 • WHERE Star1.address= Star2.address • AND Star1.name<Star2.name; • 6.2.4 Interpreting Multirelation Queries • Several ways to define the meaning of the select-from-where expressions Database Systems

17. 6.2 Queries Involving More than One Relation 1) Nested Loops (See Fig. 6.4) 2) Parallel Assignment 3) Conversion to Relational Algebra Example πA1, A5(σA2=A6 AND A1<A5(ρM(A1, A2, A3, A4)(MovieStar)× ρN(A5, A6, A7, A8)(MovieStar))) Database Systems

18. 6.2 Queries Involving More than One Relation Database Systems

19. 6.2 Queries Involving More than One Relation • 6.2.5 Union, Intersection, and Difference of Queries • Example • “The names and addresses of all female movie stars who • are also movie executives with a net worth over • $10,000,000.” • (See Fig. 6.5) • Example • (SELECT name, address FROM MovieStar) • EXCEPT • (SELECT name, address FROM MovieExec) • If necessary to get a common set of attributes, we can rename attributes. Database Systems

20. 6.2 Queries Involving More than One Relation Database Systems

21. 6.2 Queries Involving More than One Relation • Example • (SELECT title, year FROM Movies) • UNION • (SELECT MovieTitle AS title, movieYear AS year • FROM StarsIn) Database Systems

22. 6.3 Subqueries • A query that is part of another is called a subquery. • A number of ways that subqueries can be used: • 1) Subqueries can return a single constant, and this constant • can be compared with another value in a WHERE clause. • 2) Subqueries can return relations that can be used in • various way in WHERE clauses. • 3) Subqueries can appear in FROM clauses, followed by a • tuple variable that represents the tuples in the result of • the subquery. • 6.3.1 Subqueries that Produce Scalar Values • We may compare the result of a subquery to a constant or attribute. Database Systems

23. 6.3 Subqueries • Example • (See Fig. 6.6) • SELECT name • FROM MovieExec • WHERE cert#=12345 • If zero tuples or more than one tuple is produced by the subquery, it is a run-time error. • 6.3.2 Conditions Involving Relations • A number of SQL operations that we can apply to a relation R and produce a boolean result. Database Systems

24. 6.3 Subqueries Database Systems

25. 6.3 Subqueries • s: a scalar value, R: a one-column relation • 1) EXISTS R • 2) s IN R, s NOT IN R • 3) s > ALL R, s <> ALL R • 4) s > ANY R, s = ANY R • 6.3.3 Conditions Involving Tuples • If a tuple t has the same number of components as a relation R, then it makes sense to compare t and R in expressions. • Example • “all the producers of movies in which Harrison Ford stars” • (See Fig. 6.7 and Fig. 6.8) Database Systems

26. 6.3 Subqueries Database Systems

27. 6.3 Subqueries Database Systems

28. 6.3 Subqueries • A single select-from-where expression: duplicates • (See Fig. 6.9) • 6.3.4 Correlated Subqueries • Correlated query: to be evaluated many times, once for each assignment of a value to some term in the subquery that come from a tuple variable outside the subquery. • Example • “the titles that have been used for two or more movies” • (See Fig. 6.10) • When writing a correlated query, it is important that we be aware of the scoping rules for names. Database Systems

29. 6.3 Subqueries Database Systems

30. 6.3 Subqueries Database Systems

31. 6.3 Subqueries • 6.3.5 Subqueries in FROM Clauses • Example • “the producers of Harrison Ford's movies” • (See Fig. 6.11) • 6.3.6 SQL Join Expressions • Cross join (Cartesian product) • e.g., Movies CROSS JOIN StarsIn; • Join • Example • Movie JOIN StarsIn ON • title=movieTitle AND year=movieYear; Database Systems

32. 6.3 Subqueries Database Systems

33. 6.3 Subqueries • Using the whole expression as a subquery in a FROM clause and using a SELECT clause to remove the undesired attributes. • SELECT title, year, length, genre, studioName, producerC#, • starName • FROM Movies JOIN StarsIn ON • title=movieTitle AND year=movieYear; • 6.3.7 Natural Joins • Differing from a theta-join • 1) The join condition is that all pairs of attributes from the • two relations having a common name are equated. • 2) One of each pair of equated attributes is projected out. Database Systems

34. 6.3 Subqueries • Example • MovieStar NATURAL JOIN MovieExec; • 6.3.8 Outerjoins • An outerjoin operator is a way to augment the result of a join by the dangling tuples, padded with null values. • Example • MovieStar NATURAL FULL OUTER JOIN MovieExec; • (See Fig. 6.12) • NATURAL LEFT (RIGHT) OUTER JOIN Database Systems

35. 6.3 Subqueries Database Systems

36. 6.3 Subqueries • Example • Movies FULL OUTER JOIN StarsIn ON • title=movieTitle AND year=movieYear; Database Systems

37. 6.4 Full-Relation Operations • Relations that are bags • 6.4.1 Eliminating Duplicates • DISTINCT • Example • (See Fig. 6.9) • SELECT DISTINCT name Database Systems

38. 6.4 Full-Relation Operations Database Systems

39. 6.4 Full-Relation Operations • 6.4.2 Duplicates in Unions, Intersections, and Differences • Example • (SELECT title, year FROM Movies) • UNION ALL • (SELECT MovieTitle AS title, movieYear AS year • FROM StarsIn) • R INTERSECT ALL S • R EXCEPT ALL S • 6.4.3 Grouping and Aggregation in SQL • SQL provides all the capability of the γ operator through the use of aggregation operators in SELECT clauses and a special GROUP BY clause. Database Systems

40. 6.4 Full-Relation Operations • 6.4.4 Aggregation Operators • SUM, AVG, MIN, MAX, COUNT • Example • SELECT AVG(netWorth) • FROM MovieExec; • SELECT COUNT(*) • FROM StarsIn; • SELECT COUNT(DISTINCT starName) • FROM StarsIn; Database Systems

41. 6.4 Full-Relation Operations • 6.4.5 Grouping • GROUP BY • Example • SELECT studioName, SUM(length) • FROM Movies • GROUP BY studioName; • It is also possible to use a GROUP BY clause in a query about several relations. • Example • (See Fig. 6.13) Database Systems

42. 6.4 Full-Relation Operations Database Systems

43. 6.4 Full-Relation Operations • 6.4.6 Grouping, Aggregation, and Nulls • A few rules about Nulls: • 1) The value NULL is ignored in any aggregation. • 2) NULL is treated as an ordinary value when forming groups. • 3) When we perform any aggregation except count over an • empty bag of values, the result is NULL. The count of an • empty bag is 0. • Example • A B SELECT A, COUNT(B) SELECT A, SUM(B) • NULL NULL FROM R FROM R • GROUP BY A; GROUP BY A; • (NULL, 0) (NULL, NULL) Database Systems

44. 6.4 Full-Relation Operations • 6.4.7 HAVING Clauses • We could restrict the tuples prior to grouping in a way that would make undesired groups empty. • e.g., WHERE producerC#=cert# AND netWorth>=10000000 • However, sometimes we want to choose our groups based on some aggregate property of the group itself. • Example • (See Fig. 6.14) • Several rules about HAVING clauses: • 1) An aggregation in a HAVING clause applied only to the • tuples of the group being tested. Database Systems

45. 6.4 Full-Relation Operations Database Systems

46. 6.4 Full-Relation Operations 2) Any attribute of relations in the FROM clause may be aggregated in the HAVING clause, but only those attributes that are in the GROUP BY list may appear unaggregated in the HAVING clause. Database Systems

47. 6.5 Database Modifications • Insert tuples into a relation • Delete certain tuples from a relation • Update values of certain components of certain existing tuples • 6.5.1 Insertion • INSERT INTO R(A1, …, An) VALUES (v1, …, vn); • The tuple created has default values for all missing attributes. • Example • INSERT INTO StarsIn(movieTitle, movieYear, starName) • VALUES (‘The Maltese Falcon’, 1942, ‘Sydney Greenstreet’); Database Systems

48. 6.5 Database Modifications • INSERT INTO StarsIn • VALUES (‘The Maltese Falcon’, 1942, ‘Sydney Greenstreet’); • The order of the values must be the same as the standard • order of attributes for the relation. • Subquery • Example • (See Fig. 6.15) • 6.5.2 Deletion • DELETE FROM R WHERE <condition>; Database Systems

49. 6.5 Database Modifications Database Systems

50. 6.5 Database Modifications • Example • DELETE FROM StarsIn • WHERE movieTitle=’The Maltese Falcon’ AND • movieYear=1942 AND • StarName=’Sydney Greenstreet’; • DELETE FROM MovieExec • WHERE netWorth<10000000; • 6.5.3 Updates • UPDATE R SET <new-value assignments> WHERE <condition>; Database Systems