D ATABASE S YSTEMS I A DMIN S TUFF

2 Mid-term exam Tuesday, Oct 2:30pm Room 3005 (usual room) Closed book No cheating, blah blah No class on Oct 21 The amount of time we spent on each topic in class is a good indication of importance & likelihood of exam question

3 Assignment 4 solution posted (with algebra) Assignment 6 posted For procedure question, sample on next page might help

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D ATABASE S YSTEMS I W EEK 6: R EVIEW

6 R EVIEW A transaction has the following properties: Atomicity: all-or-nothing property Consistency: must leave the DB in a consistent state if DB is consistent when the transaction begins Isolation: transaction is performed as if only one transaction at a time (serial processing) Durability: effects of completed transactions are permanent What is a transaction?

7 E NTITY -R ELATIONSHIP D IAGRAMS An Entity-Relationship diagram ( ER diagram ) is a graph with nodes representing entity sets, attributes and relationship sets. Entity sets denoted by rectangles. Attributes denoted by ovals. Relationship sets denoted by diamonds. Edges ( lines ) connect entity sets to their attributes and relationship sets to their entity sets. lot dname budget did since name Works_In DepartmentsEmployees ssn

8 R ELATIONAL D ATABASE Rows are called tuples (or records ), columns called attributes (or fields ). Attributes are referenced not by column number, but by name. Order of attributes does not matter Attribute types are called domains. Domains consist of atomic values such as integers or strings. No structured values such as lists or sets The order of tuples does not matter, a relation is a set of tuples. The order of tuples resulting from a relational query is undefined.

9 R ELATIONSHIP S ETS ER Diagram Relational CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments); SQL

10 F ORMAL Q UERY L ANGUAGES Two mathematical query languages form the basis for “real” languages (e.g. SQL), and for implementation: Relational Algebra (RA) More procedural, very useful for representing execution plans, relatively close to SQL. Composed of a collection of operators A step-by-step procedure for computing the answer Relational Calculus (RC) Lets users describe what they want, rather than how to compute it. (Non-procedural, declarative.) Describes the answer, not the steps. Understanding these formal query languages is important for understanding SQL and query processing.

11 P RELIMINARIES A query is applied to relation instances, and the result of a query is also a relation instance. Inputs and Outputs of Queries are relations Query evaluated on instances of input relations Different instance (DB?) as input = different answer Schemas of input relations for a query are fixed (but query will run regardless of instance!) The schema for the result of a given query is also fixed! Determined by definition of input relations and query language constructs.

12 R ELATIONAL A LGEBRA O PERATIONS Basic operations Selection ( ) Selects a subset of rows from relation. Projection ( ) Deletes unwanted columns from relation. Cartesian product ( ) Combine two relations. Set-difference ( ) Tuples in relation 1, but not in relation 2. Union ( ) Tuples in relation 1 or in relation 2.

13 Renames relations / attributes, without changing the relation instance. relation R is renamed to S, attributes are renamed A1,..., An Rename only some attributes using the positional notation to reference attributes No renaming of attributes, just the relation R ENAMING

14 P ROJECTION Similar in concept to VIEWs S2 Other fields are projected out

15 S ELECTION S2

16 U NION Concatenates S1 and S2 Result contains ALL tuples that occur in either S1 or S2 Schemas must be identical If they have the same number of fields Fields have same domains S1 S2 SELECT * FROM S1 UNION SELECT * FROM S2 SELECT * FROM S1 UNION SELECT * FROM S2

17 I NTERSECTION Result contains ALL tuples that occur in both S1 or S2 Schemas must be identical S1 S2 SELECT * FROM S1 INTERSECT SELECT * FROM S2 SELECT * FROM S1 INTERSECT SELECT * FROM S2

18 S ET -D IFFERENCE Result contains ALL tuples that occur in S1 but not in S2 Schemas must be identical S1 S2 SELECT * FROM S1 MINUS SELECT * FROM S2 SELECT * FROM S1 MINUS SELECT * FROM S2

19 C ARTESIAN P RODUCT Field names in conflict become unnamed R1 S1

20 J OIN Condition Join Each tuple of the one relation is paired with each tuple of the other relation if the two tuples satisfy the join condition. Condition c refers to attributes of both R and S.

21 Find names of sailors who’ve reserved boat #103. Solution 1: Solution 2: Solution 3: Which is most efficient? Why? E XAMPLE Q UERIES

22 Find names of sailors who’ve reserved a red boat. Information about boat color only available in Boats; so need an extra join: A more efficient solution: A query optimizer can find the second solution given the first one. E XAMPLE Q UERIES

23 How to find number of sailors? E XAMPLE Q UERIES

24 How to find oldest sailor? E XAMPLE Q UERIES