Lecture 11: Basic SQL, Integrity constraints

Lecture 11:Basic SQL,Integrity constraints www.cl.cam.ac.uk/Teaching/current/Databases/

SQL – A language for relational databases • Originates from System R project at IBM • So widespread it has been standardised • SQL-86 (ANSI) • SQL-89 (minor revisions) • SQL-92 (major revision ANSI/ISO: current standard) • SQL:1999 (major extension - objects) • SQL:2003 (more features and XML support)

SQL components • SQL/92 is a huge standard (600+ pages) • SQL is a comprehensive database language, containing • Data definition language (DDL): to define schemas • Data manipulation language (DML): to insert, delete, modify and retrieve data • Lots of other features: e.g. views, schema evolution, access control, triggers, assertions, …

Relation instances Reserves Boats Sailors

Simple queries • Contents of entire relation SELECT * FROM Sailors; • QualificationsSELECT * FROM Sailors WHERE age>22;

Simple queries cont. • Projection of attributes SELECT rating,age FROM Sailors; • Note that SQL does not remove duplicates. We need to request this explicitly: SELECT DISTINCT rating,age FROM Sailors;

Range variables Simple queries cont. • Can query across multiple relations:SELECT S.sname FROM Sailors S, Reserves R WHERE S.sid=R.sid AND R.bid=103; • SQL allows the range variables to be dropped, e.g. SELECT sname FROM Sailors,Reserves WHERE Sailors.sid=Reserves.sid AND Reserves.bid=103;

Basic SQL query syntax SELECT [DISTINCT] target-list FROMrelation-listWHERE qualification • relation-list is a list of relation names (possibly with range variables) • target-list is a list of attributes of relations in relation-list • qualification is a Boolean expression

Semantics of SQL queries • The semantics of SQL query evaluation can be conceptually defined as: • Compute the product of relation-list • Discard those tuples that fail the qualification • Delete attributes that are not in target-list • If DISTINCT is specified, eliminate duplicate rows • NOTE: This is probably the least efficient way to evaluate a query!

Compiling to relational algebra • The querySELECT a1, …, anFROM R1, …, RmWHERE b • Can be compiled to the RA expression:a1,…,an(b(R1…Rm)) (Assuming no duplicates)

Example Find sailors who’ve reserved at least one boat SELECT S.sid FROM Sailors S, Reserves R WHERE S.sid=R.sid • Would adding DISTINCT to this query make any difference?

String matching • SQL provides powerful string matching facilities • These can be used in the WHERE clause • _ denotes any one character • % denotes zero or more characters • For example: SELECT * FROM Sailors WHERE sname LIKE ‘_ic%’;

Arithmetic • SQL provides arithmetic operators, which can be used in the SELECT clause • Use AS to name columns SELECT sname, age*5 AS OldAge FROM Sailors; • Can also be used in the WHERE clause SELECT S1.sname FROM Sailors S1, Sailors S2 WHERE S1.rating=2*S2.rating –4;

Nested Query! Set theoretic operations • SQL provides the familiar set theoretic operators: UNION, INTERSECT, IN (set membership) • SQL also allows a WHERE clause to contain another query, e.g. SELECT S.sname FROM Sailors S WHERE S.sid IN (SELECT R.sid FROM Reserves R WHERE R.bid=103);

Example Find names of sailors who’ve reserved a red or a green boat SELECT S.sname FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid=B.bid AND (B.colour=“red” OR B.colour=“green”);

Example Find names of sailors who’ve reserved a red and a green boat SELECT S.sname FROM Sailors S, Reserves R1, Reserves R2, Boats B1, Boats B2 WHERE S.sid=R1.sid AND R1.bid=B1.bid AND S.sid=R2.sid AND R2.bid=B2.bid AND B1.colour=“red” AND B2.colour=“green”;

Integrity constraints • An integrity constraint (IC) is a condition that must be true for any instance of the database • E.g. domain constraints • ICs are specified when schema is defined • ICs are checked when relations are modified • A legal instance of a relation is one that satisfies all the specified ICs • We’d like the DBMS to check for ICs

Primary key constraints • A set of fields is a (candidate) key for a relation if • No two distinct tuples can have same values in all the key fields; and • This is not true for any subset of the key • If part 2 is false, then it’s a superkey • If there’s more than one key for a relation, then one is chosen by the DBA to be the primary key

Candidate key Primary and candidate keys in SQL • Can define possibly many candidate keys, one of which is chosen as the primary key CREATE TABLE Enrolled ( sid CHAR(20), name CHAR(20), age INTEGER, grade CHAR(2),PRIMARY KEY (sid),UNIQUE (name,age));

Foreign key • A foreign key is a set of fields in one relation that is used to refer to a tuple in another relation (must correspond to a primary key of the second relation) • A bit like a pointer • For example, sid in Reserves is a foreign key referring to Sailors

Foreign keys in SQL • Only sailors listed in the Sailors relation should be allowed to make reservations CREATE TABLE Reserves (sid INTEGER, bid INTEGER, date DATE, PRIMARY KEY (sid,bid,date),FOREIGN KEY (sid)REFERENCES Sailors);

Referential integrity • If all foreign key constraints are enforced then referential integrity is achieved. • Similar to `no dangling pointers’ • Can you name a data model without referential integrity?

Semantic integrity constraints • ICs come from the semantics of the real-world enterprise that is being modelled • Another class of ICs is semantic integrity constraints. These apply to the actual data values that can be stored, e.g. “Sailor rating is between 0 and 10”

Summary You should now understand: • Simple SQL queries • SELECT-FROM-WHERE • Compilation to relational algebra • Nested queries • Integrity constraints • Primary and candidate keys

Lecture 11: Basic SQL, Integrity constraints