1. 1 Lecture 11: Basic SQL, Integrity constraints www.cl.cam.ac.uk/Teaching/current/Databases/

2. 2 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)

3. 3 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, …

4. 4 Relation instances sidbidday 22101101001 55103111201 sidsnameratingage 11Sue726 22Tim726 33Bob828 55Kim1028 Reserves Sailors bi d bnamecolou r 101ClipperRed 102HatterBlue 103Interlakegreen Boats

5. 5 Simple queries Contents of entire relation SELECT * FROM Sailors; Qualifications SELECT * FROM Sailors WHERE age>22;

6. 6 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;

7. 7 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; Range variables

8. 8 Basic SQL query syntax SELECT [DISTINCT] target-list FROM relation-list WHERE 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

9. 9 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!

10. 10 Compiling to relational algebra The query SELECT a 1, …, a n FROM R 1, …, R m WHERE b Can be compiled to the RA expression:  a 1,…,a n (  b (R 1  …  R m )) (Assuming no duplicates)

11. 11 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?

12. 12 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%’;

13. 13 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;

14. 14 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); Nested Query!

15. 15 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”);

16. 16 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”;

17. 17 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

18. 18 Primary key constraints A set of fields is a (candidate) key for a relation if 1.No two distinct tuples can have same values in all the key fields; and 2.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

19. 19 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)); Candidate key

20. 20 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

21. 21 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);

22. 22 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?

23. 23 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”

24. 24 Summary You should now understand: Simple SQL queries –SELECT-FROM-WHERE Compilation to relational algebra Nested queries Integrity constraints Primary and candidate keys