SQL: An Implementation of the Relational Algebra

Transcription

1 : An Implementation of the Relational Algebra P.J. McBrien Imperial College London P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 1 / 40

2 SQL Relation Model and Algebra proposed by C.J.Codd in 1970 IBM deleveped a prototype relational database called System R with a query language Structured English Query Language (SEQUEL) SEQUEL later renamed SQL Various commericial versions of SQL launched in late 1970 s/early 1980s DB2 Oracle P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 2 / 40

3 SQL Relation Model and Algebra proposed by C.J.Codd in 1970 IBM deleveped a prototype relational database called System R with a query language Structured English Query Language (SEQUEL) SEQUEL later renamed SQL Various commericial versions of SQL launched in late 1970 s/early 1980s DB2 Oracle SQL Language Components Data Definition Language (DDL): a relational schema with data Data Manipulation Language (DML): a relational query and update language P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 2 / 40

4 RM to SQL DDL SQL DML: Definition of Tables CREATE TABLE branch ( sortcode INTEGER NOT NULL, bname VARCHAR(20) NOT NULL, cash DECIMAL(10,2) NOT NULL ) branch sortcode bname cash CREATE TABLE account ( INTEGER NOT NULL, type VARCHAR(8) NOT NULL, cname VARCHAR(20) NOT NULL, rate DECIMAL(4,2) NULL, sortcode INTEGER NOT NULL ) account type cname rate sortcode P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 3 / 40

5 RM to SQL DDL SQL DML: SQL Data Types SQL Data Types Keyword Semantics BOOLEAN A logical value (TRUE, FALSE, or UNKNOWN) BIT 1 bit integer (0, 1, or NULL) INTEGER 32 bit integer REAL 32 bit floating point number FLOAT(n) An n bit mantissa floating point number DECIMAL(p,s) A p digit number with s digits after the decimal point CHAR(n) A fixed length string of upto n characters VARCHAR(n) A varying length string of upto n characters DATE A calendar date (day, month and year) TIME A time of day (seconds, minutes, hours) TIMESTAMP time and day together ARRAY A fixed length list of a certain datatype MULTISET A variable sized bag of a certain datatype XML XML text P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 4 / 40

6 RM to SQL DDL SQL DML: Definition of Keys branch CREATE TABLE branch sortcode bname cash ( sortcode INTEGER NOT NULL, bname VARCHAR(20) NOT NULL, cash DECIMAL(10,2) NOT NULL, CONSTRAINT branch pk PRIMARY KEY (sortcode ) ) CREATE TABLE account ( INTEGER NOT NULL, type VARCHAR(8) NOT NULL, cname VARCHAR(20) NOT NULL, rate DECIMAL(4,2) NULL, sortcode INTEGER NOT NULL, CONSTRAINT account pk PRIMARY KEY (), ) CONSTRAINT account fk FOREIGN KEY ( sortcode ) REFERENCES branch account type cname rate sortcode account(sortcode) fk branch(sortcode) Primary Key Choose the key most often used to access a table as the primary key P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 5 / 40

7 RM to SQL DDL SQL DML: Definition of Keys branch CREATE TABLE branch sortcode bname cash ( sortcode INTEGER NOT NULL, bname VARCHAR(20) NOT NULL, cash DECIMAL(10,2) NOT NULL, CONSTRAINT branch pk PRIMARY KEY (sortcode ) ) CREATE TABLE account ( INTEGER NOT NULL, type VARCHAR(8) NOT NULL, cname VARCHAR(20) NOT NULL, rate DECIMAL(4,2) NULL, sortcode INTEGER NOT NULL, CONSTRAINT account pk PRIMARY KEY (), ) CONSTRAINT account fk FOREIGN KEY ( sortcode ) REFERENCES branch account type cname rate sortcode account(sortcode) fk branch(sortcode) Declaring Primary Keys after table creation ALTER TABLE branch ADD CONSTRAINT branch pk PRIMARY KEY ( sortcod P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 5 / 40

8 RM to SQL DDL SQL DML: Definition of Keys branch CREATE TABLE branch sortcode bname cash ( sortcode INTEGER NOT NULL, bname VARCHAR(20) NOT NULL, cash DECIMAL(10,2) NOT NULL, CONSTRAINT branch pk PRIMARY KEY (sortcode ) ) CREATE TABLE account ( INTEGER NOT NULL, type VARCHAR(8) NOT NULL, cname VARCHAR(20) NOT NULL, rate DECIMAL(4,2) NULL, sortcode INTEGER NOT NULL, CONSTRAINT account pk PRIMARY KEY (), ) CONSTRAINT account fk FOREIGN KEY ( sortcode ) REFERENCES branch account type cname rate sortcode account(sortcode) fk branch(sortcode) Declaring additional keys for a table CREATE UNIQUE INDEX branch bname key ON branch(bname) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 5 / 40

9 RM to SQL DDL SQL DML: Inserting, Updating and Deleting Data INSERT INTO account VALUES (100, current, McBrien, P.,NULL,67), (101, deposit, McBrien, P.,5.25,67), (103, current, Boyd, M.,NULL,34), (107, current, Poulovassilis, A.,NULL,56), (119, deposit, Poulovassilis, A.,5.50,56), (125, current, Bailey, J.,NULL,56) account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 6 / 40

10 RM to SQL DDL SQL DML: Inserting, Updating and Deleting Data INSERT INTO account VALUES (100, current, McBrien, P.,NULL,67), (101, deposit, McBrien, P.,5.25,67), (103, current, Boyd, M.,NULL,34), (107, current, Poulovassilis, A.,NULL,56), (119, deposit, Poulovassilis, A.,5.50,56), (125, current, Bailey, J.,NULL,56) UPDATE account SET type= deposit WHERE =100 account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 account type cname rate sortcode 100 deposit McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 DELETE WHERE =100 account type cname rate sortcode 101 deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 6 / 40

11 RA to SQL DML SQL DML: An Implementation of the RA SQL SELECT statements: Rough Equivalence to RA SELECT A 1,...,A n FROM R 1,...,R m WHERE P 1 AND... AND P k π A1,...,A n σ P1... P k R 1... R m SQL SELECT implements RA π,σ and π bname, σ branch.sortcode=account.sortcode account.type= current (branch account) SELECT branch.bname, account., branch WHERE account. sortcode=branch. sortcode AND account. type= current P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 7 / 40

12 RA to SQL DML Naming columns in SQL Column naming rules in SQL You must never have an ambiguous column name in an SQL statement You can use SELECT * to indicate all columns (i.e. have projection) You can use tablename.* to imply all columns from a table SELECT branch.bname, account. sortc ode, branch WHERE account. sortc ode= branch. sortcode AND account. type= current SELECT bname, sortc ode, branch WHERE account. sortc ode= branch. sortcode AND type= current SELECT bname, account. sortc ode, branch WHERE account. sortc ode= branch. sortcode AND type= current SELECT branch.,, branch WHERE account. sortc ode= branch. sortcode AND type= current sortcode bname cash 67 Strand Goodge St Wimbledon Wimbledon P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 8 / 40

13 RA to SQL DML Quiz 1: Translating RA into SQL Which SQL query implements π bname, σ type= deposit (account branch)? A SELECT, branch WHERE type= deposit B SELECT bname,, branch WHERE type= deposit C SELECT bname, FROM branch, account WHERE branch. sortcode= account. sortcode AND type= deposit D SELECT bname,, branch WHERE branch. sortcode= account. AND type= deposit P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 9 / 40

14 RA to SQL DML Connectives Between SQL SELECT statements Binary operators between SELECT statements SQL UNION implements RA SQL EXCEPT implements RA SQL INTERSECT implements RA Note that two tables must be union compatible: have the same number and type of columns π account π movement SELECT EXCEPT SELECT FROM movement P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 10 / 40

15 Joins SQL Joins Classic SQL Join Syntax SELECT branch.,, type, cname, rate FROM branch, account WHERE branch. sortcode=account. sortcode Modern SQL Join Syntax SELECT branch.,, type, cname, rate FROM branch JOIN account ON branch. sortcode=account. sortcode Special Syntax for Natural Join SELECT FROM branch NATURAL JOIN account Ather Special Syntax for Natural Join SELECT branch.,, type, cname, rate FROM branch JOIN account USING (sortcode ) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 11 / 40

16 Joins Overview of RA and SQL correspondances RA and SQL RA Operator SQL Operator π SELECT σ WHERE R 1 R 2 FROM R 1,R 2 or FROM R 1 CROSS JOIN R 2 R 1 R 2 FROM R 1 NATURAL JOIN R 2 θ R 1 R2 FROM R 1 JOIN R 2 ON θ R 1 R 2 R 1 EXCEPT R 2 R 1 R 2 R 1 UNION R 2 R 1 R 2 R 1 INTERSECT R 2 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 12 / 40

17 Joins Try some examples yourself... medusa-s2(pjm)-4$ psql -h db -U lab -d lab bank branch -W Password: lab bank branch=> SELECT * lab bank branch-> FROM branch NATURAL JOIN account; sortcode bname cash type cname rate Strand current McBrien, P. 67 Strand deposit McBrien, P Goodge St current Boyd, M. 56 Wimbledon current Poulovassilis, A. 56 Wimbledon deposit Poulovassilis, A Wimbledon current Bailey, J. P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 13 / 40

18 Joins...and find out that t all DBMSs are the same medusa-s2(pjm)-4$ sqsh -S sqlserver -X -U lab -D lab bank branch Password: [21] sqlserver.lab bank branch.1> SELECT * [21] sqlserver.lab bank branch.2> FROM branch NATURAL JOIN account [21] sqlserver.lab bank branch.3> \go Msg 102, Level 15, State 1 Server DOWITCHER, Line 2 Line 2: Incorrect syntax near account. P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 14 / 40

19 Bags v Sets SQL: Bags and Sets SELECT sortcode π sortcode account sortcode SELECT DISTINCT sortcode π sortcode account sortcode SQL SELECT: Bag semantics By default, an SQL SELECT (equivalent to an RA π) does t eliminate duplicates, and returns a bag (or multiset) rather than a set. Any SELECT that does t cover a key of the input relation, and requires a set based answer, should use DISTINCT. P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 15 / 40

20 Bags v Sets SQL: Bags and Sets SELECT ALL sortcode π sortcode account sortcode SELECT DISTINCT sortcode π sortcode account sortcode SQL SELECT: Bag semantics By default, an SQL SELECT (equivalent to an RA π) does t eliminate duplicates, and returns a bag (or multiset) rather than a set. Any SELECT that does t cover a key of the input relation, and requires a set based answer, should use DISTINCT. P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 15 / 40

21 Bags v Sets Quiz 2: Correct use of SELECT DISTINCT (1) branch(sortcode,bname,cash) key branch(sortcode) key branch(bname) Which SQL query requires the use of DISTINCT in order to avoid the possibility of a bag being produced? A SELECT FROM branch WHERE cash >10000 B SELECT sortcode FROM branch WHERE cash >10000 C SELECT bname, cash FROM branch D SELECT cash FROM branch WHERE cash >10000 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 16 / 40

22 Bags v Sets Quiz 3: Correct use of SELECT DISTINCT (2) branch(sortcode,bname,cash) account(,type,cname,rate,sortcode) key branch(sortcode) key branch(bname) key account() Which SQL query requires the use of DISTINCT in order to avoid the possibility of a bag being produced? A SELECT FROM branch NATURAL JOIN account B SELECT branch. sortcode, type, rate FROM branch NATURAL JOIN account C SELECT branch. sortcode, FROM branch NATURAL JOIN account D SELECT branch. sortcode,, cash FROM branch NATURAL JOIN account P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 17 / 40

23 Bags v Sets Quiz 4: Operators that might produce bags If R and S are sets, which RA operator could produce a bag result if the implementation did t check for duplicates? A σr B R S C R S D R S P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 18 / 40

24 Bags v Sets Bag and Set operations in SQL RA Operator Set Based SQL Bag Based SQL π A1,...,A n SELECT DISTINCT A 1,...,A n SELECT ALL A 1,...,A n R 1... R m FROM R 1,...,R m FROM R 1,...,R m σ P1,...,P k WHERE P 1 AND...AND P k WHERE P 1 AND...AND P k R 1 R 2 R 1 UNION DISTINCT R 2 R 1 UNION ALL R 2 R 1 R 2 R 1 EXCEPT DISTINCT R 2 R 1 EXCEPT ALL R 2 R 1 R 2 R 1 INTERSECT DISTINCT R 2 R 1 INTERSECT ALL R 2 Chosing between set and bag semantics If you omit DISTINCT or ALL, then the defaults are: SELECT ALL UNION DISTINCT EXCEPT DISTINCT INTERSECT DISTINCT No FROM DISTINCT or WHERE DISTINCT? There is need for DISTINCT or ALL around FROM ( ) and WHERE (σ) cant introduce any duplicates, and any existing duplicates can be removed in the SELECT P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 19 / 40

25 Bags v Sets Project-Select-Product Queries SQL SELECT statements: Exact Equivalence to RA SELECT DISTINCT A 1,...,A n FROM R 1,...,R m WHERE P 1 AND... AND P k SQL SELECT implements RA π,σ and Omit DISTINCT when either you kwn A 1,...,A n cover a key you want a bag (rather than set) answer π A1,...,A n σ P1... P k R 1... R m P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 20 / 40

26 Bags v Sets Quiz 5: SQL EXCEPT SELECT FROM movement EXCEPT SELECT movement mid amount tdate /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/1999 account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 What is the result of the above SQL query? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 21 / 40

27 Bags v Sets Quiz 6: SQL EXCEPT ALL SELECT FROM movement EXCEPT ALL SELECT movement mid amount tdate /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/1999 account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 What is the result of the above SQL query? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 22 / 40

28 Bags v Sets Table Aliases Table and Column Aliases The SQL operator AS allows a column or table name to be renamed. Essential when needing to join a table with itself List people with a current and a deposit account SELECT current account.cname, current account. AS current, deposit account. AS deposit AS current account JOIN account AS deposit account ON current account.cname=deposit account.cname AND current account. type= current AND deposit account. type= deposit cname current deposit McBrien, P Poulovassilis, A P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 23 / 40

29 Bags v Sets Worksheet: Translating Between Relational Algebra and SQL account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 movement mid amount tdate /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/1999 movement() fk account. P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 24 / 40

30 Set Operations Set Operations: IN IN operator tests for membership of a set SELECT WHERE type= current AND IN (100,101) Can use nested SELECT to generate set SELECT WHERE type= current AND IN (SELECT FROM movement WHERE amount >500) 100 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 25 / 40

31 Set Operations Set Operations: IN IN operator tests for membership of a set SELECT WHERE type= current AND IN (100,101) Can use nested SELECT to generate set SELECT WHERE type= current AND IN (SELECT FROM movement WHERE amount >500) SELECT DISTINCT account. FROM account JOIN movement ON account. =movement. WHERE type= current AND amount >500 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 25 / 40

32 Set Operations Quiz 7: SQL Set Membership Testing SELECT WHERE type= current AND NOT IN ( SELECT FROM movement WHERE amount >500) account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 movement mid amount tdate /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/1999 What is the result of the above SQL query? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 26 / 40

33 Set Operations Quiz 7: SQL Set Membership Testing SELECT SELECT DISTINCT account. WHERE type= current JOIN movement AND NOT IN ON account.=movement. ( SELECT WHERE type= current FROM movement AND NOT amount >500 WHERE amount >500) What is the result of the above SQL query? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 26 / 40

34 Set Operations Set Operations: EXISTS Testing for Existence IN can be used to test if some value is in a relation, either listed, or produced by some SELECT statement EXISTS can be used to test if a SELECT statement returns any rows List people without a deposit account SELECT cname WHERE cname NOT IN ( SELECT cname WHERE type= deposit ) cname Boyd, M. Bailey, J. SELECT cname WHERE NOT EXISTS ( SELECT deposit account WHERE type= deposit AND account.cname=cname) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 27 / 40

35 Set Operations Set Operations: EXISTS NOT EXISTS and EXCEPT Most queries involving EXCEPT can be also written using NOT EXISTS EXCEPT relatively recent addition to SQL π account π movement SELECT EXCEPT SELECT FROM movement SELECT WHERE NOT EXISTS ( SELECT FROM movement WHERE =account.) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 28 / 40

36 Set Operations Set Operations: SOME and ALL Can test a value against members of a set V op SOME S is TRUE is there is at least one V s S such that V op V s V op ALL S is TRUE is there are values V s S such that NOT V op V s names of branches that only have current accounts SELECT bname FROM branch WHERE current =ALL (SELECT type WHERE branch. sortcode=account. sortcode ) names of branches that have deposit accounts SELECT bname FROM branch WHERE deposit =SOME (SELECT type WHERE branch. sortcode=account. sortcode ) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 29 / 40

37 Set Operations Worksheet: Set Operations branch sortcode bname cash 56 Wimbledon Goodge St Strand movement mid amount tdate /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/1999 account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 key branch(sortcode) key branch(bname) key movement(mid) key account() movement() fk account() account(sortcode) fk branch(sortcode) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 30 / 40

38 Set Operations Worksheet: Set Operations (3) Write an SQL query without using any negation (i.e. without the use of NOT or EXCEPT) that list accounts with movements on or before the 11-Jan SELECT WHERE 11 jan 1999 <ALL (SELECT tdate FROM movement WHERE movement.=account.) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 31 / 40

39 Set Operations Worksheet: Set Operations (4) Write an SQL query that lists the cname of customers that have every type of account that appears in account SELECT DISTINCT cname AS cust account WHERE NOT EXISTS ( SELECT type EXCEPT SELECT type WHERE account.cname=cust account.cname ) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 32 / 40

40 Set Operations Set Operations: NOT SOME NOT and ALL In first order classical logic: accounts with all movements less than or equal to 500 SELECT WHERE 500>=ALL ( SELECT amount FROM movement WHERE account.=movement.) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 33 / 40

41 Set Operations Set Operations: NOT SOME NOT and ALL In first order classical logic: accounts with all movements less than or equal to 500 SELECT WHERE 500>=ALL ( SELECT amount FROM movement WHERE account.=movement.) SELECT WHERE NOT 500<SOME ( SELECT amount FROM movement WHERE account.=movement.) P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 33 / 40

42 Null Null Several definitions of null have been proposed, including: 1 null represents a something that is t present in the UoD P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 34 / 40

43 Null Null Several definitions of null have been proposed, including: 1 null represents a something that is t present in the UoD 2 null represents something that might be present in the UoD, but we do t kw its value at present P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 34 / 40

44 Null Null Several definitions of null have been proposed, including: 1 null represents a something that is t present in the UoD 2 null represents something that might be present in the UoD, but we do t kw its value at present 3 null represents something that is present in the UoD, but we do t kw its value at present P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 34 / 40

45 Null Null Several definitions of null have been proposed, including: 1 null represents a something that is t present in the UoD 2 null represents something that might be present in the UoD, but we do t kw its value at present 3 null represents something that is present in the UoD, but we do t kw its value at present SQL handling of NULL SQL uses a three valued logic to process WHERE predicate Truth values are TRUE, FALSE, and UNKNOWN SQL standard vague, but handling of NULL is nearest to option 2 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 34 / 40

46 Null Quiz 8: SQL handling of NULL (1) account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 SELECT WHERE rate=null What is the result of the SQL query above? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 35 / 40

47 Null Quiz 9: SQL handling of NULL (2) account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 SELECT WHERE rate=null OR rate<>null What is the result of the SQL query above? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 36 / 40

48 Null SQL implements three valued logic P 1 OR P 2 P 2 TRUE UNKNOWN FALSE TRUE TRUE TRUE TRUE P 1 UNKNOWN TRUE UNKNOWN UNKNOWN FALSE TRUE UNKNOWN FALSE NOT P 1 TRUE FALSE P 1 UNKNOWN UNKNOWN FALSE TRUE P 1 AND P 2 P 2 TRUE UNKNOWN FALSE TRUE TRUE UNKNOWN FALSE P 1 UNKNOWN UNKNOWN UNKNOWN FALSE FALSE FALSE FALSE FALSE x=null is UNKNOWN (and t TRUE or FALSE) null=null is UNKNOWN x IS NULL returns TRUE if x has a null value x IS NOT NULL returns TRUE if x does t have a null value P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 37 / 40

49 Null Correct SQL Queries Using null Every nullable attribute can be tested to see if it is null: SELECT WHERE rate IS NULL SELECT WHERE rate IS NULL OR rate IS NOT NULL Can test for logical state by IS TRUE, IS NOT TRUE,... SELECT WHERE (rate=5.50) IS NOT TRUE P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 38 / 40

50 Null Quiz 10: SQL Might Be SELECT WHERE (rate=5.25) IS NOT FALSE account type cname rate sortcode 100 current McBrien, P. NULL deposit McBrien, P current Boyd, M. NULL current Poulovassilis, A. NULL deposit Poulovassilis, A current Bailey, J. NULL 56 What is the result of the above SQL query? A B C D P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 39 / 40

51 Null Worksheet: Null values in SQL movement mid amount tdate null /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ null 20/1/ null null 20/1/ null /1/ null /1/1999 account type cname rate sortcode 100 current McBrien, P. null deposit McBrien, P deposit Poulovassilis, A current Bailey, J. null 56 P.J. McBrien (Imperial College London) SQL: An Implementation of the Relational Algebra 40 / 40