Onsdag The concepts in a relation data model SQL DDL DML

Concepts

create table Employee (fname varchar(15) not null, minit char, lname varchar(15) not null, ssn char(9), bdate datetime, address varchar(30), sex char, salary decimal(10,2), superssn char(9), dno int not null, primary key (ssn), foreign key(superssn) references Employee(ssn), foreign key (dno) references Department(dnumber));

Create table create table Employee (……. dno int not null default 1, primary key (ssn), foreign key(superssn) references Employee(ssn) on update cascade, foreign key (dno) references Department(dnumber) on delete set default on update cascade); )

constraints On attributes not null check (dno > 0 and dno < 20) unique primary key (defines the PK, implicit not null)

Referential integrity constraints Foreign key foreign key(dno) references deparment(dnumber) Cascade foreign key(superssn) references Employee(ssn) on update cascade

Alter table alter table employee add varchar(40); alter table employee add constraint fkmr foreign key (dept) references department(dnumber);

SQL - DML Modifying the data insert into …, update... set... where..., delete from … where... Query on the data select... from... where...

insert insert into employee values ('James','E','Borg',' ',' ','450 Stone, Houston,TX','M',55000,null,1);

Update update employee set salary = where ssn = ’ ’ update employee set salary = salary * 1.2 where salary >= 30000

Deleting data Delete, deletes tuples from the database if the fulfill the where condition Deleting a tuple with referentiel integritet constraints defined with cascade, can delete tuples in other tables If there are no where condition, all tuples from at table are deleted delete from department

delete delete from employee where ssn = ” ” //delete departments which has no employees delete from department where dnumber not in (select dno from employee )

Select-From-Where Statements SELECT FROM WHERE

Single-Relation Query Starts with the FROM clause. Then the WHERE clause. At last the attributes from the SELECT clause. Algerbra operations?

* in SELECT clauses SELECT * FROM Employee WHERE ssn = ‘ ’;

Renaming Attributes SELECT fname AS firstName FROM employee WHERE ssn = ‘ ’

NULL Values Tuples in SQL relations can have NULL as a value for one or more components. Meaning depends on context. Two common cases: Missing value : e.g., we know Joe’s Bar has some address, but we don’t know what it is. Inapplicable : e.g., the value of attribute spouse for an unmarried person.

Comparing NULL’s to Values The logic of conditions in SQL is really 3-valued logic: TRUE, FALSE, UNKNOWN. When any value is compared with NULL, the truth value is UNKNOWN. But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN).

Use is or is not select Fname, lname from employee where superssn is null Instead of = or != Since sql considers each null value as being distinct from every other null value

Multirelation Queries Interesting queries often combine data from more than one relation. We can address several relations in one query by listing them all in the FROM clause. Distinguish attributes of the same name by “. ”

Example select fname from employee, dependent where fname = dependent_name and ssn = essn;

Formal Semantics Almost the same as for single-relation queries: 1.Start with the product of all the relations in the FROM clause. 2.Apply the selection condition from the WHERE clause. 3.Project onto the list of attributes and expressions in the SELECT clause. Algebra operations

Explicit Tuple-Variables Sometimes, a query needs to use two copies of the same relation. Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause. It’s always an option to rename relations this way, even when not essential.

Example Foreach employee select the name from the employee and his supervisor select e.fname,e.lname, s.fname, s.lname from employee e, employee s where e.superssn = s.ssn;

Subqueries A parenthesized SELECT-FROM-WHERE statement (subquery) can be used as a value in a number of places, including FROM and WHERE clauses. Example: in place of a relation in the Where clause, we can place another query, and then query its result.

Subqueries That Return One Tuple If a subquery is guaranteed to produce one tuple, then the subquery can be used as a value. Usually, the tuple has one component. Also typically, a single tuple is guaranteed by keyness of attributes. A run-time error occurs if there is no tuple or more than one tuple.

Example From Sells(bar, beer, price), find the bars that serve Miller for the same price Joe charges for Bud. Two queries would surely work: 1.Find the price Joe charges for Bud. 2.Find the bars that serve Miller at that price.

Query + Subquery Solution SELECT bar FROM Sells WHERE beer = ‘Miller’ AND price = (SELECT price FROM Sells WHERE bar = ‘Joe’’s Bar’ AND beer = ‘Bud’); The price at which Joe sells Bud

The IN Operator IN is true if and only if the tuple is a member of the relation. NOT IN means the opposite. IN-expressions can appear in WHERE clauses. The is often a subquery.

Example From Beers(name, manf) and Likes(drinker, beer), find the name and manufacturer of each beer that Fred likes. SELECT * FROM Beers WHERE name IN (SELECT beer FROM Likes WHERE drinker = ‘Fred’); The set of beers Fred likes

The Exists Operator EXISTS( ) is true if and only if the is not empty. Being a boolean-valued operator, EXISTS can appear in WHERE clauses.

Example Query with EXISTS select e.fname, e.lname from employee e where exists (select * from dependent where e.ssn = essn and e.sex = sex and e.fname = dependent_name)

Important Points Two single quotes inside a string represent the single-quote (apostrophe). Conditions in the WHERE clause can use AND, OR, NOT SQL is case-insensitive. In general, upper and lower case characters are the same, except inside quoted strings.

Patterns WHERE clauses can have conditions in which a string is compared with a pattern, to see if it matches. General form: LIKE or NOT LIKE Pattern is a quoted string with % = ‘any string’; _ = “any character.”

Exercises 8.13 (Company databasen) 8.14 (Company databasen FlereSQLØvelser (word doc. ) 8.10 and 8.11 create an database and queries on the database