Chapter 7: Relational Database Design

Chapter 3: Introduction to SQL Database System Concepts, 7th Ed. Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Outline Overview of The SQL Query Language SQL Data Definition Basic Query Structure of SQL Queries Additional Basic Operations Set Operations Null Values Aggregate Functions Nested Subqueries Modification of the Database Database System Concepts - 7th Edition 3.2

Silberschatz, Korth and Sudarshan History IBM Sequel language developed as part of System R project at the IBM San Jose Research Laboratory Renamed Structured Query Language (SQL) ANSI and ISO standard SQL: SQL-86 SQL-89 SQL-92 SQL:1999 (language name became Y2K compliant!) SQL:2003 Commercial systems offer most, if not all, SQL-92 features, plus varying feature sets from later standards and special proprietary features. Not all examples here may work on your particular system. Database System Concepts - 7th Edition

3.3 Silberschatz, Korth and Sudarshan SQL Parts DML -- provides the ability to query information from the database and to insert tuples into, delete tuples from, and modify tuples in the database. integrity the DDL includes commands for specifying integrity constraints. View definition -- The DDL includes commands for defining views. Transaction control includes commands for specifying the beginning and ending of transactions. Embedded SQL and dynamic SQL -- define how SQL statements can be embedded within general-purpose programming languages. Authorization includes commands for specifying access rights to relations and views. Database System Concepts - 7th Edition 3.4

Silberschatz, Korth and Sudarshan Data Definition Language The SQL data-definition language (DDL) allows the specification of information about relations, including: The schema for each relation. The type of values associated with each attribute. The Integrity constraints The set of indices to be maintained for each relation. Security and authorization information for each relation. The physical storage structure of each relation on disk. Database System Concepts - 7th Edition 3.5 Silberschatz, Korth and Sudarshan Domain Types in SQL

char(n). Fixed length character string, with user-specified length n. varchar(n). Variable length character strings, with user-specified maximum length n. int. Integer (a finite subset of the integers that is machine-dependent). smallint. Small integer (a machine-dependent subset of the integer domain type). numeric(p,d). Fixed point number, with user-specified precision of p digits, with d digits to the right of decimal point. (ex., numeric(3,1), allows 44.5 to be stores exactly, but not 444.5 or 0.32) real, double precision. Floating point and double-precision floating point numbers, with machine-dependent precision. float(n). Floating point number, with user-specified precision of at least n digits. More are covered in Chapter 4. Database System Concepts - 7th Edition 3.6 Silberschatz, Korth and Sudarshan

Create Table Construct An SQL relation is defined using the create table command: create table r (A1 D1, A2 D2, ..., An Dn, (integrity-constraint1), ..., (integrity-constraintk)) r is the name of the relation each Ai is an attribute name in the schema of relation r Di is the data type of values in the domain of attribute Ai Example: create table instructor ( ID name dept_name salary Database System Concepts - 7th Edition

char(5), varchar(20), varchar(20), numeric(8,2)) 3.7 Silberschatz, Korth and Sudarshan Integrity Constraints in Create Table Types of integrity constraints primary key (A1, ..., An ) foreign key (Am, ..., An ) references r not null SQL prevents any update to the database that violates an integrity constraint. Example: create table instructor (

ID char(5), name varchar(20) not null, dept_name varchar(20), salary numeric(8,2), primary key (ID), foreign key (dept_name) references department); Database System Concepts - 7th Edition 3.8 Silberschatz, Korth and Sudarshan And a Few More Relation Definitions create table student ( ID varchar(5),

name varchar(20) not null, dept_name varchar(20), tot_cred numeric(3,0), primary key (ID), foreign key (dept_name) references department); create table takes ( ID varchar(5), course_id varchar(8), sec_id varchar(8), semester varchar(6), year numeric(4,0), grade

varchar(2), primary key (ID, course_id, sec_id, semester, year) , foreign key (ID) references student, foreign key (course_id, sec_id, semester, year) references section); Database System Concepts - 7th Edition 3.9 Silberschatz, Korth and Sudarshan And more still create table course ( course_id varchar(8), title varchar(50), dept_name varchar(20), credits

numeric(2,0), primary key (course_id), foreign key (dept_name) references department); Database System Concepts - 7th Edition 3.10 Silberschatz, Korth and Sudarshan Updates to tables Insert insert into instructor values ('10211', 'Smith', 'Biology', 66000); Delete Remove all tuples from the student relation delete from student Drop Table drop table r Alter alter table r add A D

where A is the name of the attribute to be added to relation r and D is the domain of A. All exiting tuples in the relation are assigned null as the value for the new attribute. alter table r drop A where A is the name of an attribute of relation r Dropping of attributes not supported by many databases. Database System Concepts - 7th Edition 3.11 Silberschatz, Korth and Sudarshan Basic Query Structure A typical SQL query has the form: select A1, A2, ..., An from r1, r2, ..., rm where P Ai represents an attribute

Ri represents a relation P is a predicate. The result of an SQL query is a relation. Database System Concepts - 7th Edition 3.12 Silberschatz, Korth and Sudarshan The select Clause The select clause lists the attributes desired in the result of a query corresponds to the projection operation of the relational algebra Example: find the names of all instructors: select name from instructor NOTE: SQL names are case insensitive (i.e., you may use upper- or lower-case letters.) E.g., Name NAME name

Some people use upper case wherever we use bold font. Database System Concepts - 7th Edition 3.13 Silberschatz, Korth and Sudarshan The select Clause (Cont.) SQL allows duplicates in relations as well as in query results. To force the elimination of duplicates, insert the keyword distinct after select. Find the department names of all instructors, and remove duplicates select distinct dept_name from instructor The keyword all specifies that duplicates should not be removed. select all dept_name from instructor Database System Concepts - 7th Edition

3.14 Silberschatz, Korth and Sudarshan The select Clause (Cont.) An asterisk in the select clause denotes all attributes select * from instructor An attribute can be a literal with no from clause select '437' Results is a table with one column and a single row with value 437 Can give the column a name using: select '437' as FOO An attribute can be a literal with from clause select 'A' from instructor Result is a table with one column and N rows (number of tuples in the instructors table), each row with value A

Database System Concepts - 7th Edition 3.15 Silberschatz, Korth and Sudarshan The select Clause (Cont.) The select clause can contain arithmetic expressions involving the operation, +, , , and /, and operating on constants or attributes of tuples. The query: select ID, name, salary/12 from instructor would return a relation that is the same as the instructor relation, except that the value of the attribute salary is divided by 12. Can rename salary/12 using the as clause: select ID, name, salary/12 as monthly_salary

Database System Concepts - 7th Edition 3.16 Silberschatz, Korth and Sudarshan The where Clause The where clause specifies conditions that the result must satisfy Corresponds to the selection predicate of the relational algebra. To find all instructors in Comp. Sci. dept select name from instructor where dept_name = 'Comp. Sci.' SQL allows the use of the logical connectives and, or, and not The operands of the logical connectives can be expressions involving the comparison operators <, <=, >, >=, =, and <>. Comparisons can be applied to results of arithmetic expressions To find all instructors in Comp. Sci. dept with salary > 70000 select name from instructor

where dept_name = 'Comp. Sci.' and salary > 70000 Database System Concepts - 7th Edition 3.17 Silberschatz, Korth and Sudarshan The from Clause The from clause lists the relations involved in the query Corresponds to the Cartesian product operation of the relational algebra. Find the Cartesian product instructor X teaches select from instructor, teaches generates every possible instructor teaches pair, with all attributes from both relations. For common attributes (e.g., ID), the attributes in the resulting table are renamed using the relation name (e.g., instructor.ID)

Cartesian product not very useful directly, but useful combined with where-clause condition (selection operation in relational algebra). Database System Concepts - 7th Edition 3.18 Silberschatz, Korth and Sudarshan Examples Find the names of all instructors who have taught some course and the course_id select name, course_id from instructor , teaches where instructor.ID = teaches.ID Find the names of all instructors in the Art department who have taught some course and the course_id select name, course_id from instructor , teaches

where instructor.ID = teaches.ID and instructor. dept_name = 'Art' Database System Concepts - 7th Edition 3.19 Silberschatz, Korth and Sudarshan The Rename Operation The SQL allows renaming relations and attributes using the as clause: old-name as new-name Find the names of all instructors who have a higher salary than some instructor in 'Comp. Sci'. select distinct T.name from instructor as T, instructor as S where T.salary > S.salary and S.dept_name = 'Comp. Sci. Keyword as is optional and may be omitted instructor as T instructor T

Database System Concepts - 7th Edition 3.20 Silberschatz, Korth and Sudarshan Self Join Example Relation emp-super Find the supervisor of Bob Find the supervisor of the supervisor of Bob Can you find ALL the supervisors (direct and indirect) of Bob? Database System Concepts - 7th Edition 3.21 Silberschatz, Korth and Sudarshan String Operations

SQL includes a string-matching operator for comparisons on character strings. The operator like uses patterns that are described using two special characters: percent ( % ). The % character matches any substring. underscore ( _ ). The _ character matches any character. Find the names of all instructors whose name includes the substring dar. select name from instructor where name like '%dar%' Match the string 100% like '100 \%' escape '\' in that above we use backslash (\) as the escape character. Database System Concepts - 7th Edition 3.22 Silberschatz, Korth and Sudarshan

String Operations (Cont.) Patterns are case sensitive. Pattern matching examples: 'Intro%' matches any string beginning with Intro. '%Comp%' matches any string containing Comp as a substring. '_ _ _' matches any string of exactly three characters. '_ _ _ %' matches any string of at least three characters. SQL supports a variety of string operations such as concatenation (using ||) converting from upper to lower case (and vice versa) finding string length, extracting substrings, etc. Database System Concepts - 7th Edition 3.23 Silberschatz, Korth and Sudarshan Ordering the Display of Tuples

List in alphabetic order the names of all instructors select distinct name from instructor order by name We may specify desc for descending order or asc for ascending order, for each attribute; ascending order is the default. Example: order by name desc Can sort on multiple attributes Example: order by dept_name, name Database System Concepts - 7th Edition 3.24 Silberschatz, Korth and Sudarshan Where Clause Predicates SQL includes a between comparison operator Example: Find the names of all instructors with salary between $90,000 and $100,000 (that is, $90,000 and $100,000)

select name from instructor where salary between 90000 and 100000 Tuple comparison select name, course_id from instructor, teaches where (instructor.ID, dept_name) = (teaches.ID, 'Biology'); Database System Concepts - 7th Edition 3.25 Silberschatz, Korth and Sudarshan Set Operations Find courses that ran in Fall 2017 or in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) union (select course_id from section where sem = 'Spring' and year = 2018)

Find courses that ran in Fall 2017 and in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) intersect (select course_id from section where sem = 'Spring' and year = 2018) Find courses that ran in Fall 2017 but not in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) except (select course_id from section where sem = 'Spring' and year = 2018) Database System Concepts - 7th Edition 3.26 Silberschatz, Korth and Sudarshan Set Operations (Cont.) Set operations union, intersect, and except Each of the above operations automatically eliminates duplicates To retain all duplicates use the

union all, intersect all except all. Database System Concepts - 7th Edition 3.27 Silberschatz, Korth and Sudarshan Null Values It is possible for tuples to have a null value, denoted by null, for some of their attributes null signifies an unknown value or that a value does not exist. The result of any arithmetic expression involving null is null Example: 5 + null returns null The predicate is null can be used to check for null values. Example: Find all instructors whose salary is null. select name from instructor

where salary is null The predicate is not null succeeds if the value on which it is applied is not null. Database System Concepts - 7th Edition 3.28 Silberschatz, Korth and Sudarshan Null Values (Cont.) SQL treats as unknown the result of any comparison involving a null value (other than predicates is null and is not null). Example: 5 < null or null <> null or null = null The predicate in a where clause can involve Boolean operations (and, or, not); thus the definitions of the Boolean operations need to be extended to deal with the value unknown. and : (true and unknown) = unknown, (false and unknown) = false, (unknown and unknown) = unknown

or: (unknown or true) = true, (unknown or false) = unknown (unknown or unknown) = unknown Result of where clause predicate is treated as false if it evaluates to unknown Database System Concepts - 7th Edition 3.29 Silberschatz, Korth and Sudarshan Aggregate Functions These functions operate on the multiset of values of a column of a relation, and return a value avg: average value min: minimum value max: maximum value sum: sum of values count: number of values

Database System Concepts - 7th Edition 3.30 Silberschatz, Korth and Sudarshan Aggregate Functions Examples Find the average salary of instructors in the Computer Science department select avg (salary) from instructor where dept_name= 'Comp. Sci.'; Find the total number of instructors who teach a course in the Spring 2018 semester select count (distinct ID) from teaches where semester = 'Spring' and year = 2018; Find the number of tuples in the course relation select count (*) from course;

Database System Concepts - 7th Edition 3.31 Silberschatz, Korth and Sudarshan Aggregate Functions Group By Find the average salary of instructors in each department select dept_name, avg (salary) as avg_salary from instructor group by dept_name; Database System Concepts - 7th Edition 3.32 Silberschatz, Korth and Sudarshan Aggregation (Cont.)

Attributes in select clause outside of aggregate functions must appear in group by list /* erroneous query */ select dept_name, ID, avg (salary) from instructor group by dept_name; Database System Concepts - 7th Edition 3.33 Silberschatz, Korth and Sudarshan Aggregate Functions Having Clause Find the names and average salaries of all departments whose average salary is greater than 42000 select dept_name, avg (salary) as avg_salary from instructor group by dept_name having avg (salary) > 42000;

Note: predicates in the having clause are applied after the formation of groups whereas predicates in the where clause are applied before forming groups Database System Concepts - 7th Edition 3.34 Silberschatz, Korth and Sudarshan Nested Subqueries SQL provides a mechanism for the nesting of subqueries. A subquery is a select-from-where expression that is nested within another query. The nesting can be done in the following SQL query select A1, A2, ..., An from r1, r2, ..., rm where P as follows: From clause: ri can be replaced by any valid subquery Where clause: P can be replaced with an expression of the form:

B (subquery) B is an attribute and to be defined later. Select clause: Ai can be replaced be a subquery that generates a single value. Database System Concepts - 7th Edition 3.35 Silberschatz, Korth and Sudarshan Set Membership Database System Concepts - 7th Edition 3.36 Silberschatz, Korth and Sudarshan

Set Membership Find courses offered in Fall 2017 and in Spring 2018 select distinct course_id from section where semester = 'Fall' and year= 2017 and course_id in (select course_id from section where semester = 'Spring' and year= 2018); Find courses offered in Fall 2017 but not in Spring 2018 select distinct course_id from section where semester = 'Fall' and year= 2017 and course_id not in (select course_id from section where semester = 'Spring' and year= 2018); Database System Concepts - 7th Edition

3.37 Silberschatz, Korth and Sudarshan Set Membership (Cont.) Name all instructors whose name is neither Mozart nor Einstein select distinct name from instructor where name not in ('Mozart', 'Einstein') Find the total number of (distinct) students who have taken course sections taught by the instructor with ID 10101 select count (distinct ID) from takes where (course_id, sec_id, semester, year) in (select course_id, sec_id, semester, year from teaches where teaches.ID= 10101); Note: Above query can be written in a much simpler manner. The formulation above is simply to illustrate SQL features

Database System Concepts - 7th Edition 3.38 Silberschatz, Korth and Sudarshan Set Comparison Database System Concepts - 7th Edition 3.39 Silberschatz, Korth and Sudarshan Set Comparison some Clause Find names of instructors with salary greater than that of some (at least one) instructor in the Biology department. select distinct T.name from instructor as T, instructor as S where T.salary > S.salary and S.dept name = 'Biology';

Same query using > some clause select name from instructor where salary > some (select salary from instructor where dept name = 'Biology'); Database System Concepts - 7th Edition 3.40 Silberschatz, Korth and Sudarshan Definition of some Clause F some r t r such that (F t ) Where can be: (5 < some

0 5 6 (5 < some 0 5 ) = false (5 = some 0 5 ) = true (5 some

0 5 ) = true (since 0 5) ) = true (read: 5 < some tuple in the relation) (= some) in However, ( some) not in Database System Concepts - 7th Edition 3.41 Silberschatz, Korth and Sudarshan Set Comparison all Clause Find the names of all instructors whose salary is greater than the salary of

all instructors in the Biology department. select name from instructor where salary > all (select salary from instructor where dept name = 'Biology'); Database System Concepts - 7th Edition 3.42 Silberschatz, Korth and Sudarshan Definition of all Clause F all r t r (F t) (5 < all 0 5

6 ) = false (5 < all 6 10 ) = true (5 = all 4 5 ) = false (5 all

4 6 ) = true (since 5 4 and 5 6) ( all) not in However, (= all) in Database System Concepts - 7th Edition 3.43 Silberschatz, Korth and Sudarshan Test for Empty Relations The exists construct returns the value true if the argument subquery is nonempty. exists r r not exists r r =

Database System Concepts - 7th Edition 3.44 Silberschatz, Korth and Sudarshan Use of exists Clause Yet another way of specifying the query Find all courses taught in both the Fall 2017 semester and in the Spring 2018 semester select course_id from section as S where semester = 'Fall' and year = 2017 and exists (select * from section as T where semester = 'Spring' and year= 2018 and S.course_id = T.course_id); Correlation name variable S in the outer query Correlated subquery the inner query Database System Concepts - 7th Edition

3.45 Silberschatz, Korth and Sudarshan Use of not exists Clause Find all students who have taken all courses offered in the Biology department. select distinct S.ID, S.name from student as S where not exists ( (select course_id from course where dept_name = 'Biology') except (select T.course_id from takes as T where S.ID = T.ID)); First nested query lists all courses offered in Biology Second nested query lists all courses a particular student took

Note that X Y = X Y Note: Cannot write this query using = all and its variants Database System Concepts - 7th Edition 3.46 Silberschatz, Korth and Sudarshan Test for Absence of Duplicate Tuples The unique construct tests whether a subquery has any duplicate tuples in its result. The unique construct evaluates to true if a given subquery contains no duplicates . Find all courses that were offered at most once in 2017 select T.course_id from course as T where unique ( select R.course_id from section as R

where T.course_id= R.course_id and R.year = 2017); Database System Concepts - 7th Edition 3.47 Silberschatz, Korth and Sudarshan Subqueries in the From Clause Database System Concepts - 7th Edition 3.48 Silberschatz, Korth and Sudarshan Subqueries in the Form Clause SQL allows a subquery expression to be used in the from clause Find the average instructors salaries of those departments where the

average salary is greater than $42,000. select dept_name, avg_salary from ( select dept_name, avg (salary) as avg_salary from instructor group by dept_name) where avg_salary > 42000; Note that we do not need to use the having clause Another way to write above query select dept_name, avg_salary from ( select dept_name, avg (salary) from instructor group by dept_name) as dept_avg (dept_name, avg_salary) where avg_salary > 42000; Database System Concepts - 7th Edition 3.49 Silberschatz, Korth and Sudarshan

With Clause The with clause provides a way of defining a temporary relation whose definition is available only to the query in which the with clause occurs. Find all departments with the maximum budget with max_budget (value) as (select max(budget) from department) select department.name from department, max_budget where department.budget = max_budget.value; Database System Concepts - 7th Edition 3.50 Silberschatz, Korth and Sudarshan Complex Queries using With Clause Find all departments where the total salary is greater than the average of

the total salary at all departments with dept _total (dept_name, value) as (select dept_name, sum(salary) from instructor group by dept_name), dept_total_avg(value) as (select avg(value) from dept_total) select dept_name from dept_total, dept_total_avg where dept_total.value > dept_total_avg.value; Database System Concepts - 7th Edition 3.51 Silberschatz, Korth and Sudarshan Scalar Subquery Scalar subquery is one which is used where a single value is expected

List all departments along with the number of instructors in each department select dept_name, ( select count(*) from instructor where department.dept_name = instructor.dept_name) as num_instructors from department; Runtime error if subquery returns more than one result tuple Database System Concepts - 7th Edition 3.52 Silberschatz, Korth and Sudarshan Modification of the Database Deletion of tuples from a given relation. Insertion of new tuples into a given relation Updating of values in some tuples in a given relation

Database System Concepts - 7th Edition 3.53 Silberschatz, Korth and Sudarshan Deletion Delete all instructors delete from instructor Delete all instructors from the Finance department delete from instructor where dept_name= 'Finance; Delete all tuples in the instructor relation for those instructors associated with a department located in the Watson building. delete from instructor where dept name in (select dept name from department where building = 'Watson');

Database System Concepts - 7th Edition 3.54 Silberschatz, Korth and Sudarshan Deletion (Cont.) Delete all instructors whose salary is less than the average salary of instructors delete from instructor where salary < (select avg (salary) from instructor); Problem: as we delete tuples from instructor, the average salary changes Solution used in SQL: 1. First, compute avg (salary) and find all tuples to delete 2.

Next, delete all tuples found above (without recomputing avg or retesting the tuples) Database System Concepts - 7th Edition 3.55 Silberschatz, Korth and Sudarshan Insertion Add a new tuple to course insert into course values ('CS-437', 'Database Systems', 'Comp. Sci.', 4); or equivalently insert into course (course_id, title, dept_name, credits) values ('CS-437', 'Database Systems', 'Comp. Sci.', 4); Add a new tuple to student with tot_creds set to null insert into student values ('3003', 'Green', 'Finance', null);

Database System Concepts - 7th Edition 3.56 Silberschatz, Korth and Sudarshan Insertion (Cont.) Make each student in the Music department who has earned more than 144 credit hours an instructor in the Music department with a salary of $18,000. insert into instructor select ID, name, dept_name, 18000 from student where dept_name = 'Music' and total_cred > 144; The select from where statement is evaluated fully before any of its results are inserted into the relation. Otherwise queries like insert into table1 select * from table1 would cause problem

Database System Concepts - 7th Edition 3.57 Silberschatz, Korth and Sudarshan Updates Give a 5% salary raise to all instructors update instructor set salary = salary * 1.05 Give a 5% salary raise to those instructors who earn less than 70000 update instructor set salary = salary * 1.05 where salary < 70000; Give a 5% salary raise to instructors whose salary is less than average update instructor set salary = salary * 1.05 where salary < (select avg (salary) from instructor);

Database System Concepts - 7th Edition 3.58 Silberschatz, Korth and Sudarshan Updates (Cont.) Increase salaries of instructors whose salary is over $100,000 by 3%, and all others by a 5% Write two update statements: update instructor set salary = salary * 1.03 where salary > 100000; update instructor set salary = salary * 1.05 where salary <= 100000; The order is important Can be done better using the case statement (next slide)

Database System Concepts - 7th Edition 3.59 Silberschatz, Korth and Sudarshan Case Statement for Conditional Updates Same query as before but with case statement update instructor set salary = case when salary <= 100000 then salary * 1.05 else salary * 1.03 end Database System Concepts - 7th Edition 3.60 Silberschatz, Korth and Sudarshan

Updates with Scalar Subqueries Recompute and update tot_creds value for all students update student S set tot_cred = (select sum(credits) from takes, course where takes.course_id = course.course_id and S.ID= takes.ID.and takes.grade <> 'F' and takes.grade is not null); Sets tot_creds to null for students who have not taken any course Instead of sum(credits), use: case when sum(credits) is not null then sum(credits) else 0 end Database System Concepts - 7th Edition 3.61

Silberschatz, Korth and Sudarshan End of Chapter 3 Database System Concepts - 7th Edition 3.62 Silberschatz, Korth and Sudarshan

Recently Viewed Presentations