Database Systems Basic SQL

Outline SQL Data Definition and Data Types Specifying Constraints in SQL Basic Retrieval Queries in SQL INSERT, DELETE, and UPDATE Statements in SQL Additional Features of SQL

Basic SQL SQL language Considered one of the major reasons for the commercial success of relational databases SQL Structured Query Language Statements for data definitions, queries, and updates (both DDL and DML) SQL is not a programming language. SQL commands can be used interactively as a query language within the DBMS or SQL commands can be embedded within programming languages.

Types of SQL Commands Data Definition Language (DDL): Commands that define a database - Create, Alter, Drop Data Manipulation Language (DML) Commands that maintain and query a database. Each statement in SQL ends with a semicolon

SQL Concept & Relational Concept Terminology: Table, row, and column used for relational model terms relation, tuple, and attribute SQL Concept Relational Concept Table Relation Row Tuple Column Attribute

Schema and Catalog Concepts in SQL Catalog: A set of user schemas that typically constitute a description of a database A catalog can contain many user schemas. Schema: A collection of database objects created by a user, such as base tables, views, and constraints. A schema is created via the CREATE SCHEMA statement which should include at least the schema name and authorization identifier. CREATE SCHEMA Company Authorization JSmith;

The CREATE TABLE Command CREATE statement Main SQL command for data definition Specify a new relation Provide name Specify attributes and initial constraints Base tables (base relations) Relation and its tuples are actually created and stored as a file by the DBMS Virtual relations Created through the CREATE VIEW statement

The CREATE TABLE Command Can optionally specify schema: CREATE TABLE COMPANY.EMPLOYEE ... or CREATE TABLE EMPLOYEE ... Create the following relation: Student(SSN, Name, GPA, Age). SQL> create table Student ( 2 SSN char(9) not null, 3 Name varchar(20), 4 GPA number(3,2), 5 Age number(2), 6 primary key (SSN));

Attribute Data Types char(n) fixed length character string of length n varchar(n) variable length characters of length up to n number(n) integer and real values of up to n digits number integer and real values of up to 40 digits integer(n) integer of up to n digits intege r integer of up to 40 digits date yyyy-mm-dd (10 positions) time typically in the form HH:MM:SS

Domain Definitions CREATE DOMAIN <name> As <data-type>; CREATE DOMAIN SSN-TYPE AS CHAR(9); We can use SSN_TYPE in place of CHAR(9) This style in writing SQL statements is preferred as it makes it easier to change the data types of attributes. A domain can also have an optional default specification via default clause. CREATE DOMAIN DNO-DOM AS CHAR(2) DEFAULT 01;

Specifying Constraints in SQL Constraints are rules or regulations imposed on data to ensure their integrity and correctness. Constraints on individual columns. Constraints on a table. Constraints on multiple tables.

Constraints on Columns (1) Create table Book( ISBN char(15) primary key, Title varchar(50) unique, Price number(5,2) check (Price>0), Edition number(2) default 1 check (Edition > 0), Publisher_name varchar(30) references Publisher(Name));

Constraints on Individual Columns (2) type values must be of the given type not null no null value can be taken unique Specifies alternate (secondary) keys Dname VARCHAR(15) UNIQUE; no identical non-null values primary key Specifies one or more attributes that make up the primary key of a relation no null value and no identical values Dnumber INT PRIMARY KEY; check constraint definition values must satisfy the check condition. Dnumber INT NOT NULL CHECK (Dnumber > 0 AND Dnumber < 21);

Constraints on Individual Columns (3) Syntax for column definition: col_name data_type [default value] [constraints] Syntax for column constraints: [constraint constraint_name] [not] null | check condition | unique | primary key | references table_name [(column)] [on delete cascade] [on update cascade]

Create Table Example (4) Create table Enrollment( SSN char(9) references Students, Crs_no varchar(7) not null, Section_no number(2) not null, Grade char, primary key(SSN,Crs_no,Section_no), foreign key (Crs_no,Section_no) references Section);

Giving Names to Constraints Syntax for table constraints: [constraint constraint_name] check condition | unique (column {, column}) | primary key (column {, column}) | foreign key (column {, column}) references table_name[(column{, column)] CHECK clauses at the end of a CREATE TABLE statement Apply to each tuple individually CHECK (Dept_create_date <= Mgr_start_date);

Giving Names to Constraints Create table Graduate( SSN char(9) primary key, Name varchar(30), Address varchar(80), Degree varchar(8) check (Degree in {‘Bachelor’, ‘Master’, ‘Ph.D.’}), GPA number (3,2) not null, unique (Name, Address), constraint ic12 check (GPA>=2.0 and (Degree=‘Bachelor’ or GPA>=3.0)));

Disabling and Enabling a Constraint Disabling a constraint: alter table Graduate disable constraint ic12; Enabling a constraint: alter table Graduate enable constraint ic12;

Specifying Key and Referential Integrity Constraints FOREIGN KEY clause Default operation: reject update on violation Attach referential triggered action clause Options include SET NULL, CASCADE, and SET DEFAULT Action taken by the DBMS for SET NULL or SET DEFAULT is the same for both ON DELETE and ON UPDATE CASCADE option suitable for “relationship” relations

Basic Retrieval Queries in SQL SELECT statement One basic statement for retrieving information from a database SQL allows a table to have two or more tuples that are identical in all their attribute values Unlike relational model Multiset or bag behavior

The SELECT-FROM-WHERE Basic form of the SELECT statement:

The SELECT-FROM-WHERE Structure Logical comparison operators =, <, <=, >, >=, and <> Projection attributes Attributes whose values are to be retrieved Selection condition Boolean condition that must be true for any retrieved tuple

The SELECT-FROM-WHERE Find the SSN, Name and GPA of all students whose GPA is higher than 3.8. Select SSN, Name, GPA From Students Where GPA > 3.8; Select all EMPLOYEE SSNs. SELECT SSN FROM EMPLOYEE; Select all combinations of Employee SSNs and Department Dname. SELECT SSN, DNAME FROM EMPLOYEE, DEPARTMENT;

The SELECT-FROM-WHERE Select all attributes of employees who work for department number 5; SELECT * FROM EMPLOYEE WHERE DNO = 5; Find the names and GPAs of all students who take database systems. Select Name, GPA From Student, Enrollment, Course Where Title = `database systems` and Students.SSN = Enrollment.SSN and Enrollment.Course_no=Courses.Course_no;

Ambiguous Attribute Names Same name can be used for two (or more) attributes As long as the attributes are in different relations Must qualify the attribute name with the relation name to prevent ambiguity EMPLOYEE(SSN, Name, …., DNo) DEPARTMENT(DNo, Name, …)

Aliasing, Renaming, and Tuple Variables Aliases or tuple variables Declare alternative relation names E and S EMPLOYEE AS E(Fn, Mi, Ln, Ssn, Bd, Addr, Sex, Sal, Sssn, Dno)

Dealing with Aliasing (1) For each employee retrieve the employee’s first and last name and the first and last name of his or her immediate advisor. SELECT E.Fname, E.Lname, S.Fname, S.Lname FROM Employee E, Employee S WHERE E.SuperSSN = S.SSN; We may write Employee AS E Note that aliases can be used to shorten queries by giving short names to relations.

Dealing with Aliasing (2) We may alias attributes as well. Employee(Name, Address, Supervisor, Dept) Q1: Select NA, Adr, S, Dept FROM EMPLOYEE AS E(NA, Adr, S, D) Or this Select Name NA, Address adr, Supervisor S, Dept D FROM EMPLOYEE; Find the names and GPAs of all students who take database systems. Select Name, GPA From Student S, Enrollment E, Course C Where Title=`database systems‘ and S.SSN=E.SSN and E.Course_no=C.Course_no

Dealing with Aliasing (3) Find all pairs of students who have the same GPA. select s1.SSN, s2.SSN from Student s1, Student s2 where s1.GPA=s2.GPA and s1.SSN<s2.SSN; Question: Why use “s1.SSN < s2.SSN”?

Dealing with Aliasing (4) Find the names of all students whose GPA is higher than Tom's GPA. select s1.Name from Students s1, Students s2 where s2.Name=‘Tom’ and s1.GPA>s2.GPA; Question: Is a student qualified if his/her GPA is higher than some Tom's GPAs but not all Tom's GPAs?

Renaming SELECT Attributes SQL allows us to rename SELECT attributes This does not significantly change the results of a query, but it does allow us to control the headings of the columns displayed on reports SELECT FNAME AS FN, LNAME AS LN FROM EMPLOYEE WHERE SUPERSSN IS NULL The same tuples will be selected as for the last query, but now the columns will be labeled FN and LN instead of FNAME and LNAME.

Unspecified WHERE Clause and Use of the Asterisk Missing WHERE clause Indicates no condition on tuple selection CROSS PRODUCT All possible tuple combinations

Unspecified WHERE Clause and Use of the Asterisk (cont’d.) Specify an asterisk (*) Retrieve all the attribute values of the selected tuples

Unspecified Where Clause & Use of Asterisk To retrieve all attributes of a relation, use the shorthand * To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the attributes Find all students whose GPA is higher than 3.8. select * from Students where GPA > 3.8; The where-clause may be absent. Find the Names of all students. select Name from Students; Select all EMPLOYEE SSNs. SELECT SSN FROM EMPLOYEE;

Unspecified Where clause A missing WHERE-clause indicates no condition; hence, all tuples of the relations in the FROM-clause are selected This is equivalent to the condition WHERE TRUE Retrieve the SSN values for all employees. SELECT SSN FROM EMPLOYEE; If more than one relation is specified in the FROM-clause and there is no join condition, then the CARTESIAN PRODUCT of tuples is selected

Use of Asterisk Retrieve all the attributes of an employee and the attributes of the department he/she works in for every employee of the “Research” department. SELECT * FROM Employee, Department WHERE Dname = “Research” AND DNO = Dnumber; Specify (retrieve) the cross product of the Employee and Department relations; SELECT * FROM Employee, Department; Select all combinations of Employee SSNs and Department Dname. SELECT SSN, DNAME FROM EMPLOYEE, DEPARTMENT;

Tables as Sets in SQL SQL does not automatically eliminate duplicate tuples in query results Use the keyword DISTINCT in the SELECT clause Only distinct tuples should remain in the result Find the Names of all students without duplicate rows. Select distinct Name from Student; Select all can be used to explicitly request that all duplicate rows are kept. Is the following query an efficient query? select distinct SSN, Name from Student Retrieve the salary for every employee. duplicates may appear SELECT Salary FROM Employee; duplicates will be eliminated SELECT DISTINCT Salary FROM Employee;

Tables as Sets in SQL (cont’d.) Set operations UNION, EXCEPT (difference), INTERSECT Corresponding multiset operations: UNION ALL, EXCEPT ALL, INTERSECT ALL) union removes duplicate rows. union all keeps duplicate rows.

Substring Pattern Matching and Arithmetic Operators LIKE comparison operator Used for string pattern matching % replaces an arbitrary number of zero or more characters underscore (_) replaces a single character Standard arithmetic operators: Addition (+), subtraction (–), multiplication (*), and division (/) BETWEEN comparison operator

Substring Pattern Matching Find the course numbers and titles of all courses whose title contains “systems”. select Course_no, Title from Courses where Title like `%systems%`; % matches 0 or more characters.

Substring Pattern Matching Find all students whose name starts with M and is six-character long. select * from Students where Name like `M_ _ _ _ _`; _ matches exactly one character not like is the opposite of like. Find all students whose name contains a _. where Name like `%\_ %`;

Substring Pattern Matching Retrieve all employees whose address in Housten, Texas. SELECT Fname, Lname FROM Employee WHERE Address LIKE ’%Houston, TX%’; Find all employees who were born during the 1950s. WHERE Bdate LIKE ’195_ _ _ _ _’;

Arithmetic Operators Show resulting salaries if every employee working on the ‘productX’ project is given a 10% raise. SELECT Fname, Lname, 1.1*Salary FROM Employee, Works_on, Project WHERE SSN = ESSN AND PNO = Pnumber AND Pname=“ProductX”; Find the names of all students whose GPA is between 3.5 and 3.8, inclusive. select Name from Students where GPA between 3.5 and 3.8; not between ... and … is the opposite of between … and ...

Arithmetic Operators Employees (SSN, Name, Salary) Find the SSN, name and annual salary of each employee. select SSN, Name, 12*Salary from Employees; or select SSN, Name,12*Salary as Annual_Salary

Ordering of Query Results Use ORDER BY clause Keyword DESC to see result in a descending order of values Keyword ASC to specify ascending order explicitly ORDER BY D.Dname DESC, E.Lname ASC, E.Fname ASC

Ordering Tuples Find the names of all students and order the names in ascending order. select Name from Students order by Name asc; ascending order is the default in order by clause. We can write: order by Dname desc, Lname ASC, Fname ASC

Ordering Tuples Find all the students whose GPA is higher than 3.5 and order the result in descending order by GPA, and for students having the same GPA, order them in ascending order by their names. select * from Students where GPA > 3.5 order by GPA desc, Name asc;

Ordering Tuples Retrieve a list of employees and the projects they are working on, ordered by department and, within each department, alphabetically by last name, first name; SELECT Dname, Lname, Fname, Pname FROM Department, Employee, Works_on, Project WHERE Dnumber=DNO AND SSN=ESSN AND PNO=Pnumber ORDER BY Dname, Lname, Fname;

Discussion and Summary of Basic SQL Retrieval Queries

INSERT, DELETE, and UPDATE Statements in SQL Three commands used to modify the database: INSERT, DELETE, and UPDATE Relation schemas under consideration: Student(SSN, Name, GPA, Age) Course(CNo, Title, DName) Enrollment(SSN, CNo, Grade)

The INSERT Command Specify the relation name and a list of values for the tuple

Insert In its simplest form, it is used to add one or more tuples to a relation Attribute values should be listed in the same order as the attributes were specified in the CREATE TABLE command. Syntax: insert into table_name [(column {, column})] [values (expression {, expression})] | [Subselect] Multiple tuples can be inserted with the same INSERT command, tuples are separated by commas and enclosed between ( and ).

Insert Add a new student (123, Ali, null, 22) to the student table Student(SSN, Name, GPA, Age). insert into Students values (`123`,`Ali`,null,22); Order of values should agree with the original definition of Student. or insert into student(name, ssn, age) values (`Ali`,`123`,22) Attributes with NULL and DEFAULT values can be left out.

Insert Insert two tuples into the Students table insert into Students values (‘111223333’, ‘Smith’, 3.45, 23) (‘444556666’, ‘Maria’, 3.60, 22); Suppose table Top_Students (SSN, Name, GPA) has been created. Find all students whose GPA is 3.8 or higher and save the result in the Top_Students table. insert into Top_Students select SSN, Name, GPA from Student where GPA >= 3.8;

Insert CREATE TABLE DEPTS-INFO( DNAME VARCHAR(15), NO-OF-EMPS INTEGER, TOTTAL-SAL INTEGER); INSERT INTO DEPTS-INFO (DNAME, NO-OF-EMPS, TOTAL-SAL) SELECT DNAME, count(*), sum(SALARY) FROM DEPARTMENT, EMPOYEE WHERE DNUMBER = DNO GROUP BY DNAME;

Inserting Null Data INSERT INTO COURSE (COURSE_CODE, CREDIT_HOURS) VALUES (‘MIS499’,4); VALUES (‘MIS499’,’’,4); VALUES (‘MIS499’,NULL,4); COLUMN LIST IS NEEDED IF YOU PLAN TO LEAVE OUT A VALUE IN THE VALUE LIST COLUMN LIST CAN BE OMITTED IF YOU PUT IN A BLANK VALUE THE NULL KEYWORD CAN BE USED TO CREATE A BLANK COLUMN ALL OF THESE ASSUME THAT THE DATABASE ALLOWS THE COLUMN TO BE NULL. YOU CANNOT LEAVE PRIMARY KEYS BLANK

Delete Removes tuples from a relation Includes a WHERE-clause to select the tuples to be deleted Tuples are deleted from only one table at a time (unless CASCADE is specified on a referential integrity constraint) A missing WHERE-clause specifies that all tuples in the relation are to be deleted; the table then becomes an empty table The number of tuples deleted depends on the number of tuples in the relation that satisfy the WHERE-clause Referential integrity should be enforced

Delete Syntax: delete from table_name [where search_condition] Delete all tuples from Students. delete from Students; The schema of Students remains. Delete all students who take no courses. delete from Students where SSN not in (select SSN from Enrollment);

The DELETE Command Removes tuples from a relation Includes a WHERE clause to select the tuples to be deleted

Delete DELETE FROM Employee WHERE Lname = ‘Brown’; DELETE FROM Employee where SSN = ‘123456789’ DELETE FROM EMPLOYEE WHERE DNO IN (SELECT Dnumber FROM Department WHERE Dname = ‘Research’);

Update Used to modify attribute values of one or more selected tuples A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced Syntax: update table_name set column = {expression | null} {, column = {expression | null}} [where search_condition]

Update Increase the GPA of the student with SSN=123 by 0.5. update Student set GPA = GPA + 0.5 where SSN = '123'; Set the Grade of every student who takes database systems to be A. update Enrollment set Grade = 'A' where CNo in (select CNo from Course where title='database systems');

Update Change the location and controlling department number of project number 10 to 'Bellaire' and 5, respectively. UPDATE Project SET Plocation = ‘Bellaire’, Dnum = 5 WHERE Pnumber=10; Give all employees in the 'Research' department a 10% raise in salary. UPDATE Employee SET Salary = Salary * 1.1 WHERE DNO IN (SELECT Dnumber FROM Department WHERE Dname = ‘Research’);

The UPDATE Command Modify attribute values of one or more selected tuples Additional SET clause in the UPDATE command Specifies attributes to be modified and new values

Additional Features of SQL Techniques for specifying complex retrieval queries Writing programs in various programming languages that include SQL statements Set of commands for specifying physical database design parameters, file structures for relations, and access paths Transaction control commands

Summary SQL Comprehensive language Data definition, queries, updates, constraint specification, and view definition Covered in Chapter 4: Data definition commands for creating tables Commands for constraint specification Simple retrieval queries Database update commands