From theory to practice. Designing Tables for a postgresql Database System. psql. Reminder. A few useful commands

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1 From theory to practice Designing Tables for a postgresql Database System The Entity- Relationship model: a convenient way of representing the world. The Relational model: a model for organizing data using tables. postgresql: a database infrastructure which implements the relational model. Converting ER->Relational model is important! SQL(Structured Query Language): A language used to get information from a database. psql In order to connect to the postgresql database, we use the psql terminal In order to connect to it, print the following command in your shell: psql hdbserver public This will give you access t o your database A few useful commands \ q exit psql \ h [command] help about command \ d [] descr ibe t able/ index/ called \ dt list tables \ di list indexes \ dv list views \ df list functions Reminder The database is kept on the disk, so anything you create will be there next time you log on. sailors Reserves Main Memory DISK CPU

2 Running Commands from an.sql File Instead of typing commands into the psql terminal, you can load commands from a file (no special format is required). The f ile should end wit h.sql Invoke by: \ i filename Tables The basic element in postgresql is a table. A table has columns (attributes), and rows (tuples). Every column has a Name and Type (of the data it stores), and some columns have constraints. Some tables may have additional constraints. Id Creating Tables in SQL Name Eyal Yair Ori Dept. Sales Transport Warehouse Age Creating a Table The basic format of the CREATE TABLE command is: CREATE TABLE TableName( Column1 DataType1 ColConstraint, ColumnN DataTypeN ColConstraint, TableConstraint1, TableConstraintM Example An Example (cont.) ID INTEGER NOT NULL, F VARCHAR(20), L VARCHAR(20), Dept INTEGER If you type \ d Employee you get: Column Type Modifiers integer not null f character varying(20) l character varying(20) gender character(1) integer not null dept integer Note: postgresql is case insensitive in Column s!

3 Name bigint boolean box bytea character varying [ (n) ] character [ (n) ] circle date double precision integer line point text Examples of Data Types Aliases int8 bool varchar [ (n) ] char [ (n) ] float8 int, int4 Description signed eight-byte integer logical Boolean (true/false) rectangular box in the plane binary data ("byte array") variable-length character string fixed-length character string circle in the plane calendar date (, month, day) double precision floating-point number signed four-byte integer infinite line in the plane geometric point in the plane variable-length character string Constraints in Create Table Adding constraints to a table enables the database system to enforce data integrity. However, adding constraints also makes inserting data slower. Different types of constraints: * Not Null * Default Values * Unique * Primary Key * Foreign Key * Check Condition timestamp [ (p) ] date and time Not Null Constraint Default Values ID INTEGER NOT NULL, F VARCHAR(20), L VARCHAR(20), Dept INTEGER ID INTEGER NOT NULL, F VARCHAR(20), L VARCHAR(20), Gender CHAR(1) default( F ), Dept INTEGER Unique Constraint (Syntax 1) Unique Constraint (Syntax 2) ID INTEGER UNIQUE NOT NULL, F VARCHAR(20), L VARCHAR(20), Gender CHAR(1) default( F ), Dept INTEGER ID INTEGER NOT NULL, F VARCHAR(20), L VARCHAR(20), Gender CHAR(1) default( F ), Dept INTEGER, UNIQUE(ID)

4 Unique Constraint (Another Example) ID INTEGER NOT NULL, F VARCHAR(20), L VARCHAR(20), Gender CHAR(1) default( F ), Dept INTEGER, UNIQUE(FNAME,LNAME) Can this be written differently? Primary Key Constraint ID INTEGER PRIMARY KEY, F VARCHAR(20), L VARCHAR(20), Dept INTEGER, UNIQUE(FNAME,LNAME) Primary Key implies: * NOT NULL * UNIQUE. There can only be one primary key. Primary Key Constraint (Syntax 2) ID INTEGER, F VARCHAR(20), L VARCHAR(20), Dept INTEGER, PRIMARY KEY(FNAME,LNAME) ID INTEGER primary key, F VARCHAR(20), L VARCHAR(20), Dept INTEGER CREATE TABLE Department( DeptNumber INTEGER PRIMARY KEY, Name VARCHAR(20), ManagerId INTEGER Shouldn t all department numbers in Employee appear in Department? Employee ID FName Larry Magic Bird Foreign Key LName Johnson Gender M M Sallary Dept Foreign Key Foreign Key Constraint CREATE TABLE Employee ( ID INTEGER primary key, F VARCHAR(20), L VARCHAR(20), DeptNumber INTEGER REFERENCES Department Department Dept Name Sales Repair ManID Note 1: DeptNumber must be unique (or primary key) in Department Note 2:You can use this syntax only if the of the fields in both tables are entical Note 3: The referencing attribute (DeptNumber) can be null

5 Foreign Key Constraint (different s of attributes) ID INTEGER primary key, F VARCHAR(20), L VARCHAR(20), Dept INTEGER REFERENCES Department(DeptNumber) Foreign Key Constraint ID INTEGER primary key, F VARCHAR(20), L VARCHAR(20), DeptNumber INTEGER, FOREIGN KEY (DeptNumber) REFERENCES Department Using this syntax allows a pair or more of attributes to be a foreign key Deleting referenced values What happens to Employees in department 312 when Department 312 is removed from the Department table? Deleting a Referenced Value If nothing additional is specified, then postgresql will not allow Department 312 to be deleted if there are Employees working in (referencing to) this department. You can also specify this explicitly: FOREIGN KEY (Dept) REFERENCES Department ON DELETE RESTRICT Alternatively, if the constraint is written as FOREIGN KEY (Dept) REFERENCES Department ON DELETE CASCADE then Employees working in 312 will be deleted automatically from the Employee table, when 312 is deleted from Departments Check Conditions A check condition is a Boolean expression: And s and Or s of condit ions of t he t ype X > 5 On a column: it can refer only to the column On a table: it can refer only to multiple columns in the table Check Constraints ID INTEGER primary key, F VARCHAR(20), L VARCHAR(20), Gender CHAR(1) check(gender= F or gender= M ), DeptNumber INTEGER

6 Deleting a Table To delete the table Employee : DROP TABLE Employee; Converting ER- Diagrams to Table Definitions Mind the order of dropping when there are foreign key constraints. General Principals When converting ER diagrams to Relations, we should try to: 1. Reduce duplicate information 2. Constrain as tightly as possible Note: 1. Some scenarios can be represented in different ways. 2. Sometimes we will not be able to fully represent constraints, or will be forced to represent information more than once. Relation definition vs. Table definition We show how to translate ER-Diagrams to table definitions Sometimes, people translate ER-Diagrams to relation definitions, which are more abstract than table definitions. e.g., Employee(ID, F, L, Gender, Salary, Dept table definitions contain, in addition, constraints and datatypes Simple entity translation Actor birthday Student student General Rule: Create a table with the of the Entity. There is a column for each attribute The key in the diagram is the primary key of the table

7 Simple entity translation Actor birthday Translating Entities with Relationships birthday Actor (without constraints) Acted In Film title Relation: Actor (,, birthday, ) create table Actor( varchar(20) primary key, varchar(40), birthday date, varchar(100) Create tables for the entities as before Create a table with the of the relationship Relationship table attributes: its own attributes () + all keys of the relating entities (title, ). Q: What is the primary key of the table? A: A composite of both entity keys Q: What foreign keys are needed? A: From the relationship table to the entities type (without constraints) birthday Actor Acted In Film title How would we represent this diagram in tables? type ' ' How would you define the table for ActedIn? create table ActedIn( varchar(20) references actor, title varchar(20) references film, integer, primary key(,title) ' Answer Translating Recursive Relationships (without constraints) manager ' ' ' ' ' ' Employee worker Manages Relation: Manages (W, M) What would be the table definition? create table Manages( E varchar(20) references Employee(), M varchar(20) references Employee(), Primary key(e, M) How would you prevent someone from being his own manager?

8 Option 1: (key constraints): Option 1 Same as without key constraints (3 tables), except that t he r elat ionship pr imar y key is? title. Directed Film title (key constraints): Option 1 Directed Film title create table Directed( varchar(20) references, title varchar(40) references Film, integer, primary key (title) (key constraints): Option 2 (key constraints): Option 2 Directed Film title Directed Film title Option 2: Do not create a table for the relationship Add information columns that would have been in the relationship's table to the table of the entity with the key constraint create table Film( title varchar(40) primary key, integer, integer, Why couldn t we do this when there were no constraints? varchar(20) references (participation constraints) (participation constraints) Directed Film title Directed Film title General Rule: If both participation and key constraint exist, use Option 2 from before (only 2 tables), AND: Add the not null constraint on the referncing attribute to ensure that there will always be values for the key of the other entity create table Film( title varchar(40) primary key, integer, varchar(20), integer, Where should we add NOT NULL? foreign key () references

9 (participation constraints) Actor Acted In Film title How would we translate this? Translating Weak Entity Sets Organization Gives Award phone number money A regular table for Organization, and.. create table award( varchar(40), integer, money integer, o_ varchar(40) references Organization(), primary key(,, o_), O Organization Gives phone number picture Translating Aggregation Acted In ID Simple Entity Tables Single table Summary Primary key The entity key Remarks a column for each attr. Award Won Broadcasted title Film type Simple Relationship Key constraint 3 (2 entities +relationship) 3 as before or For the relation: Both entity keys Key of constrained ent. Foreign keys from rel. Table Foreign keys from rel. Table Won(title,,, O, Broadcasted 2 (one for each entity) Foreign key from constr. Entity Key and Participation constr. 2 Regular Constrained entity has a non-null f. key Tables Primary key Remarks Weak Entity 2: parent and weak entities Weak: its own and parent keys Foreign keys from weak ent. ISA: covers and disjoint 2: only child entities Parent key ISA: otherwise 3: parent and child entities Parent key Foreign keys from child ent. Aggregation 3: 2 aggregates and relationship For relationship: keys of both aggregates Foreign keys from relationship table

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