introduction to sql ii

8/14/2019 Introduction to SQL II

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LECTURE 1

Introduction to SQL II

Data Definition Language (DDL) Data Manipulation Language (DML)


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DDL WITH TABLES.

Creating Tables with Table constraints

Modifying Table structures


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Create Table using table and column

constraints Constraints enforce rules on data whenever a row is

inserted, updated, or deleted from a table. The constraints

have to be satisfied for the operation to succeed.

Data Integrity Constraints:

Not null: specifies that the column cannot contain a

null value.

Unique: specifies that a column or combination ofcolumns whose values must be unique for all rows in

the table.


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constraints contd Check: specifies a condition that must be true

Primary key: uniquely identifies each row of the table

Foreign key: establishes and enforces a foreign key

relationship between the column and a column of the

referenced table.

Constraints can be divided at one of the two levels

i. Column constraint:

References a single column and is defined within

the column definition.

Can define any type of integrity constraint.


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o Table constraint:

References one or more columns and is defined

after the column list. Can define any constraint except not null.

It is an issue of style to define constraints at either tablelevel or column level; however, the not null constraint

must be strictly defined at the column level.


constraints Contd


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Example: [using column constraints]

create table students (regNo varchar(15) primary key, name

varchar(20), dob date, gender char(1) not null);

OR

[using table constraints]

create table students (regNo varchar(15), name varchar(20),

dob date, gender char(1) not null, constraintstudents_regNo_pk primary key(regNo));

How would you confirm constraints defined on a table?


constraints Contd


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Modifying a Table Structure

Modifying a table involves using the ALTER TABLE

statement which could cater for three kinds of adjustments:

MODIFY column (modify [data type/size][not null]

[default]) ADD column(s) / constraint(s),

DROP column(s) / constraint(s)

Examples:

alter table students add weight float (3,1);

alter table students add height float (3,1);;

alter table students modify weight int;


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Modifying a Table Structure contd alter table students drop column height;

alter table students modify gender char(1) default F;

Alter table students add constraint students_gender_ck

check (gender in (M,F)); Alter table students add constraint students_name_uk

unique (name);

alter table students drop key students_name_uk;


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Notes:

The not null constraint is added by modifying the column

that is to be defined as not null

Example:

ALTER TABLE students

MODIFY weight int not null;

Adjustments to populated tables is more restrictive

because the adjustments should not violet the nature of

data stored in the tables.

Modifying a Table Structure Contd


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Removing a Table from a DB schema

Example:

DROP TABLE students;


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Quiz Write all SQL statements that will create the relational DB schema:

DEPTS (DeptNo, DName, Loc)

PERSONNEL (EmpNo, EName, Position, *Mgr, HireDate, Sal,Comm, *DeptNo)

Data Constraints: A department name value must never be repeated.

No person should earn a salary that is more than 5000.

Note:

A foreign key is preceded by an asterisk (*); Mgr references EmpNo.


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Quiz Contd

1. Assuming that Sex has data type char, modify the personnel

table such that it can accept values of varied lengths.

2. Modify the staff table such that each staff member has a

position.


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Manipulating Data In a Table

UPDATE statement

DELETE Statement


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DML Overview This is a language that provides a set of operations to

support the basic data manipulation operations on thedata held in the database.

There are many types of DML statements, these include;

INSERT to add data/new rows to a table

UPDATE to make changes to existing data/rowsin a table


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DML Overview contd SELECT to query data in the database (could involve

one table or many tables to be covered in another learning unit)

DELETE to remove data from a table/remove existingrows from a table

Therefore some of the operations performed are insertion,modification, retrieval and deletion.


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Create a table called emp_table with following attributes:

Example to be used


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Statement for creating emp_tablecreate table emp_table (empId varchar(20), fName

varchar(15), lName varchar(15) not null, dob date,

constraint emp_empid_pk primary key(empId));


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The Update Operation The Update operation is used to modify existing rows in a

database.

The general syntax of an update statement is;

UPDATE table

SET column = value [,column = value,]

[WHERE condition];


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Specific value(s) are modified if you specify the WHERE

clause in the UPDATE statement. This is mainly by the

use of the primary key because any other column name

may cause several rows to be updated unexpectedly .

Example:

UPDATE emp_table

SET lname = 'nakato'

WHERE empid = '93/3/3';

The Update Operation


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The Update Operation contd

All rows in the table are modified if you omit the where

clause.

Example:

UPDATE emp_tableSET lname= 'nakato';

Updating rows with values with that are tied to intergrity

constraints usually returns an error when the constraint is

violated. Example when updating a foreign key.


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Updating with the use of a Subquery

A subquery may be used to update a row or rows in a given

relation

Example:

UPDATE emp_copySET job_id='SAL_PER',

salary = (select salary

from employees

where employee_id = 100)

WHERE job_id = 'FI_ACCOUNT';


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The Update Operation contd

Updating multiple columns using a subquery; The general syntax is

UPDATE table

SET column = ( SELECT columnFROM table

WHERE condition)

[,

column = ( SELECT column

FROM table

WHERE condition)]

[WHERE condition];


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The Delete Operation

Specific row(s) are deleted if you specify the WHERE

clause in the DELETE FROM statement.

Example:

DELETE FROM emp_copyWHERE job_id= SAL_PER;

Take care, if a WHERE clause is not specified, then all

rows from the table will be deleted.Example:

DELETE FROM emp_copy;


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Deleting Rows based on Another Table This is usually done using a Subquery

Example:

DELETE FROM emp_copy

WHERE department_id = (SELECT department_id

FROM departments

WHERE location_id=2400);


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Deleting Rows: Integrity constraint Error

You can not delete a row that contains a primary key that is

used as a foreign key in another table.

Example:

DELETE FROM departments

WHERE department_id = 60;

Error: child record found violation


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Common Integrity Constraint Errors

Leaving out values for columns with NOT NULLconstraints.

Deleting a parent record that has dependencies on it.

Example:DELETE FROM dept_copyWHERE department_id = 20;

Inserting or updating a record with a foreign key valuethat does not exist in the referenced primary or unique

key of the parent table. Example:UPDATE emp_copySET department_id = 55WHERE department_id = 110;


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Primary keys and Foreign key constraints to be added to the

tables created:

Add a primary key constraint on the emp_copy table.

Add a primary key constraint on the dept_copy table

Add a foreign key constraint on the dept_copy tablewhich references the primary key of the emp_copy

table.

ExerciseContd


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ExerciseContd statements for adding

the constraints alter table emp_copy add constraint

emp_copy_employee_id_pk primary key(employee_id);

alter table dept_copy add constraintdept_copy_department_id_pk primary key(department_id);

alter table emp_copy add constraint

emp_copy_department_id_uk foreign key(department_id)references dept_copy(department_id);

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