Web app development_my_sql_09

hasen@microcis.net July 16, 2013Hassen poreya
Trainer, Cresco Solution
Afghanistan Workforce
Development Program
Database
MySQL, Modification Commands

Modification Commands
 There are three SQL commands to modify the
database:
 INSERT
 DELETE
 UPDATE

INSERT Command
 In its simplest form, it is used to add one or more
tuples to a relation.
 Tuples are the rows
 Relations are the tables.
 Attribute values should be listed in the same order
as the attributes were specified in the CREATE
TABLE command.

INSERT Command
INSERT INTO table name VALUES
(value1, value2, ...);
 Values are separated by comma ,
 Strings, Dates are enclosed by single quotation
marks ‘’
 Values have to be sorted according to the table
structure (the columns)

INSERT Command
INSERT INTO employee VALUES
(1,'John',‘ Smith', 'Berlin', '1979-
09-04', 1000.40);
INSERT INTO project VALUES (1,
'Database Design', 'Kabul');
INSERT INTO works in VALUES(1,1,40);

INSERT Command
INSERT INTO employee VALUES
(NULL, 'Anna', 'Conner', 'London',
'1960-11-10‘,1.40);
What will happen if we put null value in the id field?

INSERT Command
 If we used AUTO INCREMENT for the definition of
a column, the database system will automatically
compute a unique value for Id.
 The value is computed by incrementing the
maximum value for Id already in the database.

DELETE Command
 Removes tuples from a relation.
 Includes a WHERE clause to select the tuples to be
deleted.
 A missing WHERE clause species 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.

DELETE Command
DELETE FROM table name WHERE condition;
DELETE FROM employee WHERE id=1;
DELETE FROM project WHERE
location=‘kabul’;

INSERT Command: Integrity Constraints
 Integrity Constraints:
 One of the tasks of a database system is to make
(automatically) sure that the stored data is in a
consistent state.
 When we model the Mini-World we also model what
a consistent state means for us by defining integrity
constraints.
 In our company each employee is assigned a unique id
as the PRIMARY KEY

Integrity Constraints: PRIMARY KEY
 Primary Key
 Database system automatically rejects all new records
with a key that is already stored in the database as
below:

Integrity Constraints: UNIQUE
 UNIQUE
 Similar to primary key: has to be unique
 We do not want to use name as a primary key to
identify records, but still we want to express that it has
to be unique.

Integrity Constraints: NOT NULL
 NOT NULL
 Sometimes we want to make sure that a value is
always assigned to a column, so we use NOT NULL
 Primary keys are automatically defined as NOT NULL
(we couldn't identify an employee if there is no Id
stored)

Modification Command: UPDATE
 It is used to modify attribute values of one or more
selected tuples.
 A WHERE clause selects the tuples to be modified.
 An additional SET clause species the attributes to
be modified and their new values.

Modification Command: SELECT
 Extract data stored in some columns of some table.
 General syntax is:
SELECT column name1, column name2, ... FROM
table name;
SELECT * FROM table name;
 The asterisk (*) retrieves all attributes form the
specified table.

SELECT last name, location, salary
FROM employee

SELECT * FROM employee

SELECT Command: WHERE
 Basic form of the SQL SELECT statement is called a
mapping or a SELECT-FROM-WHERE- block.
SELECT < attributelist > FROM < tablelist > WHERE
< condition >
 < condition > is a conditional (Boolean) expression
that identifies the tuples to be retrieved by the
query.

SELECT Command: WHERE
SELECT location, salary FROM
employee WHERE last_name=Smith;

SELECT Command: ORDER BY
 Sort the result according to some column,
descending or ascending.

SELECT Command: ORDER BY
 Compare the result with the same query using
ORDER BY.

hasen@microcis.net July 16, 2013Hassen poreya
Trainer, Cresco Solution
Any Questions!

Web app development_my_sql_09

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