This document provides an overview of SQL fundamentals and Oracle 11g, including: - How to add comments to tables and columns to document the data dictionary. - The main data types in Oracle like VARCHAR2, INT, DATE, and TIMESTAMP. - Data integrity constraints that can be applied at the column or table level, including NULL/NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints. - How to add, drop, disable, enable, and use cascading constraints to modify constraints on existing schemas.

1. SQL Fundamentals
Oracle 11g
M U H A M M A D WA H E E D
O R A C L E D ATA B A S E D E VE L O P E R
E M A I L : m.w a h e e d 3 6 6 8 @ g ma i l . c om
.
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Lecture#3

2. Lab Work Video Tutorial
•Video is embedded inside the slides.
•Go through each slide for conceptual knowledge then practical is
performed in the video that is provided in last slides.
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3. COMMENTS
•Use the COMMENT statementto add to the data dictionary a comment
about a table or table column etc.
•We use following syntax for creating comments:
COMMENT ON TABLE <table_name> IS ‘<comment_statement>’;
or
COMMENT ON COLUMN <table_name>.<column_name>
IS ‘<comment_statement>';
•We use following syntax to remove/drop comments:
COMMENT ON TABLE <table_name> IS ' ';
or
COMMENT ON COLUMN <table_name>.<column_name>IS ' ';
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4. View COMMENTS
•SELECT * FROM USER_COL_COMMENTS;
•SELECT * FROM USER_TAB_COMMENTS;
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5. Data Types
Data Type Description
VARCHAR2( size ) Variable length character data with
specified size ( 1 to 4000)
INT Fixed length number data type with default
size 38.
CHAR [ size ] Fixed length character data of specified size
( 1 to 2000)
NUMBER [ p,s ] Number having precision p(1 to 38) and
scale s(-84 to 127)
Date It must be between January 1, 4712 and
December 31,9999
TIMESTAMP(s) It is date+time data type with scale
specified from user on the right of decimal
point in time. Scale is 6 by default.
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6. Data Integrity Constraints
•These are used to enforce limitations on the data entered in tables.
•It have two levels:
column level(applied on specific column definition)
table level(applied at table definition)
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7. Column Level Constraints
•Column level constraints are defined with following syntax:
CREATE TABLE <table_name>
(<column_name> <data_type> <constraint_definition>);
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8. Table Level Constraints
•Table level constraints are defined with following syntanx:
CREATE TABLE <table_name>
(<column_name> <data_type>,
<column_name> <data_type>,
CONSTRAINT<constraint_name> <constraint_definition>);
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9. Constraint Types
•There are following five constraints:
1- NULL/NOT NULL
2- UNIQUE
3- PRIMARYKEY
4- FOREIGN KEY
5- CHECK
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10. NULL/NOT NULL Constraint
•It determines whether a column can be blank or not.
•It is only possible at column level.
•By default each column is set to NULL.
•Example (column level):
CREATE TABLE student
( std_id NUMBER(4) NOT NULL,
std_name VARCHAR2(30));
•Example(table level):
ALTER TABLE student
MODIFY std_name VARCHAR2(30)NOT NULL);
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11. UNIQUE Constraint
•It is used for a field which needs to be unique but also can be
null/empty.
•Example (column level):
contact_no of student table can be empty but must be unique.
CREATE TABLE student
( std_contact_no NUMBER(4)CONSTRAINTcontact_uk UNIQUE,
);
• Example(table level):
CREATE TABLE student
(std_contact_no int,
CONSTRAINTcontact_uk UNIQUE(std_contact_no));
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12. PRIMARY KEY Constraint
•It restricts the duplication of values and doesn’t allow null values.
•Example(columnlevel):
CREATE TABLE student
( std_id int PRIMARYKEY,
std_name varchar2(100));
•Example( table level):
CREATE TABLE student
(std_id int,
CONSTRAINTstdid_pk PRIMARY KEY(std_id));
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13. FOREIGN KEY Constraint
•It is used to create the link between data of two tables.
•It is based on common field in two tables where parent table have
primary key and child table uses it as foreign key.
•One table can have multiple foreign keys.
•Example(columnlevel):
CREATE TABLE result
( std_id int REFERENCES student);
•Example(table level):
CREATE TABLE result
(std_id int,
CONSTRAINTstdid_fk FOREIGN KEY(std_id)REFERENCES
student(std_id));
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14. CHECK Constraint
•Defines a condition that each row mustsatisfy.
•It is only possible at column level.
•Example(columnlevel):
CREATE TABLE student
( age int CONSTRAINTstd_age CHECK(age>18));
•Example(table level):
CREATE TABLE result
(age int,
CONSTRAINTstd_age CHECK (age>18));
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15. Modify Constraints
•SQL provides the facility of modification of constraints on existing
schemas.
•There are four following facilities to be used on constraints :
Add
Drop
Enable
Disable
Cascade
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16. ADD Constraint
•ADD constraint:
ALTER TABLE <table_name>
ADD CONSTRAINT<constraint_name><constraint_defination>;
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17. DROP Constraint
•DROPconstraint:
ALTER TABLE <table_name>
DROPCONSTRAINT<constraint_name>;
•*Note: PRIMARYKEY as a special case uses “CASCADE” to disable its
foreign keys. For example:
ALTER TABLE student
DROPPRIMARYKEY CASCADE;
In above case foreign key constraint on result table will be deleted too.
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18. DISABLE Constraint
•DROPconstraint:
ALTER TABLE <table_name>
DISABLECONSTRAINT<constraint_name>CASCADE;
•Apply the “CASCADE” option to disable its dependent integrity
constraints.
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19. ENABLE Constraint
•It is used to ENABLE the DISABLED constraints:
ALTER TABLE <table_name>
ENABLE CONSTRAINT<constraint_name>;
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20. CASCADE Constraint
•The CASCADE constraints clause is used along with the DROPCOLUMN
clause.
•The CASCADE clause drops all referential integrity constraints that refer
to the primary and unique keys defined on the dropped columns.
•The CASCADE clause also drops all multicolumn constraints defined on
the dropped columns.
•Example:
ALTER TABLE <table_name>
DROP(<constraint_name(s)>)CASCADECONSTRAINTS;
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21. Your Suggestions/Feedback?
-Elbert Hubbard
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Email: m.waheed3668@gmail.com