SQL Concepts
Database Languages ............................................................................................................... 2
Characteristics of SQL............................................................................................................ 2
SQL Server Object Names ..................................................................................................... 2
Delimited Identifiers ............................................................................................................... 3
SQL’s DDL Statements .......................................................................................................... 3
Creating Tables ....................................................................................................................... 3
Creating User-Defined Data Types ........................................................................................ 5
Data Integrity .......................................................................................................................... 6
Constraints .............................................................................................................................. 7
The PRIMARY KEY Constraint ............................................................................................ 7
Syntax When Creating a Table ............................................................................................... 7
Column-Level Constraint ....................................................................................................... 7
Table-Level Constraint ........................................................................................................... 8
Syntax After Creating a Table ................................................................................................ 8
The FOREIGN KEY Constraint ............................................................................................. 9
Syntax When Creating a Table ............................................................................................... 9
Column-Level Constraint ....................................................................................................... 9
Table-Level Constraint ........................................................................................................... 9
Syntax After Creating a Table .............................................................................................. 10
The CHECK Constraint ........................................................................................................ 10
The DEFAULT Constraint ................................................................................................... 11
The UNIQUE Constraint ...................................................................................................... 12
SQL’s DML Statements ....................................................................................................... 13
The SELECT Statement .................................................................................................................... 13
Selecting All Columns and All Rows ................................................................................... 13
Selecting Specific Columns and All Rows ........................................................................... 13
Selecting Specific Rows ....................................................................................................... 13
Comparison Operators .......................................................................................................... 14
Operator Meaning ............................................................................................................................................. 14
Examples .............................................................................................................................. 14
Using AND ........................................................................................................................... 15
Using OR .............................................................................................................................. 16
Operator Precedence ............................................................................................................. 16
Changing Precedence ........................................................................................................... 16
Example
................................................................................................................................................. 16
Eliminating Duplicate Rows ................................................................................................. 17
Controlling the Order of Displayed Rows ............................................................................ 17
Examples .............................................................................................................................. 18
Inserting Data in Only Some Columns ................................................................................. 18
SELECT for Joining Tables ................................................................................................. 19
Cartesian Product .................................................................................................................. 19
Examples .............................................................................................................................. 19
Using Pseudonyms (Aliases) ................................................................................................ 20
Views .................................................................................................................................... 20
SQL Concepts
1
Database Languages
SQL, often known as a query language, is a combined DDL, DML and DCL used
with relational databases.



A Data Definition Language (DDL) is used to specify the data in a database.
A Data Manipulation Language (DML) is used to access the data in a
database.
A Data Control Language (DCL) is used to control access to the data in a
database.
Characteristics of SQL

The DDL statements define database objects, eg databases, tables, views,
indexes, users, constraints, user-defined data types:
CREATE, DROP, ALTER

The DML statements manipulate data:
SELECT, INSERT, DELETE, UPDATE

The DCL statements control access to data:
GRANT, DENY, REVOKE
Transact-SQL is the SQL Server implementation of SQL.
SQL Server Object Names
Standard Identifiers

Can contain from one to 128 characters, including letters, symbols (_ @ or #)
and numbers.

No embedded spaces are allowed.

The first character must be alphabetic.

A name beginning with @ denotes a local variable or parameter.

A name beginning with # denotes a temporary table or procedure.

A name beginning with ## denotes a global temporary object.
NB:
Names for temporary objects shouldn’t exceed 116 characters including # or
## as SQL Server gives them an internal numeric suffix.
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2
Delimited Identifiers
Do not comply with the rules for standard identifiers and must, therefore, always be
delimited.
You can use delimited identifiers when:

Names contain embedded spaces.

Reserved words are used for object names or portions of object names.
You must enclose delimited identifiers in square brackets or quotation marks when
you use them in Transact-SQL statements, eg:
SELECT * FROM [Blanks In Table Name]
SELECT * FROM “Blanks In Table Name”
NB:
You can always use bracketed delimiters but can only use quotation marks if
the SET QUOTED_IDENTIFIER option is on.
SQL’s DDL Statements
Creating Tables
In SQL Server you can have up to:



Two billion tables per database.
1,024 columns per table.
8060 bytes per row (this maximum length does not apply to image, text, and
ntext data types).
The CREATE TABLE statement creates a new table. The simplest format of the
statement is:
CREATE TABLE Table_name
(Column_name1 Data_type1,
Column_name2 Data_type2, ….);
Before creating a table you need to decide on the table name, column names, data
types and widths.
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3
Example
CREATE TABLE Student
(Studentno
CHAR(9)
Title
VARCHAR(6)
Fname
VARCHAR(15)
Surname
VARCHAR(15)
DOB
DATETIME
Telno
VARCHAR(12)
NOT NULL,
NULL,
NOT NULL,
NOT NULL,
NULL,
NULL);
The NULL/NOT NULL option specifies whether NULL values are allowed for a
column.
Deleting a Table
DROP TABLE Table_name;
DROP TABLE Student;
Dropping a table removes the table definition and all data. A dropped table is
unrecoverable.
Adding a Column to a Table
ALTER TABLE Table_name
ADD Column_name Data_type;
ALTER TABLE Student
ADD Email CHAR(20) NULL;
Deleting a Column from a Table
ALTER TABLE Table_name
DROP COLUMN Column_name;
ALTER TABLE Student
DROP COLUMN Email;
Altering a Column
SQL Concepts
4
ALTER TABLE Table_name
ALTER COLUMN Column_name Data_type;
ALTER TABLE Student
ALTER COLUMN Email VARCHAR(30) NULL;
Creating User-Defined Data Types
User-defined data types allow you to refine system-supplied data types. Data type
names must follow the rules for identifier names.
User-defined data types are defined for a specific database, but those that you
create in the model database are automatically included in all databases that are
subsequently created.
You define each user-defined data type in terms of a system-supplied data type,
specifying NULL or NOT NULL.
An existing user-defined data type can be used with the CREATE TABLE or ALTER
TABLE statements to define one or more columns.
Creating a User-defined Data Type
Use the sp_addtype system stored procedure.
Example
EXEC sp_addtype City_dt, 'VARCHAR(15)', NULL;
EXEC sp_addtype Studentno_dt, 'CHAR(9)',
‘NOT NULL’;
Using a User-Defined Data Type
Having created a user-defined data type, it can be used to define a column’s data
type and width, eg:
CREATE TABLE Student
(Studentno studentno_dt, …
Dropping a User-Defined Data Type
Use the sp_droptype system stored procedure.
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5
A user-defined data type cannot be dropped if tables or other database objects
reference it.
Example
EXEC sp_droptype city;
Data Integrity
Data integrity refers to the consistency and accuracy of data that is stored in a
database.
Constraints are the preferred method of enforcing data integrity in a database.
Entity (Table) Integrity
Requires that all rows in a table have a unique identifier, known as the PRIMARY
KEY.
Enforced through the use of the PRIMARY KEY constraint, which ensures
uniqueness of the column(s) and ensures that no column that is part of the primary
key can contain a NULL value.
Referential Integrity
Ensures that the relationship between a primary key and a foreign key is always
maintained.
Enforced through the use of the FOREIGN KEY constraint, which ensures that a
value in a foreign key must match an existing value in the referenced primary key or
be NULL.
Domain (Column) Integrity
Specifies a set of data values that are valid for a column and determines whether
NULLs are allowed.
Enforced through definition of the data type of a column, also through use of the
CHECK and DEFAULT constraints, NULL/NOT NULL.
The CHECK constraint restricts the data that users can enter into a particular column
to specific values.
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A DEFAULT constraint enters a value in a column when one is not specified in an
INSERT statement.
Constraints
You can create a constraint:

When the table is created, as part of the CREATE TABLE statement.

After the table has been created, by using the ALTER TABLE statement.
Always name a constraint or SQL Server will give it a complicated system-generated
name.

A column-level constraint is defined within a column definition. It must apply
to a single column.

A table-level constraint is defined at the end of the CREATE TABLE or ALTER
TABLE statement, after the definition of all columns. If a constraint references
multiple columns, it must be defined as a table-level constraint.
The PRIMARY KEY Constraint
Syntax When Creating a Table
Column-Level Constraint
Can only be used when the PRIMARY KEY consists of only one column.
CREATE TABLE Table_name
(Column_name1 Data_type1
CONSTRAINT Constraint_name PRIMARY KEY,
Column_name2 Data_type2, ….);
CREATE TABLE Student
(Studentno
CHAR(9)
CONSTRAINT Student_PK PRIMARY KEY,
Title
VARCHAR(6)
Fname
VARCHAR(15)
Surname
VARCHAR(15)
DOB
DATETIME
Telno
VARCHAR(12)
SQL Concepts
NULL,
NOT NULL,
NOT NULL,
NULL,
NULL);
7
Table-Level Constraint
Must be used when the PRIMARY KEY consists of more than one column.
May be used when the PRIMARY KEY consists of only one column.
CREATE TABLE Table_name
(Column_name1 Data_type1,
Column_name2 Data_type2, ….,
CONSTRAINT Constraint_name
PRIMARY KEY
(Column_name1, Column_name2, …));
CREATE TABLE Student
(Studentno
CHAR(9),
Title
VARCHAR(6)
Fname
VARCHAR(15)
NULL,
NOT NULL,
Surname
VARCHAR(15)
NOT NULL,
DOB
DATETIME
NULL,
Telno
VARCHAR(12)
NULL,
CONSTRAINT Student_PK PRIMARY KEY (Studentno));
CREATE TABLE CourseStudent
(Courseno
TINYINT,
Studentno CHAR(9),
CONSTRAINT CourseStudent_PK
PRIMARY KEY (Courseno, Studentno));

Only one PRIMARY KEY constraint can be defined per table.

The PRIMARY KEY constraint creates a unique index on the specified
column(s).
Syntax After Creating a Table
You must use the ALTER TABLE statement.
ALTER TABLE Table_name
ADD CONSTRAINT Constraint_name
PRIMARY KEY
(Column_name1, Column_name2, …);
Examples
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8
ALTER TABLE Student
ADD CONSTRAINT Student_PK
PRIMARY KEY (Studentno);
ALTER TABLE CourseStudent
ADD CONSTRAINT CourseStudent_PK
PRIMARY KEY (Courseno, Studentno);
The FOREIGN KEY Constraint
Syntax When Creating a Table
Column-Level Constraint
Can only be used when the FOREIGN KEY consists of only one column.
CREATE TABLE Table_name
(Column_name1 Data_type1,
Column_name2 Data_type2 CONSTRAINT
Constraint_name FOREIGN KEY
REFERENCES Table_name (Column_name),
Column_name3 Data_type3, ….);
CREATE TABLE TutorStudent
(Tutorno
INT
CONSTRAINT TutorStudent_Tutorno_FK
FOREIGN KEY
REFERENCES Tutor (Tutorno),
Studentno
CHAR(9)
CONSTRAINT TutorStudent_Studentno_FK
FOREIGN KEY
REFERENCES Student (Studentno));
Table-Level Constraint
Must be used when the FOREIGN KEY consists of more than one column.
May be used when the FOREIGN KEY consists of only one column.
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9
Example
CREATE TABLE TutorStudent
(Tutorno
INT,
Studentno
CHAR(9),
CONSTRAINT TutorStudent_Tutorno_FK
FOREIGN KEY (Tutorno)
REFERENCES Tutor (Tutorno),
CONSTRAINT TutorStudent_Studentno_FK
FOREIGN KEY (Studentno)
REFERENCES Student (Studentno));
Syntax After Creating a Table
You must use the ALTER TABLE statement.
Example
ALTER TABLE TutorStudent
ADD CONSTRAINT TutorStudent_Tutorno_FK
FOREIGN KEY (Tutorno)
REFERENCES Tutor (Tutorno);

Data type(s) in the FK column(s) must match data type(s) in the referenced
PK column(s).

A row in a referenced table cannot be deleted, nor the PK changed, if a FK refers
to the row, unless the CASCADE option is specified.

FOREIGN KEY constraints don’t create indexes automatically, so if you’ll be
joining these tables often you should create an index on FOREIGN KEY
column(s) to improve join performance.
The CHECK Constraint
Column-Level Constraint When Creating a Table
CONSTRAINT Constraint_name
CHECK (Condition);
CREATE TABLE Tutor
(Tutorno
INT, …..
Job
VARCHAR(20),
Salary
INT
CONSTRAINT Tutor_Salary_Check
CHECK (Salary BETWEEN 17000 AND 35000));
SQL Concepts
10
The CHECK Constraint:

Verifies data every time that you execute an INSERT or UPDATE statement.

Is like WHERE clauses in that you specify the conditions under which data will
be accepted.

Can reference other columns in the same table, eg Salary could reference a
value in Job.
Table-Level Constraint When Creating a Table
CONSTRAINT Constraint_name
CHECK (Condition);
CREATE TABLE Tutor
(Tutorno
INT, …..
Job
VARCHAR(20),
Salary
INT,
CONSTRAINT Tutor_Salary_Check
CHECK (Salary BETWEEN 17000 AND 35000));
The comma makes all the difference!
CHECK Constraint After Creating a Table
You must use the ALTER TABLE command.
ALTER TABLE Tutor
ADD CONSTRAINT Tutor_Salary_Check
CHECK (Salary BETWEEN 17000 AND 35000);
The DEFAULT Constraint
A DEFAULT constraint can only be defined as a column-level constraint.
CREATE TABLE Student
(Studentno
CHAR(9), …..
Termstreet
VARCHAR(30),
Termtown
VARCHAR(30)
CONSTRAINT Student_Termtime_DF
DEFAULT ‘Middlesbrough’, …..);
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11
ALTER TABLE Student
ADD CONSTRAINT Student_Termtime_DF
DEFAULT ‘Middlesbrough’ FOR TermTown;

Only one DEFAULT constraint can be defined per column.

A DEFAULT constraint applies only to INSERT statements.
The UNIQUE Constraint
A UNIQUE constraint designates a column or columns as unique so that no 2 rows in a
table can have the same value for this column or columns.
Column-Level Constraint When Creating a Table
CREATE TABLE Course
(Courseno
TINYINT,
Coursename
VARCHAR(25)
CONSTRAINT Unq_coursename UNIQUE,
Coursetype
CHAR(3));
Table-Level Constraint When Creating a Table
CREATE TABLE Course
(Courseno
TINYINT,
Coursename
VARCHAR(25),
Coursetype
CHAR(3),
CONSTRAINT Unq_coursename_type
UNIQUE (Coursename, Coursetype));
UNIQUE Constraint After Creating a Table
You must use the ALTER TABLE command.
ALTER TABLE Course
ADD CONSTRAINT Unq_coursename_type
UNIQUE (Coursename, Coursetype);

The UNIQUE constraint can allow only one NULL value.

A table can have multiple UNIQUE constraints.

The UNIQUE constraint is enforced through the creation of a unique index on
the specified column or columns.
SQL Concepts
12
SQL’s DML Statements
The SELECT Statement
A query, ie a request for information to be retrieved from the database.
The SELECT statement is made up of a number of clauses, with a minimum of two:
SELECT
FROM
the columns to retrieve
the table(s) which contain those columns

The clauses may be entered on the same line or on separate lines.

You can enter statements in UPPER CASE, lower case or a MiXtUrE of the
two. However, reserved words are, by convention, entered in UPPER CASE.
Selecting All Columns and All Rows
To display all the information from a table called Student:
SELECT * FROM Student;
Selecting Specific Columns and All Rows
To select some of the columns and all the rows:
SELECT Studentno, Surname FROM Student;
The order of column names in a SELECT statement determines the order in which
the columns are displayed.
SELECT Surname, Studentno FROM Student;
Selecting Specific Rows
Requires the WHERE clause following the FROM clause:
SQL Concepts
13
SELECT
FROM
WHERE
the columns to retrieve
the table(s) which contain those columns
certain conditions are met
SELECT * FROM Course
WHERE Courseno = 30;
SELECT Courseno, Coursename
FROM Course
WHERE Coursename = 'Computing';
Comparison Operators
Operator
Meaning
=
!= or <>
>
>=
<
<=
BETWEEN … AND …
IN (list)
LIKE
IS NULL
equal to
not equal to
greater than
greater than or equal to
less than
less than or equal to
between two values
any of a list of values
match a character pattern
is a null value
To negate the last four operators, use the operator NOT, ie NOT BETWEEN, NOT
IN, NOT LIKE and IS NOT NULL.
Examples
SELECT * FROM Course
WHERE Coursetype <> 'BSc';
SELECT Studentno, Surname FROM Student
WHERE Dob < '01/26/77';
SELECT * FROM Student
WHERE Surname > 'M';
SELECT Tutorno, Job FROM Tutor
WHERE Salary >= 20000 AND Salary <=25000;
SQL Concepts
14
SELECT Tutorno, Job FROM Tutor
WHERE Salary BETWEEN 20000 AND 25000;
SELECT Tutorno, Job FROM Tutor
WHERE Salary NOT BETWEEN 20000 AND 25000;
SELECT Courseno, Coursename FROM Course
WHERE Courseno IN (30, 40, 50, 90);
SELECT Courseno, Coursename FROM Course
WHERE Coursename
NOT IN (‘Electronics’, ‘Computing’);
SELECT * FROM Course
WHERE Coursename LIKE 'Comp%';
SELECT Courseno, Studentno
FROM CourseStudent
WHERE Studymode LIKE '_T';
NB:
Each _ represents one character.
characters.
The % represents any number of
SELECT Tutorno, Studentno
FROM TutorStudent
WHERE Studentno NOT LIKE '_ _D%';
Using AND
SELECT Tutorno, Job FROM Tutor
WHERE Salary >= 20000 AND Salary <=25000;
SELECT * FROM CourseStudent
WHERE Courseno = 50 AND Studymode <> 'FT';
A SELECT statement with AND only retrieves a row or rows if both parts of the
WHERE clause are true.
SQL Concepts
15
Using OR
A SELECT statement with OR retrieves a row or rows if either part of the WHERE
clause is true.
SELECT * FROM CourseStudent
WHERE Courseno = 50 OR Studymode <> 'FT';
SELECT Courseno, Coursename FROM Course
WHERE Courseno = 30 OR Courseno = 40;
Operator Precedence
When AND and OR appear in the same WHERE clause, AND has a higher precedence
than OR, ie all the ANDs are performed first, then all the ORs.
SELECT * FROM Tutor
WHERE Job = ‘Lecturer' AND Salary > 26788
OR Job = 'Senior Lecturer' AND Salary < 30058
OR Job = 'Principal Lecturer' AND Salary < 30058;
All the comparison operators have equal precedence, then the order of precedence
is:
NOT
AND
OR
Changing Precedence
To override the rules of precedence, place the part of the expression you want
evaluated first in parentheses ().
Example
SELECT * FROM Tutor
WHERE Salary > 26788 AND Salary < 30058
AND (Job = 'Lecturer'
OR Job = 'Senior Lecturer'
OR Job = 'Principal Lecturer');
SQL Concepts
16
If in doubt, add parentheses to clarify your SELECT statements and ensure the correct
results.
Eliminating Duplicate Rows
The default display of a result from a query is all rows, including duplicate rows.
To eliminate duplicate rows, use the DISTINCT keyword in the SELECT clause, eg:
SELECT DISTINCT Coursename
FROM Course;
SELECT DISTINCT Job
FROM Tutor;
SELECT DISTINCT Courseno, Coursename
FROM Course;
Controlling the Order of Displayed Rows
You can control the order in which rows are displayed by adding the ORDER BY
clause as the last clause of a SELECT statement.
ORDER BY lets you order rows in ascending order, in descending order and by multiple
columns.
SELECT * FROM Course
ORDER BY Coursename;
SELECT Tutorno, Job, Salary FROM Tutor
ORDER BY Salary DESC;
SELECT * FROM Student
ORDER BY Surname, Fname;
SELECT Tutorno, Job, Salary FROM Tutor
WHERE Salary BETWEEN 20000 AND 25000
ORDER BY Job, Salary DESC;
SQL Concepts
17
Inserting Data into a Table
Use INSERT to insert new rows into a table.
INSERT INTO Table_name
VALUES (Data_value1, Data_value2, ….);

Put character data and dates in single quotes.

Do not put numeric data in quotes.

Data items must be separated by commas.

Only one row may be inserted for each INSERT statement.

The columns must be in the same order as defined when the table was
created.

A NULL may be placed instead of a value by typing NULL.
Examples
INSERT INTO Course
VALUES (20, ‘Computing’, ’HNC’);
INSERT INTO Student
VALUES
('HNC102345', 'Ms', 'Freda', 'Bloggs',
'05/16/80', '[email protected]', NULL);
Inserting Data in Only Some Columns
To insert data into only some of the columns for a row you need to specify the
column names after the table name. The columns not listed will have a NULL value.
INSERT INTO Student
(Studentno, Fname, Surname)
VALUES
(‘HNC102345’, ’Freda’, ’Bloggs’);
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18
SELECT for Joining Tables
To retrieve data from more than one table, the tables must have a column containing
common values. The tables can then be joined by naming them both in the FROM
clause and by writing the join condition in the WHERE clause.
When the same column appears in more than one table you must prefix the column
name with the table name.
Cartesian Product
When a join condition is omitted or is invalid, the result is a Cartesian product.
All rows in the first table will be joined to all rows in the second table, ie all possible
combinations of rows will be displayed.
Examples
SELECT
FROM
WHERE
SELECT
FROM
WHERE
SELECT
FROM
WHERE
AND
Course.Courseno,
Course.Coursename,
Coursestudent.Studentno, Coursestudent.Studymode
Course, Coursestudent
Course.Courseno = CourseStudent.Courseno;
Course.Courseno,
Coursename,
Studentno,
Studymode
Course, Coursestudent
Course.Courseno = CourseStudent.Courseno;
Course.Courseno,
Coursename,
Studentno,
Studymode
Course, Coursestudent
Course.Courseno = CourseStudent.Courseno
Studymode = ‘FT’;
SQL Concepts
19
SELECT
FROM
WHERE
AND
ORDER BY
Tutor.Fname, Tutor.Surname,
Student.Fname, Student.Surname
Student, Tutor, Tutorstudent
Student.Studentno = Tutorstudent.Studentno
Tutor.Tutorno = Tutorstudent.Tutorno
Tutor.Surname, Tutor.Fname,
Student.Surname, Student.Fname;
SELECT
Tutor.Fname, Tutor.Surname,
Student.Fname, Student.Surname
FROM
Student, Tutor, Tutorstudent
WHERE
Student.Studentno = Tutorstudent.Studentno
AND
Tutor.Tutorno = Tutorstudent.Tutorno
AND
Tutor.Surname = ‘Onley’
ORDER BY Student.Surname, Student.Fname;
Using Pseudonyms (Aliases)
Rather than type the complete table name prefixes, you can use pseudonyms (aliases).
SELECT
T.Fname, T.Surname, S.Fname, S.Surname
FROM
Student S, Tutor T, Tutorstudent TS
WHERE
S.Studentno = TS.Studentno
AND
T.Tutorno = TS.Tutorno
AND
T.Surname = ‘Onley’
ORDER BY S.Surname, S.Fname;
Views
A view allows you to store a predefined query as an object in the database for later
use.
The tables queried in a view are called base tables.
You can name and store most SELECT statements as views.
Common examples of views are:
SQL Concepts
20

A subset of rows or columns of a base table.

A join of two or more base tables.

A statistical summary of a base table.
Develop a consistent naming convention for views to distinguish them from tables.
Views offer several advantages, eg:





Focusing the data for users.
Masking database complexity.
Simplifying management of user permissions.
Improving performance.
Organising data for export to other applications.
You specify the contents of a view by using a SELECT statement, eg:
CREATE VIEW StudentNameView
AS
SELECT Fname, Surname
FROM Student
SELECT * FROM StudentNameView;
Altering Views
ALTER VIEW StudentNameView
AS
SELECT Title, Fname, Surname
FROM Student
Deleting Views
DROP VIEW StudentNameView;
Dropping a view removes its definition and all permissions assigned to it.
If users query any views that reference the dropped view, they will get an error
message.
NB: It is best to avoid creating views based on views.
Dropping a table that references a view does not drop the view automatically.
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21
Modifying Data Through Views
Views do not maintain a separate copy of data, but show the result set of a query on
one or more base tables.
Therefore, whenever you modify data in a view, you are actually modifying the base
table.
With some restrictions you can insert, update or delete table data through a view.
In general, the view must be defined on a single table and must not include aggregate
functions or GROUP BY clauses in the SELECT statement.
You can modify views that are derived from two or more tables, but each modification
can affect only one table.
SQL Concepts
22