Sql ch 4

SQL Basics – Ch 4 – Relational Databases

SQL – RELATIONAL DATABASES
1. What is a SQL statement?
An SQL statement requests an specific action from the DBMS. This action may be
creating a new table, inserting a new record, retrieving data, deleting a record, or
modifying a database.
The main structure of an SQL statement is shown below:

Every SQL statement begins with a keyword that describes what the statement does.
CREATE, INSERT, DELETE, and COMMIT are typical verbs. The statement continues
with one or more clauses. A clause may specify the data to be acted upon by the statement
or provide more detail about what the statement is supposed to do. Every clause also
begins with a keyword, such as WHERE, FROM, INTO, and HAVING. Some clauses are
optional; others are required. Many clauses contain table or column names; some may
contain additional keywords, constants, or expressions.
Keywords will always be shown in uppercase e..g., SELECT, FROM, etc
Variable items such as table name, are shown in lowercase letters.

2. What are the different types of SQL statements?
SQL statements can be divided into the following categories:.
(i) Data Manipulation Statements - SELECT, INSERT, DELETE, UPDATE
(ii) Data Definition statements – CREATE TABLE, DROP TABLE, ALTER
TABLE, CREATE VIEW, DROP VIEW, CREATE INDEX, DROP INDEX,
CREATE SCHEMA, DROP SCHEMA, CREATE DOMAIN, ALTER DOMAIN,
DROP DOMAIN
(iii) Access Control – GRANT, REVOKE

Prof. Mukesh N. Tekwani Page 1

SQL Basics– Ch 4 – Relational Databases

(iv) Transaction Control – COMMIT, ROLLBACK, SET TRANSACTION
(v) Programmatic SQL statements – DECLARE, EXPLAIN, OPEN, CLOSE,
FETCH, PREPARE, EXECUTE, DESCRIBE.

3. What are table names and column names?
(i) The objects in a SQL-based database are identified by assigning them unique names.
Names are used in SQL statements to identify the database object on which the
statement should act.

(ii) The most fundamental named objects in a relational database are table names (which
identify tables), column names (which identify columns), and user names (which
identify users of the database).

(iii) The original ANSI/ISO standard specified that SQL names must contain 1 to 18
characters, must begin with a letter, and may not contain any spaces or special
punctuation characters. The SQL2 standard increased the maximum to 128 characters.

Table Name:
When we specify a table name in a SQL statement, SQL assumes that you are referring to
one of your own tables (that is, a table that you created). The table names must be short
and descriptive.

Column Name:
When we specify a column name in a SQL statement, SQL can normally determine from
the context which column we want. However, if the statement involves two columns with
the same name from two different tables, we must use a qualified column name to
uniquely identify the column we want. A qualified column name specifies the name of the
table containing the column and the name of the column, separated by a period (.). For
example, the column named SALES in the SALESREPS table has the qualified column
name:

SALESREPS.SALES

Qualified column names can generally be used in a SQL statement wherever a simple
(unqualified) column name can appear.

4 Describe the different data types used in SQL.
The ANSI SQL standard specifies the various data types that can be stored in a SQL-based
database and manipulated by the SQL language. These data types are:
(i) Integer - Columns holding this type of data store counts, quantities, ages, and
so on. Integer columns are also used to contain I.D. numbers, such as
customer, employee, and order numbers.
INT or INTEGER
(ii) Decimal numbers. Columns with this data type store numbers that have
fractional parts and must be calculated exactly, such as rates and percentages.
They are also used to store money amounts.

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(iii) Floating point numbers. Columns with this data type are used to store
scientific numbers that can be calculated approximately, such as weights and
distances. Floating point numbers can represent a larger range of values than
decimal numbers but can produce round-off errors in computations.

FLOAT (precision)

(iv) Fixed-length character strings. Columns holding this type of data store names
of people and companies, addresses, descriptions, and so on.

CHAR(len) or CHARATER (len)

(v) Variable-length character strings. This data type allows a column to store
character strings that vary in length from row to row, up to some maximum
length.

VARCHAR (len) or CHARACTER VARYING (len) or CHAR VARYING (len)

(vi) Money amounts. Many SQL products support a MONEY or CURRENCY
type, which is usually stored as a decimal or floating point number.

(vii) Dates and times. SQL can also support date/time values. Various combinations
of dates, times, timestamps, time intervals, and date/time arithmetic are
supported.

DATE , TIME

(viii) Boolean data. Some SQL products support logical (TRUE or FALSE) values
as an explicit type, and some permit logical operations (comparison, AND/OR,
and so on) on the stored data within SQL statements.

(ix) Long text. Many SQL-based databases support columns that store long text
strings (typically up to 32,000 or 65,000 characters, and in some cases even
larger). This allows the database to store entire documents, product
descriptions, technical papers, resumes, and similar unstructured text data. The
DBMS usually restricts the use of these columns in interactive queries and
searches.

(x) Unstructured byte streams. Several DBMS products allow unstructured,
variable-length sequences of bytes to be stored and retrieved. Columns
containing this data are used to store compressed video images, executable
code, and other types of unstructured data.

5 What are constants in SQL? Explain the different types of constants used in SQL.
In some SQL statements a numeric, character, or date data value must be expressed in text



form. For example, in this INSERT statement, which adds a salesperson to the database:
INSERT INTO SALESREPS (EMPL_NUM, NAME, QUOTA,
HIRE_DATE, SALES)
VALUES (115, 'Dennis Irving', 175000.00, '21-JUN-90',
0.00)
the value for each column in the newly inserted row is specified in the VALUES clause.
Constant data values are also used in expressions, such as in this SELECT statement:
SELECT CITY
FROM OFFICES
WHERE TARGET > (1.1 * SALES) + 10000.00

Numeric Constants:
Integer and decimal constants are written as ordinary decimal numbers in SQL statements,
with an optional leading plus or minus sign.
21 -375 2000.00 +497500.8778
We must not put a comma between the digits of a numeric constant.
Floating point constants (also called approximate numeric literals) are specified using the
E notation. Here are some valid SQL floating point constants:
1.5E3 -3.14159E1 2.5E-7 0.783926E21

String Constants:
These contain character data enclosed within single quotes mark. For example,
'New Delhi.' 'New York' 'Western'
If a single quote is to be included in the constant text, it is written within the constant as
two consecutive single quote characters. Thus this constant value:
'I can''t' becomes the seven-character string "I can't".
Date and Time Constants:
In SQL constant values for dates, times, and time intervals are specified as string
constants. Here are some examples of legal SQL Server date constants:
March 15, 1990 Mar 15 1990 3/15/1990 3-15-90 1990 MAR 15

6 What are symbolic constants in SQL?
SQL language includes special symbolic constants that return data values maintained by
the DBMS itself. For example, the symbolic constant CURRENT_DATE gives the value



of the current date and can be used in queries such as the following, which lists the
salespeople whose hire date is still in the future.
SELECT NAME, HIRE_DATE
FROM SALESREPS
WHERE HIRE_DATE > CURRENT_DATE
A symbolic constant can appear in a SQL statement anywhere that an ordinary constant of
the same data type could appear.

7 How are expressions used in SQL statements?
Expressions are used in the SQL to calculate values that are retrieved from a database and
to calculate values used in searching the database. For example, this query calculates the
sales of each office as a percentage of its target:
SELECT CITY, TARGET, SALES, (SALES/TARGET) * 100
FROM OFFICES

and this query lists the offices whose sales are more than $50,000 over target:
SELECT CITY
FROM OFFICES
HERE SALES > TARGET + 50000.00

The ANSI SQL standard specifies four arithmetic operations that can be used in
expressions: addition , subtraction), multiplication, and division.

8 Explain automatic data type conversion in SQL statements.
The ANSI standard specifies automatic data type conversion from integers to decimal
numbers, and from decimal numbers to floating point numbers. We can thus mix these
data types in a numeric expression.

9 What are built-in functions in SQL?
Most SQL implementations include a number of useful built-in functions. These facilities
often provide data type conversion facilities. For example, built-in MONTH() and YEAR()
functions take a DATE or TIMESTAMP value as their input and return an integer that is
the month or year portion of the value. This query lists the name and month of hire for
each salesperson in the sample database:
SELECT NAME, MONTH(HIRE_DATE)



FROM SALESREPS
and this one lists all salespeople hired in 1988:
SELECT NAME, MONTH(HIRE_DATE)
FROM SALESREPS
WHERE YEAR(HIRE_DATE) = 1988
Built-in functions also are often used for data reformatting. TO_CHAR() function, for
example, takes a DATE data type and a format specification as its arguments and returns a
string containing a formatted version of the date. In the results produced by this query:
SELECT NAME, TO_CHAR(HIRE_DATE,'DAY MONTH DD, YYYY')
FROM SALESREPS
The hire dates will all have the format "Wednesday June 14, 1989" because of the built-in
function.

10 Explain the term “primary key”.
(i) The rows of a relational table are unordered, and so we cannot select a specific row
by its position in the table. There is no "first row," "last row," or "thirdth row" of a
table.
(ii) In a well-designed relational database every table has some column or combination
of columns whose values uniquely identify each row in the table. This column (or
columns) is called the primary key of the table.
(iii) E.g., in a list of students, we cannot use the lastname field as the primary key since
two or more students may have the same lastname. We may combine the lastname
and firstname and create a primary key. But in practice, "Roll number," such as is
often chosen as primary keys. In a table of orders placed by customers, the
ORDERNO can be used as a primary key.
(iv) The primary key has a different unique value for each row in a table, so no two rows
of a table with a primary key are exact duplicates of one another. A table where
every row is different from all other rows is called a relation in mathematical terms.
The name "relational database" comes from this term, because relations (tables with
distinct rows) form the basis of a relational database.
The following figure shows a table with two columns being used for a primary key.
This key is called a composite key.



11 Explain the term “foreign key”.
(i) A column in one table whose value matches the primary key in some other table is
called a foreign key.
(ii) In the figure shown below, the REP_OFFICE column is a foreign key for the
OFFICES table. Although REP_OFFICE is a column in the SALESREPS table, the
values that this column contains are office numbers. They match values in the
OFFICE column, which is the primary key for the OFFICES table. Together, a
primary key and a foreign key create a parent/child relationship between the tables
that contain them.

(iii) Just as a combination of columns can serve as the primary key of a table, a foreign
key can also be a combination of columns. In fact, the foreign key will always be a
compound (multi-column) key when it references a table with a compound primary
key. The number of columns and the data types of the columns in the foreign key
and the primary key must be identical to one another.
(iv) A table can contain more than one foreign key if it is related to more than one other
table. Figure below shows the three foreign keys in the ORDERS table of the
sample database:



The CUST column is a foreign key for the CUSTOMERS table, relating each order
to the customer who placed it.
The REP column is a foreign key for the SALESREPS table, relating each order to
the salesperson who took it.
The MFR and PRODUCT columns together are a composite foreign key for the
PRODUCTS table, relating each order to the product being ordered.
(v) Foreign keys are an important part of the relational model because they create
relationships among tables in the database.

12 What are Codd’s twelve rules for databases?
Rule 1: The information rule. All information in a relational database is represented
explicitly at the logical level and in exactly one way—by values in tables.
Rule 2: Guaranteed access rule. Each and every datum (atomic value) in a relational
database is guaranteed to be logically accessible by using a combination of table name,
primary key value, and column name. This rule stresses the importance of primary key to
locate data. The table name locates the correct table, the column name finds the correct
column, and the primary key value finds the row containing an individual data item of
interest.
Rule 3: Systematic treatment of null values. Null values (distinct from an empty character
string or a string of blank characters and distinct from zero or any other number) are
supported in a fully relational DBMS for representing missing information and
inapplicable information in a systematic way, independent of the data type. Rule 3 requires



support for missing data through NULL values.
Rule 4: Dynamic online catalog based on the relational model. The database description
is represented at the logical level in the same way as ordinary data, so that authorized users
can apply the same relational language to its interrogation as they apply to the regular data.
Rule 5: Comprehensive data sublanguage rule. A relational system may support several
languages. However, there must be at least one language whose statements can be
expressed, by some well-defined syntax. This language should also support the following
activities:
• Data definition
• View definition
• Data manipulation (interactive and by program)
• Integrity constraints
• Authorization
• Transaction boundaries (begin, commit, and rollback)
What this rule means is that the RDBMS should have its own extension of SQL. This
extension should support the above activities
Rule 6: View updating rule. All views that are theoretically updateable are also updateable
by the system.
Rule 7: High-level insert, update, and delete. The capability of handling a base relation or
a derived relation as a single operand applies not only to the retrieval of data but also to the
insertion, update, and deletion of data. The RDBMS should not only support retrieval of
data as relational sets, but should also support insertion, deletion, and updation of data as a
relational set. If the database uses a single record at a time and procedural techniques to
manipulate the data, then it cannot be called a relational database.
Rule 8: Physical data independence. Application programs are not disturbed if any
changes are made in either storage representations or access methods.
Rule 9: Logical data independence. Application programs and terminal activities remain
logically unimpaired when information preserving changes of any kind that theoretically
permit unimpairment are made to the base tables. Thus, this rule allows dynamic changes
in the logical database design by splitting or joining base tables, provided it does not lead
to loss of information.
Rule 10: Integrity independence. Integrity constraints specific to a particular relational
database must be definable in the relational data sublanguage and storable in the catalog,


not in the application programs. Every database must support the following rules:
Entity integrity: No component of a primary key can have a NULL value.
Referential Integrity: For every unique ‘non-null’ foreign key value, there should
be a matching primary key.
Rule 11: Distribution independence. A relational DBMS has distribution independence. It
means that if a distributed database than it should be possible to execute all operations on
it without being restricted by the physical locations of data.
Rule 12: Non-subversion rule. A low-level language (i.e. a language working on the
principle of a single record at a time), must in no way be used to bypass the integrity rules
and constraints imposed in the higher level (multiple records at a time).


Sql ch 4

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