19 structured query language

8/3/2019 19 Structured Query Language

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Structured Query Language

(SQL)

CIS*2450Advanced Programming Concepts


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SQL consists of a set of commands for defining,accessing, and managing relational databases.

In 1970, E.F. Codd at IBM Research Labs in SanJose, California published A Relational Model ofData for Large Shared Data Banks(Communications of the ACM, Vol. 13, No. 6,June 1970) in which he described a set of abstractprinciples for database management - therelational model. The field of relational databasetechnology has its origins in that paper.


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This research described several relationallanguages which implement some/all of the

features of the abstract relational model.One of these languages which was createdin 1974-75 was the Structured EnglishQuery Language (SEQUEL), defined by

Donald Chamberlin and others at IBMLabs. The first IBM prototype was calledSEQUEL-XRM (1974-75).


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A revised version of SEQUEL-XRM was defined

in 1976-77 and named SEQUEL/Z. The name

was changed to SQL for legal reasons. A prototype of this became operational in 1977

called System R.

Due to the success of System R, vendors rushed to

create their own SQL products. The Oracle

database was released before IBM's own product.


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In 1982, the American National Standards

Institute (ANSI) chartered its Database Committee

(X3H2) to develop a standard for a relationallanguage. In 1986, the X3H2 proposal was

ratified by ANSI which consisted essentially of

the IBM dialect of SQL.

In 1987, the ANSI standard was accepted as aninternational standard by the International

Organization for Standards (ISO).


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The original standard is also known as SQL/86.

Enhancements were made over time

SQL/89 - included an Integrity Enhancement Feature SQL/92 - ISO and ANSI developed a revised standard

also known as SQL2

SQL/99 - SQL3 incorporates object-oriented access

A consortium of vendors known as the SQL AccessGroup has been working to enhance interoperability

across different systems


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SQL as a Standard

Since SQL is such a pervasive standard, let

us review some information on standards -

their good points and their not so goodpoints.


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Are Standards Good? -- Good

Points Standards reduce training costs.

They promote application portability.

Standards promote application longevity.

Standards have a reasonably long life, so

applications using them should as well.


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Are Standards Good? -- Good

Points Intersystem Communications

are more easily achieved.

Different database management systems can functionequally well on a single database if they support thesame standard interface.

Customer Choice

If products support the same interface then customers

can focus on the implementation that best meets theirown needs without having to choose among differentinterfaces.


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Are Standards Good? -- Bad

Points Standards can stifle creativity - system

implementers may be prevented from

providing the best solution because thestandard prescribes some alternative.

SQL has some flaws in its design, some of

which are considered severe.


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SQL Overview

SQL is used to define, manipulate, and

control data in relational databases.

A relational database is one which is

perceived as a collection of tables by the

user.

A table is an unordered collection of rows.


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A Suppliers-and-Part Database

Example Tables

S : suppliers

P : parts (and where to ship an order)

SP : shipment of parts by suppliers


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Table S

S1 Smith 20 London

S2 Jones 10 Paris

S3 Blake 30 Paris

S4 Clark 20 London

SNO SNAME STATUS CITY

Primary Key = SNO


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Table P

P1 Nut Red 12 London

P2 Bolt Green 17 Paris

P3 Screw Blue 17 Rome

P4 Screw Red 14 London

PNO PNAME COLOUR WEIGHT CITY

Primary Key = PNO


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Table SP

S1 P1 300

S1 P2 200

S2 P1 300

S3 P2 200

S4 P4 400

SNO PNO QTY

Primary Key = SNO andPNO


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Tables

Tables can be thought of as files, with rowsrepresenting records and the columns as fields.

The SQL standard always uses the terms table,row and column.

SQL statements can be invoked eitherinteractively or from within an application.

Interactive SQL generally displays the results on thescreen.

Invocation from a program means the results are madeavailable as input to the program.


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The Select Statement

SQL Query:

SELECT S.CITY

FROM SWHERE S.SNO = 'S4'

Result:

CITY-------

London


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SELECT

SELECT is used for data retrieval.

FROM indicates the table from which to retrieve

the data. WHERE is used to describe column features that

are desired in the retrieved data.

No WHERE statement will cause all rows to bereturned.

You can use all of the standard comparisons(=,=).

Literal strings must appear in single quotes.


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Creating Tables

Empty tables are constructed using theCREATE TABLE statement.

Data must be entered later using INSERT.CREATE TABLE S ( SNO CHAR(5),

SNAME CHAR(20),

STATUS DECIMAL(3),

CITY CHAR(15),

PRIMARY KEY (SNO) )


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Creating Tables

A table name and unique column namesmust be specified.

Columns which are defined as primary keyswill never have two rows with the same keyvalue.

Primary key may consist of more than onecolumn (values unique in combination)called composite key.


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Creating Tables

CREATE TABLE SP ( SNO CHAR(5),

PNO CHAR(5),

QTY DECIMAL(5),

PRIMARY KEY (SNO,PNO),

FOREIGN KEY (SNO) REFERENCES S,

FOREIGN KEY (PNO) REFERENCES P )

Foreign keys refer to other tables.

Any key in SP.SNO must also appear in S.SNO andany key in SP.PNO must appear in P.PNO - a shipmentcannot exist unless the supplier and part also exist.


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Data Manipulation

There are four basic SQL data manipulation

operations.

SELECT - retrieves data

INSERT - add a new row

UPDATE - change values in existing records

DELETE - remove row(s)


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INSERT

INSERT

INTO SP ( SNO, PNO, QTY )

VALUES ( 'S4', 'P1', 1000 )

Row added to Table SP.


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UPDATE

UPDATE S

SET STATUS = 2 * S.STATUS

WHERE S.CITY = 'London'

Status doubled for suppliers in London (S1

and S4)


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DELETE

DELETE

FROM P

WHERE P.WEIGHT > 15

Rows deleted from P where WEIGHT > 15

(P2 and P3)


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SELECT

SELECT has the general form

SELECT-FROM-WHERE.

The result is another (new) table.


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SELECT

SELECT DISTINCT

P.COLOUR, P.CITY

FROM P

WHERE P.WEIGHT > 10

AND P.CITY 'Paris'

results in the table,

COLOUR CITY

----------------

Red London

Blue Rome

[ Red London ] - eliminated

because of

DISTINCT

statement whichremoves multiple copies of rows


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SELECT

DISTINCT - no duplicate rows.

No WHERE - all rows of FROM table are

returned.

SELECT * is short for select the entire row

(all columns).

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