1 structured query language (sql) cis*2450 advanced programming concepts
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Structured Query Language
• SQL consists of a set of commands for defining, accessing, and managing relational databases.
• In 1970, E.F. Codd at IBM Research Labs in San Jose, California published “A Relational Model of Data for Large Shared Data Banks” (Communications of the ACM, Vol. 13, No. 6, June 1970) in which he described a set of abstract principles for database management - the relational model. The field of relational database technology has its origins in that paper.
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Structured Query Language
• This research described several relational languages which implement some/all of the features of the abstract relational model. One of these languages which was created in 1974-75 was the “Structured English Query Language” (SEQUEL), defined by Donald Chamberlin and others at IBM Labs. The first IBM prototype was called SEQUEL-XRM (1974-75).
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Structured Query Language
• 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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Structured Query Language
• In 1982, the American National Standards Institute (ANSI) chartered its Database Committee (X3H2) to develop a standard for a relational language. 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 an international standard by the International Organization for Standards (ISO).
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Structured Query Language
• 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 Access Group 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 good points.
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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 function
equally well on a single database if they support the same standard interface.
• Customer Choice– If products support the same interface then customers
can focus on the implementation that best meets their own needs without having to choose among different interfaces.
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Are Standards Good? -- Bad Points
• Standards can stifle creativity - system implementers may be prevented from providing “the best” solution because the standard 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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Tables
• Tables can be thought of as files, with rows representing records and the columns as fields.
• The SQL standard always uses the terms table, row and column.
• SQL statements can be invoked either interactively or from within an application.– Interactive SQL generally displays the results on the
screen.– Invocation from a program means the results are made
available as input to the program.
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The Select Statement
• SQL Query:SELECT S.CITYFROM 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 be
returned. – 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 the CREATE 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 names must be specified.
• Columns which are defined as primary keys will never have two rows with the same key value.
• Primary key may consist of more than one column (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 and
any key in SP.PNO must appear in P.PNO - a shipment cannot 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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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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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 which
removes multiple copies of rows