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Structured Query Language (SQL)

1. Computing a Join

2. Outer Join3. Exist and Not exist

4. Combining the result tables: Union, Intersect Except

5. Adding data to the database

6. Updating data in the database7. Deleting data in the database

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Computing a Join

A join is a subset ofcartesian product

The Cartesian product oftwo tables is another table consisting ofall

possible pairs ofrows from the two tables

The columns ofthe product table are all the columns ofthe first table

followed by all the columns ofthe second table.

Ifwe specify a two-table query without a WHERE clause, SQL produces the

Cartesian product ofthe two tables as the query result

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Outer Join

The join operation combines data from two tables by forming pairs of

related rows where the matching columns in each table have the same

value. Ifone row ofa table is unmatched, the row is omitted from the

result table.

The Outer join retains rows that do not satisfy the join condition

Ifwe want to include the unmatched rows in the result table, we can use

an Outer join.

There are three types ofOuter join:

Left Outer Join

Right Outer Join

Full Outer Join

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Left Outer Join

Example List all branch offices and any properties that are in the

same city.

Query SELECT b.*, p.*

FROM Branch1 b LEFT JOIN PropertyForRent1 p

ON b.bCity = p.pCity;

Outcome

SELECT b.*, p.*

FROM Branch1 b, PropertyForRent1 p

WHERE b.bCity = p.pCity;

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Right Outer Join

Example List all properties and any branch offices that are in the

same city.

Query SELECT b.*, p.*

FROM Branch1 b RIGHT JOIN PropertyForRent1 p

ON b.bCity = p.pCity;Outcome

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Full Outer join

Example List the branch offices and properties that are in the same

city along with any unmatched branches or properties.

Query SELECT b.*, p.*

FROM Branch1 b FULL JOIN PropertyForRent1 p

ON b.bCity = p.pCity;

Outcome

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EXISTS and NOT EXISTS

The keywords EXISTS and NOT EXISTS are designed for use only with

subqueries

They produce a simple true/false result

EXISTS is true ifand only ifthere exists at least one row in the result table

returned by the subquery

It is false ifthe subquery returns an empty result table

NOT EXISTS is the opposite ofEXISTS. Since EXISTS and NOT EXISTS check

only for the existence or non-existence ofrows in the subquery result

table the subquery can contain any number ofcolumns

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Example Find all staff who work in a London branch office

Query SELECT staffNo, fName, lName, position

FROM Staff s

WHERE EXISTS (SELECT *

FROM Branch b

WHERE s.branchNo = b.branchNo

AND city = London

);

Outcome

This query could be rephrased as Find all staffsuch that there exists aBranch row containing his/her branch number, branchNo, and the branchcity equal to London. The test for inclusion is the existence ofsuch a row.Ifit exists, the subquery evaluates to true

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Combining Result Tables (UNION, INTERSECT, EXCEPT)

In SQL, we can use the normal set operations ofUnion, Intersection, and

Difference to combine the results oftwoor more queries into a singleresult table:

The Union oftwo tables, A and B, is a table containing all rows that are in

either the first table A or the second table B or both.

The Intersection oftwo tables, A and B, is a table containing all rows that are

common to both tables A and B. The Difference oftwo tables, A and B, is a table containing all rows that are in

table A but are not in table B.

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Use ofUNION

Example Construct a list of all cities where there is either a branch

office or a property. Query (SELECT city

FROM Branch

WHERE city IS NOT NULL )

UNION

(SELECT cityFROM PropertyForRent

WHERE city IS NOT NULL)

Result

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Use ofINTERSECT

Example Construct a list of all cities where there is both a branch

office and a property. Query (SELECT city

FROM Branch

WHERE city IS NOT NULL )

INTERSECT

(SELECT cityFROM PropertyForRent

WHERE city IS NOT NULL)

Result

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Use ofEXCEPT

Example Construct a list of all cities where there is a branch office

but no properties. Query (SELECT city

FROM Branch

WHERE city IS NOT NULL )

EXCEPT

(SELECT cityFROM PropertyForRent

WHERE city IS NOT NULL)

Result

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Database Updates

SQL is a complete data manipulation language that can be used for

modifying the data in the database as well as querying the database.

The commands for modifying the database are not as complex as the

SELECT statement.

SQL statements that modify the contents ofthe tables in the database:

INSERT adds new rows ofdata to a table;

UPDATE modifies existing data in a table;

DELETE removes rows ofdata from a table.

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Adding data to the database (INSERT)

There are twoforms ofthe INSERT statement.

1-Single row 2-Multiple rows

Single row format: INSERT INTO TableName [(columnList)]

VALUES (dataValueList)

Example INSERT INTO Staff

VALUES (SG16, Alan, Brown, Assistant, M, DATE 1957-05-25, 8300, B003);

TableName may be either a base table or an updatable view (see Section 6.4), and

columnList represents a list ofone or more column names separated by commas.

The columnList is optional; ifomitted, SQL assumes a list ofall columns in their originalCREATE TABLE order.

Ifspecified, then any columns that are omitted from the list must have been declared

as NULL columns when the table was created, unless the DEFAULT option was used

when creating the column

The dataValueList must match the columnListas follows: the number ofitems in each list must be the same;

there must be a direct correspondence in the position ofitems in the two lists, so that the firstitem in dataValueList applies to the first item in columnList, the second item in dataValueListapplies to the second item in columnList, and so on

the data type ofeach item in dataValueList must be compatible with the data type of thecorresponding column.

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INSERT . . . VALUES

INSERT INTO Staff

VALUES (SG16, Alan, Brown, Assistant, M, DATE 1957-05-25,

8300,B003);

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INSERT using defaults

INSERT INTO Staff (staffNo, fName, lName, position, salary, branchNo)

VALUES (SG44, Anne, Jones, Assistant, 8100, B003);

INSERT INTO Staff

VALUES (SG44, Anne, Jones, Assistant, NULL, NULL, 8100, B003);

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INSERT . . . SELECT

INSERT INTO StaffPropCount

(SELECT s.staffNo, fName, lName, COUNT(*)

FROM Staff s, PropertyForRent p

WHERE s.staffNo = p.staffNo

GROUP BY s.staffNo, fName, lName)

UNION(SELECT staffNo, fName, lName, 0

FROM Staff s

WHERE NOT EXISTS (SELECT *

FROM PropertyForRent p

WHERE p.staffNo = s.staffNo));

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Modifying data in the database (UPDATE)

The UPDATE statement allows the contents ofexisting rows in a named

table to be changed.

The format ofthe command is:

UPDATE TableName

SET columnName1 = dataValue1 [, columnName2 = dataValue2 . . . ]

[WHERE searchCondition]

TableName can be the name of a base table or an updatable view

The SET clause specifies the names ofone or more columns that are to be

updated.

The WHERE clause is optional; ifomitted, the named columns are

updated for all rows in the table.

Ifa WHERE clause is specified, only those rows that satisfy the

searchCondition are updated.

The new dataValue(s) must be compatible with the data type(s) for the

corresponding column(s).

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UPDATE all rows UPDATE Staff

SET salary = salary*1.03;

UPDATE specific rows UPDATE Staff

SET salary = salary*1.05

WHERE position = Manager;

UPDATE multiple columns UPDATE Staff

SET position = Manager, salary = 18000

WHERE staffNo = SG14; ERE position = Manager;