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SQL Complex Joins

& ANSI ISO 1999 Join Syntax

Northern Arizona University

College of Business

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Queries Using Multiple Aliases for a Table

The query processing software of a relational DBMS is designed

to take only a single pass through each table appearing in a

select statement.

If multiple passes are needed, we may use a sub select

the first sub select example we used was an example of this. We

went through the Flight table once to determine an average fare.

Then the outer select looked through the flight table again

comparing individual rows to this average.

Alternatively, we may include the same table more than once ina select statement by using different aliases.

When multiple aliases for a table are created each is treated as a

separate table when the select statement is executed and we can,

in effect, search through the same table more than once.

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Multiple table Alias Example -

The Dual Relationship Between Airports and Flights The airline database provides a classic example of a case

where multiple aliases for a table are needed. There is a dual

relationship between Airport and Flight. That is, each Flight has

a relationship to two different Airports, one that it comes form,

and one that is going to. Note that a standard where clause to join the Airport and Flight

tables would not work here. There is no single row of the Airport

table which matches both origin and destination.

What is needed is two copies of the airport table, one

associated with the origin airport and the other associated withthe destination airport.

The next slide shows this for a query where we wanted the

airport code and full airport name for both the origin and

destination airport.

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Select with Multiple Copies of the Airport Table

1 select flight_no, orig, oa.air_location as orig_airport_name,

2 dest, da.air_location as dest_airport_name

3 from airport oa, airport da, flight f

4 where oa.air_code = orig

5* and da.air_code = dest

FLIGHT_NO ORI ORIG_AIRPORT_NAME DES DEST_AIRPORT_NAME

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

101 FLG Flagstaff AZ PHX Phoenix AZ

102 PHX Phoenix AZ MPS Minneapolis/St Paul MN

103 MPS Minneapolis/St Paul MN PHX Phoenix AZ

104 PHX Phoenix AZ FLG Flagstaff AZ

15 PHX Phoenix AZ LAX Los Angeles CA



. . . . .

The select statement below creates the

logical relationships shown to the right.

The 2 copies of the AIRPORT table

appear only in temporary storage as the

select statement is executed. Physical

storage of the AIRPORT table in the database

is unaffected.

AIRPORT

OAAIRPORT

DA

FLIGHT

origin destination

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supervised by >

Using Multiple Table Aliases -

An Intra-table Relationship Example

Multiple copies of a table are also often needed when a table contains

an intra-table relationship (some rows of the table have a relationship to

other rows of the same table).

The example below shows an employee table with this type ofrelationship. The Sup_ID column is used to link an employees row to

the row of the employees supervisor.

An employee is supervised by at most one other employee, while a

supervisory employee can supervise more than one employees

E_ID E_NAME HIRE_DATE SALARY SUP_ID

---- --------------- --------- --------- ----5678 Gates 10-JAN-91 80000000

3456 Jones 12-FEB-03 80000 5678

4567 Smythe 18-MAR-98 75000 5678

1234 Adams 14-OCT-04 32500 3456

2345 Bates 12-DEC-99 35000 3456

6789 Lewis 12-NOV-02 38000 4567

7890 Earle 08-OCT-02 38000 4567

EMPLOYEE

< supervises

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Intra-Table Relationship Query Example

Based on the table on the previous slide, to retrieve

information about each employees supervisor,

their name for example, along with information

about the employee, we would need to create an

alias (S) for the employee table to search for rows

corresponding to the supervisor of the employeefound in the original (E) copy of the table.

Employee

S

Employee

E

1 select e.e_id, e.e_name, s.e_id as super_id, s.e_name as super_name

2 from employee e, employee s

3* where e.sup_id = s.e_id

E_ID E_NAME SUPER SUPER_NAME---- --------------- _ID ---------------

3456 Jones 5678 Gates

4567 Smythe 5678 Gates

1234 Adams 3456 Jones

2345 Bates 3456 Jones

6789 Lewis 4567 Smythe

7890 Earle 4567 Smythe

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In-line View or Derived Table

in the FROM

Clause

Oracle (and IBMs DB2) now support the use

of in-line views

An in-line view is just a sub-select that isplaced in the FROM tables component of a

query and given an alias

Sub-select is performed first and its results

treated as a temporary table when executing

the outer select

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In-line view example

select flight_no, orig, f.dest, fare, meal fromflight f, (select dest, min(fare) as min_fare from flight

where orig = 'PHX' group by dest) fm

where orig = 'PHX'

and f.dest = fm.destand fare = fm.min_fare

DEST MIN_FARE

LAX 49

SFO 109

FLG 48.5

MPS 156FLIGHT_NO ORIG DEST FARE MEAL

102 PHX MPS 156 L

104 PHX FLG 48.5 S

15 PHX LAX 49 B31 PHX LAX 49 S

33 PHX LAX 49 S

35 PHX LAX 49 S

40 PHX LAX 49

600 PHX SFO 109 B

604 PHX SFO 109 B

606 PHX SFO 109 L

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ANSI ISO Join Operators

The ANSI standard for SQL was

modified in 1999 to provide support for

alternative syntax for a number of SQL

statement clauses

Oracle implemented most of these

extensions beginning with version 9i(we are using version 10g)

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ANSI ISO Join Available in 9i+

FROM table1 JOIN table2ONjoin_condition

Replaces the join condition in the WHERE

clause as well as the FROM E.g. FROM flight f JOIN ticket t ON f.flight_no =

t.flight_no

Use of table aliasing to resolve column

names is unchanged E.G Select f.flight_no, orig, flight_date

from flight f join ticket t

on f.flight_no = t.flight_no

where orig = PHX;

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Multi-table joins using ON Clause

Add additional table name and onclause

in sequence. E.g.SELECT *

FROM flight f join ticket t on f.flight_no = t.flight_no

join passenger p on p.itinerary_no=t.itinerary_no

where f.flight_no = 101;

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ANSI ISO Natural JOIN

When Performing an equi-join on tables havingexactly one common column (the same name and data type and length

in each table)

Can use the phrase NATURAL JOIN and skip the joincondition E.g. Select *

FROM flight f natural join ticket t;

Or - for multi-table joins

Select flight_no, orig, dest, flight_date, itinerary_no,

Pass_namefrom flight natural join (ticket natural join passenger)

NOTES: aliases not need for the key columns,

primary keys of all tables must be included in column listfor multi-table joins

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Equi - Join Concept

The joins we have used thus fare areequi-joins, an equi-join

Produces a row of output only when a

match value for the condition is found ineach table.

But what if there are rows in one of thetables that have no matching values in the

related table? When an equi-join is used that unmatched

row of data is not included in the output.

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Outer - Join Concept

An outer join

Produces a row of output when a matching value

for the condition tested is found in each table.

Includes one row for each unmatched row of datain the outer joined table leaving any attributes from

the unmatched table blank.

Traditional Oracle outer join syntax

Places (+) next to the side of a join condition thatis allowed to be null (have no matching rows)

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Outer join Example

select pass_name, f_name || ' ' || l_name as ff_name, cur_year_miles

from passenger p, frequent_flyer ff

where p.ff_no = ff.ff_no (+) ;

SAMPLE RESULTS

PASS_NAME FF_NAME CUR_YEAR_MILES

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

ALICE ELLIS Alice Ellis 11200

HOMER ELLIS Alice Ellis 11200

WILL GATES Tip Felt 0

TIP FELT Tip Felt 0

DAVID PETERSON David Peterson 8100

GLORIA ANDERSON

ANDY ANDERSON

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OUTER JOINS ANSI-ISO

SELECT

col_listFROM table1 LEFTOUTER JOIN table2

ONjoin_condition

Also a RIGHTOUTER JOIN and a FULL OUTER

JO

IN E.G Select f.flight_no, orig, flight_date

from flight fleft outer join ticket t

on f.flight_no = t.flight_no

where orig = PHX; Parallel structure for RIGHTOUTER JOIN

FULL OUTER JOIN puts a row in the output if either table has

an unmatched value and places nulls in the fields from the

omitted table

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ASI Syntax Outer Join Example

select pass_name, f_name || ' ' || l_name as ff_name, cur_year_miles

from passenger p left outer join frequent_flyer ff

on p.ff_no = ff.ff_no;

SAMPLE RESULTS

PASS_NAME FF_NAME CUR_YEAR_MILES

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

ALICE ELLIS Alice Ellis 11200

HOMER ELLIS Alice Ellis 11200

WILL GATES Tip Felt 0

TIP FELT Tip Felt 0

DAVID PETERSON David Peterson 8100

GLORIA ANDERSONANDY ANDERSON

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