Querying multiple tables pertemuan 4
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Querying Multiple Tables Pertemuan 4. Matakuliah: T0413/Current Popular IT II Tahun: 2007. AGENDA: •Joining Tables •Joining a Table to Itself •More Complex Uses of Joins. Book: Mastering SQL by Martin Gruber Sybex (2000) Chapter : 10. Joining Tables.

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Querying Multiple Tables Pertemuan 4

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Querying multiple tables pertemuan 4

Querying Multiple TablesPertemuan 4

Matakuliah: T0413/Current Popular IT II

Tahun: 2007


Agenda joining tables joining a table to itself more complex uses of joins

AGENDA:•Joining Tables•Joining a Table to Itself•More Complex Uses of Joins

Book:

Mastering SQL by Martin Gruber

Sybex (2000)

Chapter : 10


Joining tables

Joining Tables

  • Ability to define relationships between multiple tables and draw information from the tables in terms of these relationships, all within a single statement  using JOIN.

  • Table and Column Names:

    • Consists of table name followed by a dot and then the column name.

    • Example: Salespeople.snum

      Customers.city

      Orders.odate


Joining tables cont d

Joining Tables (cont’d)

  • Making a Natural Join

    • They express relationships that already exist in the structure of the database.

    • SELECT Customers.cname, Salespeople.sname

      FROM Customers, Salespeople

      WHERE Salespeople.snum = Customers.snum;

  • Making an Unnatural Join

    • Finding relationships based on the data content rather than the database design.

    • SELECT Customers.cname, Salespeople.sname, Salespeople.city

      FROM Customers, Salespeople

      WHERE Salespeople.city = Customers.city;


Joining tables cont d1

Joining Tables (cont’d)

  • Compound and Join Predicates

    • SELECT Customers.cname, Salespeople.sname, Salespeople.city

      FROM Customers, Salespeople

      WHERE Salespeople.city IN (‘London’, ‘Barcelona’)

      AND Salespeople.city = Customers.city;

    • Compound predicate

      • Join Predicate Salespeople.city = Customers.city

      • Conventional Predicate  Salespeople.city IN (‘London’, ‘Barcelona’)


Joining tables cont d2

Joining Tables (cont’d)

  • Equijoins and Non – equijoins

    • Joins that uses predicates based on equalities are called equijoins.

    • Most of the example in this discussion are all based on equality expressions

    • Because the conditions in the WHERE clause, for example:

      • City = ‘London’

      • Salespeople.sum = Orders.snum

    • Equijoins are the most common sort of join.


Joining tables cont d3

Joining Tables (cont’d)

  • OUTER Joins, to perform it, there are 3 approaches:

    • Using subquery with EXIST operator

    • Combine two queries using UNION operator

    • Using special operator designed for use with OUTER joins.

  • Joins of More Than Two Tables

    • SELECT sname, cname, onum

      FROM Salespeople, Customers, Orders

      WHERE Orders, snum = Salespeople.snum

      AND Orders.cnum = Customers.cnum;

    • SELECT onum, cname, Orders.cnum, Orders.snum

      FROM Salespeople, Customers, Orders

      WHERE Orders.snum = Salespeople.snum AND Orders.cnum = Customers.cnum

      AND Customers.city <> Salespeople.city;


Joining a table to itself

Joining a Table to Itself

  • Joining table to itself, it’s similar to joining it to another table, but the tables are identical. The table are not being duplicated/copied.

  • Aliases

    • To differentiate between copies of a single table

    • To assign alternate names for tables and/or columns in a statement

    • Example:

      SELECT first.cname, second.cname, first.rating

      FROM Customers first, Customers second

      WHERE first.rating = second.rating

      AND first.cname < second.cname;


More complex uses of joins

More Complex Uses of Joins

  • Finding patterns in the data

  • Joining tables that are not used in the output

  • Special join operators

    • CROSS JOINs

      • Equivalent to a join without a join predicate, straight Cartesian product.

      • SELECT Salespeople.snum, Customers.city

        FROM Salespeople CROSS JOIN Customers;

    • NATURAL JOINs

      • Joins based on matching foreign key values to parent key values.

      • SELECT a.snum, a.sname, b.cnum, b.amt

        FROM Salespeople a NATURAL JOIN Orders b;

        Equal to :

        SELECT a.snum, a.sname, b.cnum, b.amt

        FROM Salespeople a, Orders b WHERE a.snum = b.snum;


More complex uses of joins cont d

More Complex Uses of Joins (cont’d)

  • Specified Joins (ON and USING)

    • Specified joins are joins that directly specify by creating a predicate that states how the join is to be done (the ON form) or by listing a set of columns that are to be equijoined (the USING form).

    • SELECT Customers.cname, Salespeople.sname, Salespeople.city

      FROM Salespeople, Customers

      ON Salespeople.city = Customers.city

      WHERE Salespeople.city IN (‘London’, ‘Barcelona’);

    • SELECT Customers.cname, Salespeople.sname, Salespeople.city

      FROM Salespeople, Customers

      USING (city)

      WHERE Salespeople.city IN (‘London’, ‘Barcelona’);


More complex uses of joins cont d1

More Complex Uses of Joins (cont’d)

  • UNION JOINs

    • SELECT a.snum, a.sname, b.cnum, b.cname

      FROM Salespeople a UNION JOIN Customers b

      WHERE a.city = ‘London’ OR b.city = ‘London’;

  • INNER JOINs

    • It is a join that excludes no unmatched rows from either table. Opposite of OUTER JOINs.

    • Syntax:

      table A [NATURAL] [INNER] JOIN table B

      [{ON predicate} | {USING column list}];

    • Example:

      SELECT onum, odate, amt, o.snum, sname

      FROM Salespeople s NATURAL INNER JOIN Orders o;


More complex uses of joins cont d2

More Complex Uses of Joins (cont’d)

  • OUTER JOINs

    • Join that includes unmatched rows from either or both of the joined tables.

    • Say that the first table name is table A, and the second table name is table B.

    • LEFT OUTER JOINs

      It includes all rows from table A, matched or not, plus the matching values from table B if applicable.

    • RIGHT OUTER JOINs

      It includes all rows from table B, matched or not, plus the matching values from table A if applicable.

    • FULL OUTER JOINs

      It is a combination of LEFT and RIGHT OUTER. All rows from both tables are shown, merged where matches are found.


Querying multiple tables pertemuan 4

End of

Querying Multiple Tables

Thank you


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