Using SQL Queries to Insert, Update, Delete, and View Data: Joining Multiple Tables © Abdou Illia...

Using SQL Queries to Insert, Update, Delete, and View Data:

Joining Multiple Tables

© Abdou Illia MIS 4200 - Spring 2014

Monday 2/24/2014

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Lesson C Objectives

After completing this lesson, you should be able to:Create SQL queries that join multiple tablesCreate nested SQL queriesCombine query results using SET operatorsCreate and use database views

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Joining Multiple Tables

a Join– Combines data from multiple tables using foreign key

references Syntax

SELECT column1, column2, …

FROM table1, table2

WHERE table1.joincolumn = table2.joincolumn

AND search_condition(s);

SELECT s_id, s_last, f_lastFROM student, facultyWHERE student.f_id = faculty.f_idAND f_last IN (‘Marx’, ‘Zhulin’);

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Joining Multiple Tables (continued)Must qualify column name in SELECT clause

– Specify name of table that contains column followed by period then column name

– Example: SELECT s_id, s_last, student.f_idJoin condition

– Specifies table names to be joined and column names on which to join tables

– Example: WHERE student.f_id = faculty.f_id

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Inner JoinsSimplest type of join Also called: Equality join, Equijoin, Natural joinVALUES in one table equal to values in other tableQuery design diagram helps get the query right

SELECT s_id, s_last, s_first, student.f_id, f_lastFROM student, facultyWHERE student.f_id = faculty.f_id;

Could be replaced by:

FROM Student NATURAL JOIN faculty;

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Display column, search column, join column

Display columns: appear in SELECT clause Search columns: appear in search conditionJoin columns: primary key and foreign key column

on which you join the tables.Linkage table: contains join column to link other

tables through foreign key values.

SELECT f_lastFROM faculty, course_section, termWHERE faculty.f_id = course_section.f_idAND course_section.term_id = term.term_idAND term_desc = 'Summer 2007';

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Deriving a SQL Query From a Query Design Diagram

4 tables, 3 links All 4 tables must be named in the

FROM clause Query must have 3 join conditions

because there are 3 links Always 1 fewer join condition than

number of tables that query joins. If you omit one join condition, the

query creates a Cartesian product (every row in one table is joined with every row in other table) with more row than expected.

SELECT course_name, gradeFROM student, enrollment, course_section, courseWHERE student.s_id = enrollment.s_idAND enrollment.c_sec_id = course_section.c_sec_idAND course_section.course_no = course.course_noAND s_last = 'Jones'AND s_first = 'Tammy';

Search conditions

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Outer JoinsInner joins return row only if values exist in all joined

tablesOuter joins return

all rows from one table (called inner table) andonly matching rows from second table (outer table)

Syntax: inner_table.join_col = outer_table.join_col(+)

(+) operator signals Oracle to insert NULL for columns from the outer table with no matching rows in the inner table.

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Self-join

Query that joins table to itselfMust create table alias

– Alternate name assigned to table in query’s FROM clause– Syntax: FROM table1 alias1, table1 alias2 …

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Creating Nested Queries Nested query

– Consists of a main query and one or more subqueries

– Main query• First query that appears in SELECT command

– Subquery • Retrieves values that main query’s search condition must match

Subquery is evaluated first. Then, DBMS substitute subquery’s output into main query.

Creating Nested Queries

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Q: What would happen if a subquery generated more values than the main query is expecting?

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Creating subqueries that return multiple values

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Using Multiple Subqueries Within a Nested Query

Use AND and OR operators – To join search conditions associated with subqueries

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Using SET operators to combine Query Results

UNION– Queries must have

same number of display column in their SELECT clause

– Corresponding display columns must have same data type

Note: S_LAST, S_FIRST, S_PHONE used as display title even though there are faculty members names displayed along with students.

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Using SET operators to combine Query Results

INTERSECT– Queries must have

same number of display column in their SELECT clause

– Corresponding display columns must have same data type

– Suppresses duplicates

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Using SET operators to combine Query Results

MINUS– Queries must have

same number of display column in their SELECT clause

– Corresponding display columns must have same data type

– Suppresses duplicates– Finds difference

between two query results

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Creating and Using Database Views Source query

– Used to create view– Specify subset of single table’s columns or rows or join

multiple tables Updatable views

– Can be used to update database Syntax

CREATE VIEW view_name AS source_query;– OrCREATE OR REPLACE VIEW view_name AS source_query;

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Removing Views

DROP VIEW command – Remove view from user schema

– Syntax•DROP VIEW view_name;