1. © Abdou Illia MIS 4200 - Spring 2014
Using SQL Queries to Insert, Update, Delete, and View Data: Joining Multiple Tables
Monday 2/24/2014
© Abdou Illia MIS Spring 2014

2. Lesson C Objectives
After completing this lesson, you should be able to:
Create SQL queries that join multiple tables
Create nested SQL queries
Combine query results using SET operators
Create and use database views

3. 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_last
FROM student, faculty
WHERE student.f_id = faculty.f_id
AND f_last IN (‘Marx’, ‘Zhulin’);

4. 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_id
Join condition
Specifies table names to be joined and column names on which to join tables
Example: WHERE student.f_id = faculty.f_id

5. FROM Student NATURAL JOIN faculty;
Inner Joins
Simplest type of join
Also called: Equality join, Equijoin, Natural join
VALUES in one table equal to values in other table
Query design diagram helps get the query right
SELECT s_id, s_last, s_first, student.f_id, f_last
FROM student, faculty
WHERE student.f_id = faculty.f_id;
Could be replaced by:
FROM Student NATURAL JOIN faculty;

6. Display column, search column, join column
Display columns: appear in SELECT clause
Search columns: appear in search condition
Join 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_last
FROM faculty, course_section, term
WHERE faculty.f_id = course_section.f_id
AND course_section.term_id = term.term_id
AND term_desc = 'Summer 2007'; 6

7. 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, grade
FROM student, enrollment, course_section, course
WHERE student.s_id = enrollment.s_id
AND enrollment.c_sec_id = course_section.c_sec_id
AND course_section.course_no = course.course_no
AND s_last = 'Jones'
AND s_first = 'Tammy';
Search conditions

8. Outer Joins
Inner joins return row only if values exist in all joined tables
Outer joins return
all rows from one table (called inner table) and
only 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.

9. Self-join Query that joins table to itself Must create table alias
Alternate name assigned to table in query’s FROM clause
Syntax: FROM table1 alias1, table1 alias2 …

10. 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.

11. Creating Nested Queries
Q: What would happen if a subquery generated more values than the main query is expecting? 11

12. Creating subqueries that return multiple values12

13. Using Multiple Subqueries Within a Nested Query
Use AND and OR operators
To join search conditions associated with subqueries

14. 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.

15. 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 15

16. 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 16

17. 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; Or
CREATE OR REPLACE VIEW view_name

18. Removing Views DROP VIEW command Remove view from user schema Syntax
DROP VIEW view_name;