Using SQL Queries to Insert, Update, Delete, and View Data: Joining Multiple Tables Wednesay 9/24/2014 Abdou Illia MIS 4200 - Fall 2014 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 2 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 ); 3 1
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 4 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; 5 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 6 AND term_desc = 'Summer 2007'; 2
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 7 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. 8 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 9 3
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. 10 Creating Nested Queries Q: What would happen if a subquery generated more values than the main query is expecting? 11 Creating subqueries that return multiple values 12 4
Using Multiple Subqueries Within a Nested Query Use AND and OR operators To join search conditions associated with subqueries 13 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. 14 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 5
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 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 AS source_query; 17 Removing Views DROP VIEW command Remove view from user schema Syntax DROP VIEW view_name; 18 6