Agenda for 02/16/2006 Answer any questions about SQL project. Do you want to see any of the answers for the queries? Discuss additional formatting options available via SQLPlus. Learn how to use more than one table in a query. Discuss how DBMS processes multiple tables. Explain the different types of joins. Describe join conditions.

Sample Database ERD Assume referential integrity is NOT maintained in this database

Accessing data from multiple tables Why do you want to access data from multiple tables in a single query? To provide more complete information in a result table. To support decision making. What happens when multiple tables are accessed in a single query?

Vocabulary Words Select List Result Table Join Cartesian Product/Cross Join Join Condition Multiple Table Access SELECT * FROMemp, time;

Eliminate columns to enhance readability SELECTname, contractid, datetime, amount/60 FROMemp, time; Does this SQL code produce a reasonable response? Let’s eliminate some columns from the select list to make a more readable result table.

Combining Tables Based on a Shared Column Option 1 (older syntax - using WHERE clause) SELECTname, contractid, datetime, amount/60 FROMtime, emp WHEREtime.empid = emp.empid; The shared column is usually the foreign key that sustains the relationship between the tables on the ERD.

Combining Tables Based on a Shared Column Option 2 (newer syntax – using join condition) SELECTname, contractid, datetime, amount/60 FROMtime INNER JOINemp ONtime.empid = emp.empid

Combining tables on a shared column that has the Same name: Natural Join SELECTname, contractid, datetime, amount/60 FROMtime NATURAL JOINemp

Viewing all rows from one of the tables in a join Option 1 (older syntax using WHERE clause) Syntax for a left outer-join in Oracle: SELECTname, time.empid contractid, datetime, amount/60 FROMtime, emp WHEREtime.empid = emp.empid (+) All data in the table on the left side of the condition will be displayed because the plus sign (+) is on the right side of the condition.

Viewing all rows from one of the tables in a join Option 2 (newer syntax using join condition) SELECTname, time.empid contractid, datetime, amount/60 FROMtime LEFT OUTER JOINemp ONemp.empid = time.empid Imagine that the time table is “left” (because it is declared first) and the emp table is “right” (because it is declared second)

Viewing all rows from the other table in a join Option 1 (older syntax using WHERE clause) Syntax for a right outer-join in Oracle: SELECTname, time.empid contractid, datetime, amount/60 FROMtime, emp WHEREtime.empid (+) = emp.empid

Right outer join – Viewing all rows in other table Option 2 (newer syntax using join condition) SELECT name, time.empid, contractid, datetime, amount/60 FROM time RIGHT OUTER JOINemp ON time.empid = emp.empid ; Remember that the time table is “left” (because it is declared first) and the emp table is “right” (because it is declared second)

Full outer join – Viewing all rows in both tables Option 2 (newer syntax using join condition – option 1 is not available) SELECTname, time.empid, contractid, datetime, amount/60 FROM time FULL OUTER JOINemp ON time.empid = emp.empid;

SELECTemp.name, time.empid, time.worktypeid, work.description, datetime, amount/60 FROMtime INNER JOINemp ONtime.empid = emp.empid INNER JOINwork ONtime.worktypeid = work.worktypeid; Combining more than two tables into a single result table

Displaying all data in TIME table COLUMN emp_name heading “Employee Name” COLUMN descr heading “Type of Work” SELECT NVL(emp.name,'NOT IN EMPLOYEE TABLE') emp_name, time.empid, time.worktypeid, NVL(work.description,'NOT IN WORK TABLE’) descr, datetime, amount/60 FROMtime LEFT OUTER JOINemp ONtime.empid = emp.empid LEFT OUTER JOINwork ONtime.worktypeid = work.worktypeid ORDER BYemp.name;

Summarizing Data COLUMN descr heading “Type of Work” SELECT NVL(work.description, ‘No Description’) descr, round(sum(amount/60),2) FROM time LEFT OUTER JOIN work ON work.worktypeid = time.worktypeid GROUP BY work.description ; The GROUP BY statement is frequently used with a result table created by multiple underlying tables

Recursive Relationship with Employee to Assign Manager

Self-join also called a recursive join Option 1 – (older syntax using WHERE clause) SELECTworker.empid "worker#", worker.name "worker name", manager.empid "manager#", manager.name "manager name" FROMemp worker, emp manager WHEREworker.mgrid = manager.empid; What change is necessary to make all employees appear whether there is a manager?

Self-join also called a recursive join Option 2 – (newer syntax using join condition) SELECTworker.empid "worker#", worker.name "worker name", manager.empid "manager#", manager.name "manager name" FROMemp worker INNER JOINemp manager ONworker.mgrid = manager.empid; What change is necessary to make all employees appear whether there is a manager?