Step 0 How to begin and what you need to do before you start?

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1 Written and cooked by Janusz R. Getta, School of Computing and Information Technology, University of Wollongong Building 3, room 2120, ext 4339, jrg ËÁÌ½½ Ø Å Ò Ñ ÒØ Ò Ë ÙÖ ØÝ Ê Ô º ÀÓÛ ØÓ ÑÔÐ Ñ ÒØ ÑÔÐ Ó Ò ÕÙ Ö Table of contents Step 0 How to begin and what you need to do before you start? Step 1 How to create a simple join query? Step 2 How to create a simple join query with JOIN operation? Step 3 How to create a simple join query with USING clause? Step 4 How to create a simple join query with NATURAL JOIN operation? Step 5 How to create a join query with a composite join condition? Step 6 How to create a join query with a composite join condition and JOIN operation? Step 7 How to create a join query with a composite join condition and USING clause? Step 8 How to create a join query that joins three relational tables? Step 9 How to create a join query that joins three relational tables with JOIN operation? Step 10 How to create join query that joins three relational tables with USING clause? Step 11 How to create a non-equality join query? Step 12 How to create a non-equality join query with JOIN operation? Step 13 How to create a non-equality join query with USING clause? Step 0 How to begin and what you need to do before you start? A printable copy of this experiment in pdf format is available here ( e5-4.pdf). Logon to your Win 7 system and start VirtualBox. Next, either import your appliance from your USB drive and start imported virtual machine or use one of the brand new virtual machines available on your Win 7 system. Logon to Ubuntu operating system as csit115 user. Start Terminal icon to open Terminal application. We use command line interface to communicate with mysql database server. Next, create a new folder to save SQL scripts used in the present experiment and make the folder as the current folder of csit115 user. Then start start a command based interface to MySQL database server and to logon as a user csit115. Next, connect to a database csit115. Finally, download and uncompress SQL scripts in zip format (www. uow.edu.au/~jrg/115/howto/home5.zip) used in this experiment. The scripts should be uncompressed into the folder created earlier. Next, execute a script file dbcreate5-4.sql ( to create a sample relational database. To remove the database from your account you may use a script dbdrop5-4.sql ( HOWTO/dbdrop5-4.sql) Do not execute the script now! A conceptual schema of the sample database is available here ( schema5.pdf). 1

2 It is strongly recommended to save all SELECT statements implemented in this lab into SQL script files to record the results of your work. The scripts could be reused in the solutions of the assignments. A decision whether SELECT statement is correct or not is an interesting problem. How do we know, that SELECT statement implements the same query as expressed in a natural language? Sometimes execution of SELECT statement produces an enigmatic reply: no rows selected i.e. an empty answer. Does it mean that the results are correct or incorrect? If we expect a nonempty answer then it is a signal that something could be wrong. The examples of SELECT statement presented in this and the other experiments do not produce an empty result! Ofcoursenotineverycaseanyemptyanswermeansanincorrectquery, e.g. aquerywithaconditionemployee_age > 500 always returns an empty result (unless a database is corrupted). A simple method to verify correctness of SELECT statement is to run a query on a sample database that that contains the rows we are looking for. Then, if we all we are looking for is retrieved then SELECT statement is usually correct. Another method is to modify the constants in a search condition and observe how the results change from one query to another query. Yet another method is to decompose a complex query into the sub-queries and check correctness of the individual subqueries. Step 1 How to create a simple join query? This action implements a query, that finds the budgets and names of departments that have at least one location in CHICAGO. The query accesses the values of an attribute CITY in DEPTLOC table and BUDGET in DEPARTMENT table. To bring these values together we have to join the rows from DEPARTMENTtable with the rows from DEPTLOC table. We start form an old fashioned implementation of join operation. A script joinold.sql ( contains the following SELECT statement. SELECT DEPARTMENT.DNAME, BUDGET FROM DEPTLOC, DEPARTMENT WHERE DEPTLOC.DNAME = DEPARTMENT.DNAME AND CITY = CHICAGO ; Execute the script file and verify the results. FROM clause contains the names of tables involved in a query. A condition DEPTLOC.DNAME = DEPARTMENT.DNAME means that we are interested in all combinations of rows from DEPTLOC and DEPARTMENT tables such that a value of attribute DNAME in a row from DEPTLOC table is equal to a value of attribute DNAME in DEPARTMENT table. We have to qualify a name of attribute DNAME with a name of relational table DEPARTMENT to avoid ambiguity because table DEPTLOC has an attribute with the same name. Step 2 How to create a simple join query with JOIN operation? The same query implemented in a more modern way uses JOIN operation in FROM clause. A script file joinop.sql ( contains the following SELECT statement. SELECT DEPARTMENT.DNAME, BUDGET FROM DEPTLOC JOIN DEPARTMENT ON DEPTLOC.DNAME = DEPARTMENT.DNAME WHERE CITY = CHICAGO ; 2

3 SELECT statement in step 2 is equivalent to SELECT statement in step 1. In both cases the tables DEPTLOC and DEPARTMENT are joined over an attribute DNAME. A statement in step 2 explicitly applies JOIN operation in FROM clause. ON clause in the same statement determines a join condition. Finally, WHERE clause filters the rows. If a join condition is an equality condition then it is possible to further compress SELECT statement listed above. Step 3 How to create a simple join query with USING clause? This action implements the same query as in two previous actions. This time join condition is replaced with USING clause. A script file joinusing.sql ( contains the following SE- LECT statement. SELECT DNAME, BUDGET FROM DEPTLOC JOIN DEPARTMENT USING (DNAME) WHERE CITY = CHICAGO ; USING clause determines the names of attributes in a join condition like ON clause in the previous action. Additionally, USING clause assumes equality as a comparison operation in a join condition. Application of USING clause is possible when the names of attributes used in a join condition are the same, and equality is a comparison operation. If join is performed on more than one attribute all elementary join conditions must be glued with AND operation. If join is performed over all pairs of the same attributes in both tables then it is still possible to compress SELECT statement given above. Step 4 How to create a simple join query with NATURAL JOIN operation? This action implements the same query as in three previous actions. A join condition is represented as NATURAL JOIN operation. A script file joinnat.sql ( contains the following SELECT statement. SELECT DNAME, BUDGET FROM DEPTLOC NATURAL JOIN DEPARTMENT WHERE CITY = CHICAGO ; A natural join operation assumes that join is performed over all pairs of identical attributes from both tables and that equality is used as a comparison operation and that elementary join conditions are glued with AND operation. 3

4 Step 5 How to create a join query with a composite join condition? This action implements SELECT statement, that joins two relational tables over more than one attribute. It implements a query, that finds the names of all departments located at the same location( CITY, STREET, BLDG, LEVEL) as location of employee with number A file joinloc1.sql ( contains the following statement: SELECT DISTINCT DEPTLOC.DNAME FROM DEPTLOC, EMPLOYEE WHERE DEPTLOC.CITY = EMPLOYEE.CITY AND DEPTLOC.STREET = EMPLOYEE.STREET AND DEPTLOC.BLDG = EMPLOYEE.BLDG AND DEPTLOC.BLEVEL = EMPLOYEE.BLEVEL AND ENUM = 7782; Step 6 How to create a join query with a composite join condition and JOIN operation? This action uses JOIN operation to implement the same query as above. A script joinloc2.sql ( contains the following statement. SELECT DISTINCT DEPTLOC.DNAME FROM DEPTLOC JOIN EMPLOYEE ON DEPTLOC.CITY = EMPLOYEE.CITY AND DEPTLOC.STREET = EMPLOYEE.STREET AND DEPTLOC.BLDG = EMPLOYEE.BLDG AND DEPTLOC.BLEVEL = EMPLOYEE.BLEVEL WHERE ENUM = 7782; Step 7 How to create a join query with a composite join condition and USING clause? This action uses JOIN operation with USING clause to implement the same query as above. A script joinloc3.sql ( contains the following statement. SELECT DISTINCT DEPTLOC.DNAME FROM DEPTLOC JOIN EMPLOYEE USING (CITY, STREET, BLDG, BLEVEL) WHERE ENUM = 7782; 4

5 It is not possible to express a query above with NATURAL JOIN operation because we cannot use DNAME attribute in a join condition. Step 8 How to create a join query that joins three relational tables? A query implemented in this action joins three relational tables. It finds the names of all cities that host at least one department that has BUDGET < 2000 and the same department has at least one employee with SALARY > A script join3.sql ( contains the following SELECT statement. SELECT DISTINCT DEPTLOC.CITY FROM EMPLOYEE, DEPARTMENT, DEPTLOC WHERE EMPLOYEE.DNAME = DEPARTMENT.DNAME AND DEPARTMENT.DNAME = DEPTLOC.DNAME AND EMPLOYEE.SALARY > 1000 AND DEPARTMENT.BUDGET < 2000; Step 9 How to create a join query that joins three relational tables with JOIN operation? This action implements the same query using JOIN operation. A script file join3join.sql ( contains the following statement. SELECT DISTINCT DEPTLOC.CITY FROM EMPLOYEE JOIN DEPARTMENT ON EMPLOYEE.DNAME = DEPARTMENT.DNAME JOIN DEPTLOC ON DEPARTMENT.DNAME = DEPTLOC.DNAME WHERE EMPLOYEE.SALARY > 1000 AND DEPARTMENT.BUDGET < 2000; Step 10 How to create join query that joins three relational tables with USING clause? This action implements the same query with JOIN operation in FROM clause and USING clause. A script file join3using.sql ( contains the following statement: SELECT DISTINCT DEPTLOC.CITY FROM EMPLOYEE JOIN DEPARTMENT USING (DNAME) JOIN DEPTLOC USING (DNAME) WHERE EMPLOYEE.SALARY > 1000 AND DEPARTMENT.BUDGET < 2000; 5

6 Step 11 How to create a non-equality join query? This action implements SELECT statement with non-equality join condition. It implements a query, that finds the numbers and names of all employees such that their salary is higher than 10 * BUDGET of the department they are working at. A script joinnoteq.sql ( contains the following statement: SELECT ENUM, ENAME FROM EMPLOYEE, DEPARTMENT WHERE EMPLOYEE.DNAME = DEPARTMENT. DNAME AND EMPLOYEE.SALARY > 10 * BUDGET; Step 12 How to create a non-equality join query with JOIN operation? This action uses JOIN operation in FROM clause to implement the same query as above. A script file joinnoteqjoin.sql ( contains the following statement. SELECT ENUM, ENAME FROM EMPLOYEE JOIN DEPARTMENT ON EMPLOYEE.DNAME = DEPARTMENT. DNAME AND EMPLOYEE.SALARY > 10 * BUDGET; Step 13 How to create a non-equality join query with USING clause? This action implements the same query as above using SELECT statement with JOIN operation in FROM clause and USING clause. A script file joinnotequsing.sql ( contains a statement SELECT ENUM, ENAME FROM EMPLOYEE JOIN DEPARTMENT USING (DNAME) WHERE EMPLOYEE.SALARY > 10 * BUDGET; References MySQL 5.7 Reference Manual, SELECT statement ( 6

Step 0 How to begin and what you need to do before you start?

Step 0 How to begin and what you need to do before you start? Written and cooked by Janusz R. Getta, School of Computing and Information Technology, University of Wollongong Building 3, room 2120, ext 4339, jrg@uow.edu.au, http://www.uow.edu.au/ jrg ËÁÌ½½ Ø Å Ò Ñ