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 Ò Ø ÕÙ Ö Table of contents Step 0 How to begin and what you need to do before you start? Step 1 How to implement a nested query (1)? Step 2 How to implement a nested query with a negated set membership condition? Step 3 How to implement a nested query (2)? Step 4 How to implement a nested query with an equality condition? Step 5 How to implement a query with a nested HAVING clause? Step 6 How to implement a nested query with a multiple column condition? Step 7 How to implement a nested query? 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 ( e6-3.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/home6.zip) used in this experiment. The scripts should be uncompressed into the folder created earlier. Next, execute a script file dbcreate6-3.sql ( to create a sample relational database. To remove the database from your account you may use a script dbdrop6-3.sql ( HOWTO/dbdrop6-3.sql) Do not execute the script now! A conceptual schema of the sample database is available here ( schema6.pdf). Step 1 How to implement a nested query (1)? We start form implementation of a query, that finds the names and budgets of all departments that have at least one location in CHICAGO. 1

2 Implementation of this query with JOIN operation is the following. SELECT DEPARTMENT.DNAME, BUDGET JOIN DEPTLOC ON DEPARTMENT.DNAME = DEPTLOC.DNAME WHERE CITY = CHICAGO ; Implementation of the same query as nested SELECT statement requires a decomposition of a query into two queries. The first one finds the names of departments that have at least one location in CHICAGO(see below). Q1:SELECT DNAME WHERE CITY = CHICAGO ; The second query finds the budgets (and names) of departments found in the first query. Q2:SELECT DNAME.BUDGET WHERE DNAME IN (Q1); A script file nestchicago.sql ( contains a statement given below. SELECT DNAME, BUDGET WHERE DNAME IN (SELECT DNAME WHERE CITY = CHICAGO ); Step 2 How to implement a nested query with a negated set membership condition? Nested queries with a negated set membership condition are a bit harder to comprehend. As a simple example consider a query, that finds the names of departments with no locations in CHICAGO. A strong temptation is to implement a query as the following SELECT statement. SELECT DNAME WHERE CITY <> CHICAGO ; Unfortunately this ad hoc solution is not correct! Thisis because the query findsall locationswhere a value ofattributecityis not equaltochicago.ifadepartment has two locations, one in NEW YORK and the other in CHICAGO then its name will be included in an answer because it is associated with CITY different from CHICAGO. Such result is obviously incorrect. To implement a correct solution we start from a query, that finds the names of all departments that have at least one location in CHICAGO. Q1:SELECT DNAME 2

3 WHERE CITY = CHICAGO ; Next, we find the names of all other departments, i.e. the names of departments which are not included (NOT IN) in a set of department names found in the previous query. Q2:SELECT DNAME, BUDGET WHERE DNAME NOT IN (Q1); A script file nestnochicago.sql ( contains the following statement. SELECT DNAME, BUDGET WHERE DNAME NOT IN (SELECT DNAME WHERE CITY = CHICAGO ); Step 3 How to implement a nested query (2)? Nested query is a nice replacement for self join queries, i.e. queries that compare the rows in a relational table with the other rows in the same table. For instance, a query, that finds the names all employees who are managers can be easily expressed as a nested query. First, we find the numbers of all managers. Q1:SELECT MANAGER ; Next, we find the names of managers, i.e. the names of all employees such that their ENUM is included in the result of the previous query. Q2:SELECT ENAME, ENUM WHERE ENUM IN (Q2); A script file nestin.sql ( contains the following statement. SELECT ENAME, ENUM WHERE ENUM IN (SELECT MANAGER ); Step 4 How to implement a nested query with an equality condition? This action implements a query, that finds full information about the direct manager of employee Smith. 3

4 A script file nestsmith.sql ( contains the following statement. SELECT * WHERE ENUM = (SELECT MANAGER WHERE ENAME = SMITH ); Nested queries have the limitations, which make them less universal than join queries. For example, consider a query, that finds the names, budgets of departments together with the names of all cities where the departments have their locations. An attempt to execute a query SELECT DNAME, BUDGET, CITY WHERE DNAME IN (SELECT DNAME ); Fails with a message ORA-00904: "CITY": invalid identifier This is because an attribute CITY is not included in a schema of relational table DEPARTMENT. Of course a query: SELECT DNAME, BUDGET, CITY WHERE DNAME IN (SELECT DNAME ); fails because of very similar reasons. This time, an attribute BUDGET is not included in a schema of relational table DEPTLOC. These experiments lead to a more general conclusion. Implementation of a query as a nested SELECT statement is possible when all attributes in the outermost SELECT clause are included in a schema of relational table listed in the outermost FROM clause. Otherwise, the only option is a join query. Step 5 How to implement a query with a nested HAVING clause? Nested SELECT statement can be used in HAVING clause of statement with GROUP BY clause. A typical example is a query, that finds name of departments with the largest number of employees. An attempt to implement such a query as SELECT DNAME, MAX(COUNT(ENUM)) 4

5 GROUP BY DNAME; fails with a message ORA-00937: not a single-group group function A query above is incorrect because MAX function returns only one value. On the other hand SELECT DNAME provides more than one value. A correct solution finds the largest number of employees per department in the first query: Q1:SELECT MAX(COUNT(ENUM)) GROUP BY DEPARTMENT; Then, the next query performs grouping of employees by departments and finds the total number of employees in each group: Q2:SELECT DNAME, COUNT(ENUM) GROUP BY DNAME; Finally, we use a query Q1 in HAVING clause attached to query Q2 to find a group of employees from one department that has the same cardinality as the result found in Q1. (Q2) HAVING COUNT(ENUM) = Q1 A script file nestgrphave.sql ( contains the following statement. SELECT DNAME, COUNT(ENUM) GROUP BY DNAME HAVING COUNT(ENUM) = (SELECT MAX(total) FROM ( SELECT COUNT(ENUM) total GROUP BY DNAME) ECNT); Step 6 How to implement a nested query with a multiple column condition? In this step we implement a query, that finds the names of departments located in the same city and at the same street as one of SALES departments (building number and level number doesn t matter). The query can be implemented as a nested query with a multiple column condition. We use nested queries with multiple column conditions when two or more relational tables are involved in a query and the tables are logically linked through the composite primary-foreign keys. The query selects a row from DEPTLOC table and compares it with the other rows from the same table selected by the inner query and such that values of attributes CITY and STREET are identical. 5

6 Before we implement the query, we have to perform a small modification of a sample database. The modification is needed to reveal a difference between the queries with multiple column conditions and single column conditions. A script file updtdeptloc.sql ( contains UPDATE statement that performs the modification. UPDATE DEPTLOC SET CITY = BOSTON, STREET = 30 WHERE DNAME = IT ; COMMIT; A script file nestinsales1.sql ( contains a statement given below. SELECT DNAME WHERE (CITY, STREET) IN (SELECT CITY, STREET WHERE DNAME = SALES ) AND DNAME <> SALES ; Step 7 How to implement a nested query? An interesting experiment is to compare the results produced by the statement used in the previous step with the results produced by a statement with the single attribute conditions implemented in this step. A script file nestinsales2.sql ( contains the following statement. SELECT DNAME WHERE CITY IN (SELECT CITY WHERE DNAME = SALES ) AND STREET IN (SELECT STREET WHERE DNAME = SALES ) AND DNAME <> SALES ; The results produced by the queries implemented in the steps 6 and 7 are different because a set of pairs is not necessarily the same as the results of cross product of the individual domains. It means that a set of pairs (CITY,STREET) selected from DEPTLOC table is not the same as a set of values selected from a column CITY and involved in a cross product with a set of values selected from a column STREET. 6

7 References MySQL 5.7 Reference Manual, SELECT statement ( 7

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