School of Computing, Engineering and Information Sciences University of Northumbria. Set Operations

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1 Set Operations Aim: To understand how to do the equivalent of the UNION, DIFFERENCE and INTERSECT set operations in SQL. Outline of Session: Do some example SQL queries to learn to differentiate between UNION, MINUS and INTERSECT operations, and how they can be used. Do some SQL queries requiring the use of UNION operations. Do some SQL queries requiring the use of MINUS operations. Do some SQL queries requiring the use of INTERSECT operations.

2 RELATIONS AS SETS Recall that a relation is a set of tuples. Consequently relational database management systems support the standard set operations : the Union of two relations, i.e. the set of tuples which are found in either or both of them; the (set) Difference between two relations, i.e. the set of tuples which are found in the first but not the second; the Intersection of two relations, i.e. the set of tuples which are found in both of them. For these operations to be meaningful, it is necessary that the two relations concerned be type compatible, i.e. should have the same number of attributes, and that the corresponding attribute in each relation should have the same data type. In Oracle SQL, these operations are implemented by means of the operators UNION, MINUS and INTERSECT respectively. (MINUS is the Oracle SQL equivalent of Difference). The syntax of an SQL set operation can be summarised as : <select statement> { UNION MINUS INTERSECT } <select statement> ; Thus we can in effect link two SELECT statements of any kind with a UNION, MINUS or INTERSECT (as long as the SELECT statements produce type compatible tables). Moreover, since the result of such a set operation is considered to be the equivalent of another SELECT statement, we can repeat this process any number of times; e.g. have 3 SELECT statements with 2 set operators between them, 4 SELECT statements with 3 set operators between them, and so on. However there is an important difference between the type compatibility of relations and that of SQL tables : in addition to the same number of attributes, type compatible relations use attribute names to determine correspondence, and so corresponding attributes have the same name and data type; in addition to the same number of columns, type compatible SQL tables have corresponding columns in the same left-to-right position, regardless of the column name; i.e. the first column in each table must have the same data type, likewise the second columns, the third columns, and so on. Relational attributes do not have a position (because relations are also sets of attributes, and items in a set do not have positions ), and so are identified by their name; in this sense they can be in any position in the relation. SQL tables use column names in a relational way for some things; but for set operations, SQL uses column position, i.e. whereabouts a column is in a left-to-right sequence within its table. 2

3 TABLES TYPES : THEIR EFFECT ON QUERY DESIGN The tables in the Company database are not type compatible with each other i.e. each table has a different number of columns from the others, and the types of their columns vary as well. In reality, this is typical of most databases. Given that we can only do a set operation on two tables that are type compatible, there are 2 possible query design strategies for solving this problem : 1. The two relations to be unioned/differenced/intersected together must be two (sub) tables from the same database table. In other words, the two tables will be (different) Restrictions of the same database table, picked out with WHERE phrases in SQL. 2. The two tables to be unioned/differenced/intersected together come from any two of those in the database, but what is actually unioned/differenced/intersected together must be Projections of these two tables such that the Projections are type compatible. Of course even where the first strategy is applied, Projections of Restrictions of the same table can be used if not all the table s columns are required in the result. Note that the logical equivalent of the first query design strategy can be carried out without using a set operator. EXAMPLE QUERIES Suppose we want to know which employees are married or work on project P2. We could use two separate SELECT statements to answer each part of this query :- SQL> SELECT EMP_NO FROM ALLOC WHERE PROJ_NO = 'P2'; To find out in a single query who is married or works on project P2 (or both), we can combine the previous two using UNION : 2 UNION To find out in a single query who is married and does not work on project P2, we can combine the previous two using MINUS (= Oracle s equivalent of DIFFERENCE) : 2 MINUS To find out in a single query who is married and works on project P2, we can combine the previous two using INTERSECT : 3

4 2 INTERSECT Exercise: carry out these examples. Which of the two query design strategies is being applied? What happens if you reverse the positions of the tables in each of the set operations? UNION QUERIES Exercises : design and write an SQL query using UNION for each of the following queries; in each case note which query design strategy you use, and in the case of strategy 1, write an alternative version which does not use UNION. 1. Get the names of employees who are married or earn over 30, Get the names of departments with budgets in excess of 140,000 or that are managed by employee E8. 3. Find the Project Numbers of projects which have a deadline before 01-Jan-2008 or have employee E3 working on them. 4. List the Employee Numbers of employees who either manage the Sales Department or work on project P4. 5. Get the names of employees with their salaries and of departments with their budgets. How much sense does this query make? MINUS QUERIES Exercises : design and write an SQL query using MINUS for each of the following queries; in each case note which query design strategy you use, and in the case of strategy 1, write an alternative version which does not use MINUS. 1. Get the names of employees known to be single who do not earn more than 13, Get the salaries of every employee apart from those working for department D2. 3. Find the Employee Numbers of employees who do not manage a department. 4. Get the Employee Numbers of those employees who are not working on any projects. 5. Get the Employee Numbers of employees paid more than 15,000 apart from those who manage departments with a budget of 100,000 or less. 4

5 INTERSECT QUERIES Exercises : design and write an SQL query using INTERSECT for each of the following queries; in each case note which query design strategy you use, and in the case of strategy 1, write an alternative version which does not use INTERSECT. 1. Get the names of employees who are married and earn more than 15, Get the names of departments not managed by employee E5 that have budgets of more than 96, List the Employee Numbers of department managers who are paid less than 12, Get the Project Numbers of projects that started after 10-Jun-2005 and have employee E4 working on them. 5. Get the Employee Numbers of managers who are also working on projects. Logically a query using INTERSECT can be written using MINUS instead. Exercise : re-write the first 2 queries above using MINUS instead of INTERSECT. 5