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# Chapter-9.ppt

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# Chapter-9.ppt

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### Chapter-9.ppt

1. 1. Chapter 9 Subqueries
2. 2. Objectives Describe the types of problems that subqueries can solve Define subqueries List the types of subqueries Write single-row and multiple-row subqueries
3. 3. Using a Subquery to Solve a Problem Who has a GPA higher than Owen’s? Which students have a GPA higher than Owen’s GPA? Main Query What is Owen’s GPA? Subquery ?
4. 4. Subqueries The subquery (inner query) executes once before the main query. The result of the subquery is used by the main query (outer query). SELECT select_list FROM table WHERE expr operator (SELECT select_list FROM table);
5. 5. Using a Subquery “Who has a GPA higher than Owen’s?” LastName GPA Lee 3.82 Tham 3.89 SELECT LastName, GPA from Student where GPA > (Select GPA from Student Where LastName = 'Owen'); 3.34
6. 6. Guidelines for Using Subqueries Enclose subqueries in parentheses. Place subqueries on the right side of the comparison operator. Do not add an ORDER BY clause to a subquery. Use single-row operators with single-row subqueries.
7. 7. Types of Subqueries Single-row subquery Multiple-row subquery Main query Subquery returns DIC DIC DCS Main query Subquery returns
8. 8. Single-Row Subqueries Return only one row Use single-row comparison operators Operator = > >= < <= <> Meaning Equal to Greater than Greater than or equal to Less than Less than or equal to Not equal to
9. 9. Retrieving data using Single-Row Subqueries SELECT LastName, CourseID from Student where CourseID = Select CourseID from Student Where LastName = 'Lewis'); Who study the same course as Lewis? OUTPUT: LastName CourseID Lewis DICT Nicosia DICT Maser DICT DICT
10. 10. Retrieving data using Single-Row Subqueries SELECT LastName,GroupLeader from Student where GroupLeader = (Select GroupLeader from Student Where LastName = 'Williams'); S010 Who has the same group leader as Williams? OUTPUT: LastName GroupLeader Mikulski S010 Faga S010 Williams S010
11. 11. Retrieving data using Single-Row Subqueries SELECT LastName,DateEnrolled from Student where DateEnrolled < (Select DateEnrolled from Student Where StudID = 'S009'); Who have been enrolled earlier than student S009? 01-Feb-02 OUTPUT: LastName DateEnrolled Kebel 23-Jun-01 Lee 05-Jan-02 Lewis 03-Mar-00 Law 01-Apr-01 Faga 25-Jun-01 Owen 17-Sep-01 Ng 01-Apr-01 Roche 30-Mar-00 Jann 01-Apr-01
12. 12. Retrieving data using Single-Row Subqueries with GROUP functions Display all the students that earn the minimum GPA SELECT LastName,GPA from Student where GPA = (Select min(GPA) from student); 1.88 OUTPUT: LastName GPA Ng 1.88 Roche 1.88
13. 13. Multiple-Row Subqueries Return more than one row Use the IN multiple-row comparison operator to compare an expression to any member in the list that a subquery returns
14. 14. Using Group Functions in a Multiple-Row Subquery Display all students who earn the same GPA as the minimum GPA for each course 1.88,1.89,2.22,3 SELECT LastName,GPA, CourseID from Student where GPA IN (select min(GPA) from student Group By CourseID); OUTPUT: LastName GPA CourseID Mikulski 1.89 DCS Faga 2.22 DIC Ng 1.88 DIT Maser 3 DICT Roche 1.88 DIT
15. 15. Using Group Functions in a Multiple-Row Subquery Display all students who enrolled the same course as Law and Lewis. DICT, DIT SELECT LastName, CourseID from Student where CourseID IN (select CourseID from Student where LastName IN ('Lewis','Law')) order by CourseID; OUTPUT: LastName CourseID Maser DICT Nicosia DICT Lewis DICT Roche DIT Ng DIT Law DIT Lee DIT
16. 16. Summary A subquery is a SELECT statement that is embedded in a clause of another SQL statement. Subqueries are useful when a query is based on unknown criteria. Subqueries have the following characteristics:  Can pass one row of data to a main statement that contains a single-row operator, such as =, <>, >, >=, <, or <=  Can pass multiple rows of data to a main statement that contains a multiple-row operator, such as IN  Can contain group functions