oracle10g英文原厂培训les

1、Using Subqueries to Solve Queries ObjectivesAfter completing this lesson, you should be able to do the following:Define subqueries Describe the types of problems that subqueries can solveList the types of subqueriesWrite single-row and multiple-row subqueriesUsing a Subqueryto Solve a ProblemWho has

2、 a salary greater than Abels?Which employees have salaries greater than Abels salary?Main query:What is Abels salary?Subquery:The subquery (inner query) executes once before the main query (outer query).The result of the subquery is used by the main query.SELECTselect_listFROMtableWHEREexpr operator

3、 (SELECTselect_list FROMtable);Subquery SyntaxSELECT last_nameFROM employeesWHERE salary (SELECT salary FROM employees WHERE last_name = Abel);Using a Subquery11000Guidelines for Using SubqueriesEnclose subqueries in parentheses.Place subqueries on the right side of the comparison condition.The ORDE

4、R BY clause in the subquery is not needed unless you are performing Top-N analysis.Use single-row operators with single-row subqueries, and use multiple-row operators withmultiple-row subqueries.Types of SubqueriesSingle-row subqueryMultiple-row subqueryMain querySubquery returnsST_CLERKST_CLERKSA_M

5、ANMain querySubquery returnsSingle-Row SubqueriesReturn only one rowUse single-row comparison operatorsOperatorMeaning =Equal to Greater than =Greater than or equal to Less than =Less than or equal to Not equal toSELECT last_name, job_id, salaryFROM employeesWHERE job_id = (SELECT job_id FROM employ

6、ees WHERE employee_id = 141)AND salary (SELECT salary FROM employees WHERE employee_id = 143);Executing Single-Row SubqueriesST_CLERK2600SELECT last_name, job_id, salaryFROM employeesWHERE salary = (SELECT MIN(salary) FROM employees);Using Group Functions in a Subquery2500SELECT department_id, MIN(s

7、alary)FROM employeesGROUP BY department_idHAVING MIN(salary) (SELECT MIN(salary) FROM employees WHERE department_id = 50);The HAVING Clause with SubqueriesThe Oracle server executes subqueries first.The Oracle server returns results into the HAVING clause of the main query.2500SELECT employee_id, la

8、st_nameFROM employeesWHERE salary = (SELECT MIN(salary) FROM employees GROUP BY department_id);What Is Wrong with This Statement?ERROR at line 4:ORA-01427: single-row subquery returns more thanone rowSingle-row operator with multiple-row subquerySELECT last_name, job_idFROM employeesWHERE job_id = (

9、SELECT job_id FROM employees WHERE last_name = Haas);Will This Statement Return Rows?no rows selectedSubquery returns no values.Multiple-Row SubqueriesReturn more than one rowUse multiple-row comparison operatorsOperatorMeaningINEqual to any member in the listANYCompare value to each value returned

10、by the subquery ALLCompare value to every value returned by the subquery SELECT employee_id, last_name, job_id, salaryFROM employeesWHERE salary ANY (SELECT salary FROM employees WHERE job_id = IT_PROG)AND job_id IT_PROG;Using the ANY Operator in Multiple-Row Subqueries9000, 6000, 4200SELECT employe

11、e_id, last_name, job_id, salaryFROM employeesWHERE salary ALL (SELECT salary FROM employees WHERE job_id = IT_PROG)AND job_id IT_PROG;Using the ALL Operatorin Multiple-Row Subqueries9000, 6000, 4200SELECT emp.last_nameFROM employees empWHERE emp.employee_id NOT IN (SELECT mgr.manager_id FROM employe

12、es mgr);no rows selectedNull Values in a SubquerySELECTselect_listFROMtableWHEREexpr operator (SELECT select_list FROM table);SummaryIn this lesson, you should have learned how to:Identify when a subquery can help solve a questionWrite subqueries when a query is based on unknown valuesPractice 6: Overvie