Subqueries allow queries to be nested within other queries. They can return either single rows or multiple rows. Single-row subqueries use comparison operators like = or >, while multiple-row subqueries use operators like IN, ANY, or ALL. Subqueries execute first before the outer or main query uses the results. This allows the main query to filter or compare based on unknown values from the subquery.

1. Subqueries

2. ObjectivesAfter completing this lesson, you should be able to do the following:Describe the types of problems that subqueries can solveDefine subqueriesList the types of subqueriesWrite single-row and multiple-row subqueries

3. Using a Subquery to Solve a Problem“Who has a salary greater than Jones’?”Main Query“Which employees have a salary greater than Jones’ salary?”?Subquery?“What is Jones’ salary?”

4. SubqueriesSELECTselect_listFROMtableWHEREexpr operator (SELECTselect_list FROMtable);The subquery (inner query) executes once before the main query.The result of the subquery is used by the main query (outer query).

5. 2975SQL> SELECT ename2 FROM emp3 WHERE sal > 4 (SELECT sal5 FROM emp6 WHERE empno=7566);Using a SubqueryENAME----------KINGFORDSCOTT

6. Guidelines for Using SubqueriesEnclose 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.Use multiple-row operators with multiple-row subqueries.

7. Main queryreturnsSubqueryMultiple-row subqueryMain queryCLERKMANAGERreturnsSubqueryMultiple-column subqueryMain queryreturnsCLERK 7900MANAGER 7698SubqueryTypes of SubqueriesSingle-row subqueryCLERK

8. Single-Row SubqueriesReturn only one rowUse single-row comparison operatorsOperator=> >= < <= <>MeaningEqual toGreater than Greater than or equal to Less than Less than or equal toNot equal to

9. CLERK1100Executing Single-Row SubqueriesSQL> SELECT ename, job2 FROM emp3 WHERE job = 4 (SELECT job5FROM emp6WHERE empno = 7369)7 AND sal > 8 (SELECT sal9FROMemp10WHEREempno = 7876); ENAME JOB---------- ---------MILLER CLERK

10. Using Group Functions in a Subquery800SQL> SELECTename, job, sal2 FROMemp3 WHEREsal = 4 (SELECTMIN(sal)5FROMemp);ENAME JOB SAL---------- --------- ---------SMITH CLERK 800

11. HAVING Clause with Subqueries800The Oracle Server executes subqueries first.The Oracle Server returns results into the HAVING clause of the main query.SQL> SELECTdeptno, MIN(sal)2 FROMemp3 GROUP BYdeptno4 HAVINGMIN(sal) >5 (SELECTMIN(sal)6FROMemp7WHEREdeptno = 20);

12. What Is Wrong with This Statement?SQL> SELECT empno, ename2 FROM emp3 WHERE sal = 4 (SELECT MIN(sal)5FROM emp6GROUP BY deptno);Single-row operator with multiple-row subqueryERROR:ORA-01427: single-row subquery returns more thanone rowno rows selected

13. Will This Statement Work?SQL> SELECT ename, job2 FROM emp3 WHERE job = 4 (SELECTjob5FROMemp6WHEREename='SMYTHE');no rows selectedSubquery returns no values

14. Multiple-Row SubqueriesReturn more than one rowUse multiple-row comparison operatorsOperator IN ANY ALLMeaningEqual to any member in the listCompare value to each value returned by the subquery Compare value to every value returned by the subquery

15. Using ANY Operator in Multiple-Row Subqueries13001100800950SQL> SELECT empno, ename, job2 FROM emp3 WHERE sal < ANY 4 (SELECTsal5FROMemp6WHEREjob = 'CLERK')7 AND job <> 'CLERK'; EMPNO ENAME JOB--------- ---------- ---------7654 MARTIN SALESMAN 7521 WARD SALESMAN

16. Using ALL Operator in Multiple-Row Subqueries1566.666721752916.6667SQL> SELECT empno, ename, job2 FROM emp3 WHERE sal > ALL 4 (SELECTavg(sal)5FROMemp6GROUP BYdeptno); EMPNO ENAME JOB--------- ---------- ---------7839 KING PRESIDENT7566 JONES MANAGER7902 FORD ANALYST7788 SCOTT ANALYST

17. SummarySubqueries are useful when a query is based on unknown values.SELECTselect_listFROMtableWHEREexpr operator (SELECT select_listFROMtable);

18. Practice OverviewCreating subqueries to query values based on unknown criteriaUsing subqueries to find out what values exist in one set of data and not in another