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Mastering SQL: Joining, Sequences, Views & Access Control

Learn to query, join, and view data from multiple tables, create sequences, views, and dynamic SQL queries. Control user access efficiently.

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Mastering SQL: Joining, Sequences, Views & Access Control

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  1. CH3 Part2 • Displaying Data from Multiple Tables (Join) • Sub queries • Creating Sequences • Creating Views • Dynamic SQL Queries • Controlling User Access

  2. Displaying Data from Multiple Tables(Join)

  3. Objectives • After completing this lesson, you should be able to do the following: • Write SELECT statements to access data from more than one table using equality and nonequality joins • View data that generally does not meet a join condition by using outer joins • Join a table to itself

  4. Obtaining Data from Multiple Tables EMP DEPT EMPNO ENAME ... DEPTNO------ ----- ... ------ 7839 KING ... 10 7698 BLAKE ... 30 ... 7934 MILLER ... 10 DEPTNO DNAME LOC ------ ---------- -------- 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON EMPNO DEPTNO LOC ----- ------- -------- 7839 10 NEW YORK 7698 30 CHICAGO 7782 10 NEW YORK 7566 20 DALLAS 7654 30 CHICAGO 7499 30 CHICAGO ... 14 rows selected.

  5. What Is a Join? • Use a join to query data from more than one table. • Write the join condition in the WHERE clause. • Prefix the column name with the table name when the same column name appears in more than one table. SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2;

  6. Cartesian Product • A Cartesian product is formed when: • A join condition is omitted • A join condition is invalid • All rows in the first table are joined to all rows in the second table • To avoid a Cartesian product, always include a valid join condition in a WHERE clause.

  7. “Cartesianproduct: 14*4=56 rows” Generating a Cartesian Product EMP (14 rows) DEPT (4 rows) EMPNO ENAME ... DEPTNO------ ----- ... ------ 7839 KING ... 10 7698 BLAKE ... 30 ... 7934 MILLER ... 10 DEPTNO DNAME LOC ------ ---------- -------- 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON ENAME DNAME ------ ---------- KING ACCOUNTING BLAKE ACCOUNTING ... KING RESEARCH BLAKE RESEARCH ... 56 rows selected.

  8. Types of Joins • Equijoin • Non-equijoin • Outer join • Self join

  9. Foreign key Primary key What Is an Equijoin? EMP DEPT EMPNO ENAME DEPTNO ------ ------- ------- 7839 KING 10 7698 BLAKE 30 7782 CLARK 10 7566 JONES 20 7654 MARTIN 30 7499 ALLEN 30 7844 TURNER 30 7900 JAMES 30 7521 WARD 30 7902 FORD 20 7369 SMITH 20 ... 14 rows selected. DEPTNO DNAME LOC ------- ---------- -------- 10 ACCOUNTING NEW YORK 30 SALES CHICAGO 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 20 RESEARCH DALLAS 20 RESEARCH DALLAS ... 14 rows selected.

  10. Retrieving Records with Equijoins SQL> SELECT emp.empno, emp.ename, emp.deptno, 2 dept.deptno, dept.loc 3 FROM emp, dept 4 WHERE emp.deptno=dept.deptno; EMPNO ENAME DEPTNO DEPTNO LOC ----- ------ ------ ------ --------- 7839 KING 10 10 NEW YORK 7698 BLAKE 30 30 CHICAGO 7782 CLARK 10 10 NEW YORK 7566 JONES 20 20 DALLAS ... 14 rows selected.

  11. Qualifying Ambiguous Column Names • Use table prefixes to qualify column names that are in multiple tables. • Improve performance by using table prefixes. • Distinguish columns that have identical names but reside in different tables by using column aliases.

  12. Additional Search ConditionsUsing the AND Operator EMP DEPT EMPNO ENAME DEPTNO ------ ------- ------- 7839 KING 10 7698 BLAKE 30 7782 CLARK 10 7566 JONES 20 7654 MARTIN 30 7499 ALLEN 30 7844 TURNER 30 7900 JAMES 30 7521 WARD 30 7902 FORD 20 7369 SMITH 20 ... 14 rows selected. DEPTNO DNAME LOC ------ --------- -------- 10 ACCOUNTING NEW YORK 30 SALES CHICAGO 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 30 SALES CHICAGO 20 RESEARCH DALLAS 20 RESEARCH DALLAS ... 14 rows selected.

  13. Using Table Aliases • Simplify queries by using table aliases. SQL> SELECT emp.empno, emp.ename, emp.deptno, 2 dept.deptno, dept.loc 3 FROM emp, dept 4 WHERE emp.deptno=dept.deptno; SQL> SELECT e.empno, e.ename, e.deptno, 2 d.deptno, d.loc 3 FROM emp e, dept d 4 WHERE e.deptno= d.deptno;

  14. ORD CUSTID ORDID ------- ------- 101 610 102 611 104 612 106 601 102 602 106 604 106 605 ... 21 rows selected. ITEM ORDID ITEMID ------ ------- 610 3 611 1 612 1 601 1 602 1 ... 64 rows selected. Joining More Than Two Tables CUSTOMER NAME CUSTID ----------- ------ JOCKSPORTS 100 TKB SPORT SHOP 101 VOLLYRITE 102 JUST TENNIS 103 K+T SPORTS 105 SHAPE UP 106 WOMENS SPORTS 107 ... ... 9 rows selected.

  15. “salary in the EMP table is between low salary and high salary in the SALGRADE table” Non-Equijoins EMP SALGRADE EMPNO ENAME SAL ------ ------- ------ 7839 KING 5000 7698 BLAKE 2850 7782 CLARK 2450 7566 JONES 2975 7654 MARTIN 1250 7499 ALLEN 1600 7844 TURNER 1500 7900 JAMES 950 ... 14 rows selected. GRADE LOSAL HISAL ----- ----- ------ 1 700 1200 2 1201 1400 3 1401 2000 4 2001 3000 5 3001 9999

  16. Retrieving Records with Non-Equijoins SQL> SELECT e.ename, e.sal, s.grade 2 FROM emp e, salgrade s 3 WHERE e.sal 4 BETWEEN s.losal AND s.hisal; ENAME SAL GRADE ---------- --------- --------- JAMES 950 1 SMITH 800 1 ADAMS 1100 1 ... 14 rows selected.

  17. No employee in theOPERATIONS department Outer Joins EMP DEPT ENAME DEPTNO----- ------KING 10 BLAKE 30 CLARK 10 JONES 20 ... DEPTNO DNAME ------ ---------- 10 ACCOUNTING 30 SALES 10 ACCOUNTING 20 RESEARCH ... 40 OPERATIONS

  18. Outer Joins • You use an outer join to also see rows that do not usually meet the join condition. • Outer join operator is the plus sign (+). SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column(+)= table2.column; SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column = table2.column(+);

  19. Using Outer Joins SQL> SELECT e.ename, d.deptno, d.dname 2 FROM emp e, dept d 3 WHERE e.deptno(+) = d.deptno 4 ORDER BY e.deptno; ENAME DEPTNO DNAME ---------- --------- ------------- KING 10 ACCOUNTING CLARK 10 ACCOUNTING ... 40 OPERATIONS 15 rows selected.

  20. “MGR in the WORKER table is equal to EMPNO in the MANAGER table” Self Joins EMP (WORKER) EMP (MANAGER) EMPNO ENAME MGR----- ------ ---- 7839 KING 7698 BLAKE 7839 7782 CLARK 7839 7566 JONES 7839 7654 MARTIN 7698 7499 ALLEN 7698 EMPNO ENAME----- -------- 7839 KING 7839 KING 7839 KING 7698 BLAKE 7698 BLAKE

  21. Joining a Table to Itself SQL> SELECT worker.ename||' works for '||manager.ename 2 FROM emp worker, emp manager 3 WHERE worker.mgr = manager.empno; WORKER.ENAME||'WORKSFOR'||MANAG ------------------------------- BLAKE works for KING CLARK works for KING JONES works for KING MARTIN works for BLAKE ... 13 rows selected.

  22. Summary SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2; • Equijoin • Non-equijoin • Outer join • Self join

  23. Sub queries

  24. Objectives • After completing this lesson, you should be able to do the following: • Describe the types of problems that subqueries can solve • Define subqueries • List the types of subqueries • Write single-row and multiple-row subqueries

  25. 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?”

  26. Subqueries SELECT select_list FROM table WHERE expr operator (SELECT select_list FROM table); • The subquery (inner query) executes once before the main query. • The result of the subquery is used by the main query (outer query).

  27. 2975 Using a Subquery SQL> SELECT ename 2 FROM emp 3 WHERE sal > 4 (SELECT sal 5 FROM emp 6 WHERE empno=7566); ENAME ---------- KING FORD SCOTT

  28. 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. • Use multiple-row operators with multiple-row subqueries.

  29. Main query returns Subquery • Multiple-row subquery Main query CLERK MANAGER returns Subquery • Multiple-column subquery Main query returns CLERK 7900MANAGER 7698 Subquery Types of Subqueries • Single-row subquery CLERK

  30. 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

  31. CLERK 1100 Executing Single-Row Subqueries SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4 (SELECT job 5 FROM emp 6 WHERE empno = 7369) 7 AND sal > 8 (SELECT sal 9 FROM emp 10 WHERE empno = 7876); ENAME JOB ---------- --------- MILLER CLERK

  32. 800 Using Group Functions in a Subquery SQL> SELECT ename, job, sal 2 FROM emp 3 WHERE sal = 4 (SELECT MIN(sal) 5 FROM emp); ENAME JOB SAL ---------- --------- --------- SMITH CLERK 800

  33. 800 HAVING Clause with Subqueries • The Oracle Server executes subqueries first. • The Oracle Server returns results into the HAVING clause of the main query. SQL> SELECT deptno, MIN(sal) 2 FROM emp 3 GROUP BY deptno 4 HAVING MIN(sal) > 5 (SELECT MIN(sal) 6 FROM emp 7 WHERE deptno = 20);

  34. What Is Wrong with This Statement? SQL> SELECT empno, ename 2 FROM emp 3 WHERE sal = 4 (SELECT MIN(sal) 5 FROM emp 6 GROUP BY deptno); Single-row operator with multiple-row subquery ERROR: ORA-01427: single-row subquery returns more thanone row no rows selected

  35. Will This Statement Work? SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4 (SELECT job 5 FROM emp 6 WHERE ename='SMYTHE'); no rows selected Subquery returns no values

  36. Multiple-Row Subqueries • Return more than one row • Use multiple-row comparison operators Operator IN ANY ALL Meaning Equal to any member in the list Compare value to each value returned by the subquery Compare value to every value returned by the subquery

  37. 1300 1100 800 950 Using ANY Operator in Multiple-Row Subqueries SQL> SELECT empno, ename, job 2 FROM emp 3 WHERE sal < ANY 4 (SELECT sal 5 FROM emp 6 WHERE job = 'CLERK') 7 AND job <> 'CLERK'; EMPNO ENAME JOB --------- ---------- --------- 7654 MARTIN SALESMAN 7521 WARD SALESMAN

  38. 1566.6667 2175 2916.6667 Using ALL Operator in Multiple-Row Subqueries SQL> SELECT empno, ename, job 2 FROM emp 3 WHERE sal > ALL 4 (SELECT avg(sal) 5 FROM emp 6 GROUP BY deptno); EMPNO ENAME JOB --------- ---------- --------- 7839 KING PRESIDENT 7566 JONES MANAGER 7902 FORD ANALYST 7788 SCOTT ANALYST

  39. Summary • Subqueries are useful when a query is based on unknown values. SELECT select_list FROM table WHERE expr operator (SELECT select_list FROM table);

  40. Creating Sequences

  41. What Is a Sequence? • Automatically generates unique numbers • Is a sharable object • Is typically used to create a primary key value • Replaces application code • Speeds up the efficiency of accessing sequence values when cached in memory

  42. The CREATE SEQUENCE Statement • Define a sequence to generate sequential numbers automatically. CREATE SEQUENCE sequence [INCREMENT BY n] [START WITH n] [{MAXVALUE n | NOMAXVALUE}] [{MINVALUE n | NOMINVALUE}] [{CYCLE | NOCYCLE}] [{CACHE n | NOCACHE}];

  43. Creating a Sequence • Create a sequence named DEPT_DEPTNO to be used for the primary key of theDEPT table. • Do not use the CYCLE option. SQL> CREATE SEQUENCE dept_deptno 2 INCREMENT BY 1 3 START WITH 91 4 MAXVALUE 100 5 NOCACHE 6 NOCYCLE; Sequence created.

  44. Confirming Sequences • Verify your sequence values in the USER_SEQUENCES data dictionary table. • The LAST_NUMBER column displays the next available sequence number. SQL> SELECT sequence_name, min_value, max_value, 2 increment_by, last_number 3 FROM user_sequences;

  45. NEXTVAL and CURRVAL Pseudocolumns • NEXTVAL returns the next available sequence value. It returns a unique value every time it is referenced, even for different users. • CURRVAL obtains the current sequence value. NEXTVAL must be issued for that sequence before CURRVAL contains a value.

  46. Using a Sequence • Insert a new department named “MARKETING” in San Diego. • View the current value for the DEPT_DEPTNO sequence. SQL> INSERT INTO dept(deptno, dname, loc) 2 VALUES (dept_deptno.NEXTVAL, 3 'MARKETING', 'SAN DIEGO'); 1 row created. SQL> SELECT dept_deptno.CURRVAL 2 FROM dual;

  47. Using a Sequence • Caching sequence values in memory allows faster access to those values. • Gaps in sequence values can occur when: • A rollback occurs • The system crashes • A sequence is used in another table • View the next available sequence, if it was created with NOCACHE, by querying the USER_SEQUENCES table.

  48. Modifying a Sequence • Change the increment value, maximum value, minimum value, cycle option, or cache option. SQL> ALTER SEQUENCE dept_deptno 2 INCREMENT BY 1 3 MAXVALUE 999999 4 NOCACHE 5 NOCYCLE; Sequence altered.

  49. Guidelines for Modifying a Sequence • You must be the owner or have the ALTER privilege for the sequence. • Only future sequence numbers are affected. • The sequence must be dropped and re-created to restart the sequence at a different number.

  50. Removing a Sequence • Remove a sequence from the data dictionary by using the DROP SEQUENCE statement. • Once removed, the sequence can no longer be referenced. SQL> DROP SEQUENCE dept_deptno; Sequence dropped.

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