COIS 420 - Practice04
- 1. 4 Displaying Data from Multiple Tables
- 2. 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 4-2
- 3. Obtaining Data from Multiple Tables EMP DEPT EMPNO ENAME ... DEPTNO DEPTNO DNAME LOC ------ ----- ... ------ ------ ---------- -------- 7839 KING ... 10 10 ACCOUNTING NEW YORK 7698 BLAKE ... 30 20 RESEARCH DALLAS ... 30 SALES CHICAGO 7934 MILLER ... 10 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. 4-3
- 4. What Is a Join? Use a join to query data from more than one table. SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2; • 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. 4-4
- 5. 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. 4-5
- 6. Generating a Cartesian Product EMP (14 rows) DEPT (4 rows) EMPNO ENAME ... DEPTNO DEPTNO DNAME LOC ------ ----- ... ------ ------ ---------- -------- 7839 KING ... 10 10 ACCOUNTING NEW YORK 7698 BLAKE ... 30 20 RESEARCH DALLAS ... 30 SALES CHICAGO 7934 MILLER ... 10 40 OPERATIONS BOSTON ENAME DNAME ------ ---------- KING ACCOUNTING “Cartesian BLAKE ACCOUNTING product: ... KING RESEARCH 14*4=56 rows” BLAKE RESEARCH ... 56 rows selected. 4-6
- 7. Types of Joins Equijoin Non-equijoin Outer join Self join 4-7
- 8. What Is an Equijoin? EMP DEPT EMPNO ENAME DEPTNO DEPTNO DNAME LOC ------ ------- ------- ------- ---------- -------- 7839 KING 10 10 ACCOUNTING NEW YORK 7698 BLAKE 30 30 SALES CHICAGO 7782 CLARK 10 10 ACCOUNTING NEW YORK 7566 JONES 20 20 RESEARCH DALLAS 7654 MARTIN 30 30 SALES CHICAGO 7499 ALLEN 30 30 SALES CHICAGO 7844 TURNER 30 30 SALES CHICAGO 7900 JAMES 30 30 SALES CHICAGO 7521 WARD 30 30 SALES CHICAGO 7902 FORD 20 20 RESEARCH DALLAS 7369 SMITH 20 20 RESEARCH DALLAS ... ... 14 rows selected. 14 rows selected. Foreign key Primary key 4-8
- 9. 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. 4-9
- 10. 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. 4-10
- 11. Additional Search Conditions Using the AND Operator EMP DEPT EMPNO ENAME DEPTNO DEPTNO DNAME LOC ------ ------- ------- ------ --------- -------- 7839 KING 10 10 ACCOUNTING NEW YORK 7698 BLAKE 30 30 SALES CHICAGO 7782 CLARK 10 10 ACCOUNTING NEW YORK 7566 JONES 20 20 RESEARCH DALLAS 7654 MARTIN 30 30 SALES CHICAGO 7499 ALLEN 30 30 SALES CHICAGO 7844 TURNER 30 30 SALES CHICAGO 7900 JAMES 30 30 SALES CHICAGO 7521 WARD 30 30 SALES CHICAGO 7902 FORD 20 20 RESEARCH DALLAS 7369 SMITH 20 20 RESEARCH DALLAS ... ... 14 rows selected. 14 rows selected. 4-11
- 12. 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; 4-12
- 13. Joining More Than Two Tables CUSTOMER ORD NAME CUSTID CUSTID ORDID ----------- ------ ------- ------- JOCKSPORTS 100 101 610 TKB SPORT SHOP 101 102 611 VOLLYRITE 102 104 612 JUST TENNIS 103 106 601 K+T SPORTS 105 102 602 ITEM SHAPE UP 106 106 604 ORDID ITEMID WOMENS SPORTS 107 106 605 ------ ------- ... ... ... 610 3 9 rows selected. 21 rows selected. 611 1 612 1 601 1 602 1 ... 64 rows selected. 4-13
- 14. Non-Equijoins EMP SALGRADE EMPNO ENAME SAL GRADE LOSAL HISAL ------ ------- ------ ----- ----- ------ 7839 KING 5000 1 700 1200 7698 BLAKE 2850 2 1201 1400 7782 CLARK 2450 3 1401 2000 7566 JONES 2975 4 2001 3000 7654 MARTIN 1250 5 3001 9999 7499 ALLEN 1600 7844 TURNER 1500 7900 JAMES 950 ... “salary in the EMP 14 rows selected. table is between low salary and high salary in the SALGRADE table” 4-14
- 15. 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. 4-15
- 16. Outer Joins EMP DEPT ENAME DEPTNO DEPTNO DNAME ----- ------ ------ ---------- KING 10 10 ACCOUNTING BLAKE 30 30 SALES CLARK 10 10 ACCOUNTING JONES 20 20 RESEARCH ... ... 40 OPERATIONS No employee in the OPERATIONS department 4-16
- 17. 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(+); 4-17
- 18. 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. 4-18
- 19. Self Joins EMP (WORKER) EMP (MANAGER) EMPNO ENAME MGR EMPNO ENAME ----- ------ ---- ----- -------- 7839 KING 7698 BLAKE 7839 7839 KING 7782 CLARK 7839 7839 KING 7566 JONES 7839 7839 KING 7654 MARTIN 7698 7698 BLAKE 7499 ALLEN 7698 7698 BLAKE “MGR in the WORKER table is equal to EMPNO in the MANAGER table” 4-19
- 20. 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. 4-20
- 21. Summary SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2; Equijoin Non-equijoin Outer join Self join 4-21