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Displaying Data from Multiple Tables

Displaying Data from Multiple Tables. 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

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Displaying Data from Multiple Tables

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  1. 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

  3. Obtaining Data from Multiple Tables Employee Department employee_nbr name ... dept_nbr------ ------ ... ------7839 King ... 10 7698 Blake ... 30 ... 7934 Miller ... 10 dept_nbrdept_name location -------- --------- -------- 10 Accounting New York 20 Research Dallas 30 Sales Chicago 40 Operations Boston employee_nbrdept_nbr location ------------ -------- -------- 7839 10 New York 7698 30 Chicago 7782 10 New York 7566 20 Dallas 7654 30 Chicago 7499 30 Chicago ... 14 rows selected.

  4. 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;

  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.

  6. “Cartesianproduct: 14*4=56 rows” Generating a Cartesian Product Employee (14 rows) Department(4 rows) employee_nbr name ...dept_nbr----------- ----- ...-------- 7839 King ... 10 7698 Blake ... 30 ... 7934 Miller... 10 dept_nbrdept_name location -------- ------- -------- 10 Accounting New York 20 Research Dallas 30 Sales Chicago 40 Operations Boston name dept_name ------ ---------- King Accounting Blake Accounting ... King Research Blake Research ... 56 rows selected.

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

  8. Foreign key Primary key What Is an Equijoin? Employee Department employee_nbr name dept_nbr ------------ ---- -------- 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. dept_nbrdept_name location -------- ---------- -------- 10 Accounting New York 30 Sales Chicago 10 AccountingNew 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.

  9. Retrieving Records with Equijoins MySQL> SELECT employee.employee_nbr,employee.name, -> employee.dept_nbr, -> dept.dept_nbr, dept.location -> FROM employee, dept -> WHERE employee.dept_nbr=dept.dept_nbr; employee_nbr name dept_nbrdept_nbr location ----------- ------- -------- -------- ---------- 7839 King 10 10 New York 7698 Blake 30 30 Chicago 7782 Clark10 10 New York 7566 Jones 20 20 Dallas ... 14 rows selected.

  10. Inner Join Syntax • The Keyword INNER is optional MySQL> SELECT employee_nbr, name, -> department.dept_nbr, location -> FROM employee [INNER] JOIN department -> ON employee.dept_nbr = department.dept_nbr;

  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 Employee Department employee_nbr name dept_nbr ------------ ---- -------- 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. dept_nbrdept_name location -------- --------- -------- 10 Accounting New York 30 SalesChicago 10 Accounting New York 20 Research Dallas 30 SalesChicago 30 SalesChicago 30 SalesChicago 30 SalesChicago 30 SalesChicago 20 Research Dallas 20 Research Dallas ... 14 rows selected.

  13. Using AND with Composite Keys • If the Primary key is a composite key use the AND operator in the ON clause to set the composite key columns in equal to the foreign key columns. MySQL> SELECT customer_case_id, orders.order_id, -> payment_id, payment_amt -> FROM orders INNER JOIN payments -> ON orders.cust_id = payments.cust_id -> AND orders.order_id = payments_order_id;

  14. Using Table Aliases • Simplify queries by using table aliases. MySQL> SELECT employee.employee_nbr,employee.name, -> employee.dept_nbr, -> dept.dept_nbr, dept.location -> FROM employee, dept -> WHERE employee.dept_nbr = dept.dept_nbr; MySQL> SELECT employee_nbr, e.name, e.dept_nbr, -> d.dept_nbr, d.location -> FROM employee e, dept d -> WHERE e.dept_nbr = d.dept_nbr;

  15. orders cust_idorder_id ------- -------- 101 610 102 611 104 612 106 601 102 602 106 604 106 605 ... 21 rows selected. payments item order_iditem_ID ------ ------- 610 3 611 1 612 1 601 1 602 1 ... 64 rows selected. cust_idorder_idpayment_amt ------- -------- ----------- 100 610 1050 103 601 1230 106 604 1155 Joining More Than Two Tables customer Name cust_id ----------- ------- Jocksports 100 TKB Sport Shop 101 Vollyrite 102 Just Tennis 103 K+T Sports 105 Shape Up 106 Women's Sports 107 ... ... 9 rows selected.

  16. Joining More Than Two Tables • If you have N tables to join you will need at least N-1 join statements MySQL> SELECT customer.cust_id, name, -> orders.order_id, item_id -> FROM customer JOIN orders -> ON customer.cust_id = orders.cudt_id -> JOIN item -> ON orders.order_id = item.order_id;

  17. “salary in the employee table is between low salary and high salary in thesalary_grade table” Non-Equijoins Employee Salary_grade employee_nbr name salary ----------- ----- ---- 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 low_salaryhigh_salary ----- ---------- ----------- 1 700 1200 2 1201 1400 3 1401 2000 4 2001 3000 5 3001 9999

  18. Retrieving Records with Non-Equijoins MySQL> SELECT name, salary, grade -> FROMemployee, salary_grade -> WHERE salary -> BETWEEN low_salaryANDhigh_salary; name salary grade ---------- --------- --------- James 950 1 Smith 800 1 Adams 1100 1 ... 14 rows selected.

  19. No employee in theOperations department Outer Joins Employee Department name dept_nbr----- ------King 10 Blake 30 Clark 10 Jones 20 ... dept_nbrdept_name ------ --------- 10 Accounting 30 Sales 10 Accounting 20 Research ... 40 Operations

  20. Outer Joins • You use an outer join to also see rows that do not usually meet the join condition. • Outer joins require the keyword LEFT or RIGHT. • RIGHT or LEFT keyword specify the table from which the missing data is to come • The only difference between a RIGHT and LEFT outer join ... ... ... SELECT table1.column, table2.column FROM table1 LEFT [OUTER] JOIN table2 ON table1.column = table2.column; SELECT table1.column, table2.column FROM table1 RIGHT [OUTER] JOIN table2 ON table1.column = table2.column;

  21. Using Outer Joins MySQL> SELECT name,department.dept_nbr, dept_name -> FROM employee RIGHT OUTER JOIN department -> ON employee.dept_nbr = department.dept_nbr -> ORDER BY employee.dept_nbr; name dept_nbrdept_name --------- --------- ------------- King 10 Accounting Clark 10 Accounting ... 40 Operations 15 rows selected.

  22. “MGR in the Worker table is equal to employee_nbrin the Manager table” Self Joins Employee(Worker) Employee (Manager) employee_nbrname MGR---------------- --- 7839 King 7698 Blake 7839 7782 Clark 7839 7566 Jones 7839 7654 Martin 7698 7499 Allen 7698 employee_nbrname---------------- 7839 King 7839 King 7839 King 7698 Blake 7698 Blake

  23. Joining a Table to Itself MySQL> SELECT worker.name||' works for '||manager.name -> FROM employee worker INNER JOIN employee manager -> ON worker.manager= manager.employee_nbr; WORKER.name||'WORKSFOR'||MANAG ------------------------------- Blake works for King Clark works for King Jones works for King Martin works for Blake ... 13 rows selected.

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

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