SQL Joins - Oracle SQL Fundamentals
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The document discusses SQL joins in Oracle 11g, explaining concepts such as Cartesian products and various types of joins including equijoins, non-equijoins, outer joins, and self joins. It provides examples of each type of join, highlighting the importance of using valid join conditions to avoid Cartesian products. The document is intended for individuals seeking to enhance their SQL skills, particularly in multi-table queries.
SQL Joins - Oracle SQL Fundamentals
- 1. SQL Fundamentals Oracle 11g M U H A M M A D WA H E E D O R AC L E DATA BA S E D E V E LO P E R E M A I L : m .wa h e e d 3 6 6 8 @ g m a i l . co m Lecture#7 SQL Joins/SELECT on Multiple Tables
- 3. Cartesian Product(cont’d) •A Cartesian product is formed when - A join condition is omitted - A join condition is invalid - All rows are joined to the all rows in second table •To avoid it, use valid join condition in WHERE clause 3
- 4. Cartesian Product(cont’d) •Example: i- SELECT s.std_id, s.std_name,r.sub_id,r.obt_marks FROM student s, result r; ii- SELECT s.std_id, s.std_name,r.sub_id,r.obt_marks FROM student s, result r WHERE s.std_id = r.result_id; 4
- 5. Types of Joins There are following commonly used joins: •Equijoin •Non-equijoin •Outer join •Self join *Note: join condition is always mentioned in WHERE clause 5
- 7. Equijoin 7
- 8. Equijoin(cont’d) •Example: SELECT s.std_id, s.std_name, r.obt_marks FROM student s, result r WHERE s.std_id = r.std_id; 8
- 9. Equijoin(cont’d) •Example(additional search with join): SELECT s.std_id, s.std_name, r.obt_marks FROM student s, result r WHERE s.std_id = r.std_id AND r.obt_marks>65; 9
- 10. Equijoin(cont’d) •Example(more than two tables in join): SELECT s.std_id, s.std_name,r.sub_id,r.obt_marks FROM student s, result r, subject ss WHERE s.std_id = r.std_id AND ss.sub_id = r.sub_id AND r.obt_marks>65; 10
- 11. Non-Equijoin •A join condition containing something other than equality operator. •Example: SELECT t.tch_id, t.tch_name,t.salary,s.sal_grade FROM teacher t, salary_grade s WHERE t.salary BETWEEN s.min_sal AND s.max_sal; 11
- 12. Outer Join •A join condition which is used to display those rows that doesn’t meet join criteria. •Outer join operator is plus sign ‘+’. •There are two types of outer join - Left outer join - Right outer join 12
- 13. Outer Join(cont’d) •Example(left join): SELECT s.std_id, s.std_name, r.obt_marks FROM student s, result r WHERE s.std_id (+)= r.std_id; •Example(right join): SELECT s.std_id, s.std_name, s.dept_id FROM student s, department d WHERE s.dept_id = d.dept_id(+); 13
- 14. Self Join •A join condition where a table having it’s own primary key as foreign key in itself. •Example: manager_id of employee table is employee_id itself. 14
- 15. Self Join(cont’d) •Example: SELECT a.emp_id,a.emp_name,a.manager_id FROM employee a,employee b WHERE a.manager_id = b.emp_id; 15
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