SQL Subqueries - Oracle SQL Fundamentals
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The document provides information on SQL subqueries in Oracle 11g, detailing their syntax and types, including single-row and multiple-row subqueries. It explains the execution order, guidelines for use, and provides examples for clarity. Additionally, it highlights common issues and illegal queries related to subquery usage.
SQL Subqueries - 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#8 SQL Subqueries
- 2. Subqueries 2
- 4. Subqueries(cont’d) •The subquery(inner query) is executed once before the main query. •The result of subquery is used by the main query(outer query). 4
- 5. Subqueries(cont’d) There are following types of subqueries: •Single row •Multiple row 5
- 6. Subqueries(cont’d) Guidelines for using subqueries: •Enclose subquery in parenthesis •Place subquery on the right side of comparison condition •Use single row operators with single row queries and multiple with multiple queries 6
- 7. Subqueries(cont’d) There are two types of subqueries: •Single row subqueries •Multiple row subqueries 7
- 8. Subqueries(cont’d) Single row subqueries: •Returns only one row •Use single row comparison operators i.e. =, < , > , <= ,>=, <> 8
- 9. Subqueries(cont’d) Single row subqueries: •Example: SELECT * FROM teacher WHERE dept_id = (SELECT dept_id FROM department WHERE dept_name = ‘UIIT’); 9
- 10. Subqueries(cont’d) More than one single row subqueries: •Example: SELECT * FROM teacher WHERE dept_id = (SELECT dept_id FROM department WHERE dept_name = ‘UIIT’) AND salary > (SELECT AVG(salary) FROM teacher) ; 10
- 11. Subqueries(cont’d) HAVING clause single row subqueries: •Example: SELECT dept_id, MIN(salary) FROM employee GROUP BY dept_id HAVING MIN(salary) > (SELECT MIN(salary) FROM employee WHERE dept_name = ‘UIIT’); 11
- 12. Subqueries(cont’d) 12 Illegal queries are as following: • You cannot use multiple rows return queries for single row operators
- 13. Subqueries(cont’d) 13 Empty result subqueries are as following: • ‘no rows selected’ because subquery returns nothing
- 14. Multiple-Row Subqueries(cont’d) •Return more than one row •Uses multiple row comparison operators i.e. 14
- 15. Multiple-Row Subqueries(cont’d) Use of IN operator in multiple-row subqueries 15
- 16. Multiple-Row Subqueries(cont’d) Use of ANY operator in multiple-row subqueries: • ‘<ANY’ means less than maximum of returned values • ‘>ANY’ means more than the minimum of returned values •‘=ANY’ is equivalent to ‘IN’ operator 16
- 17. Multiple-Row Subqueries(cont’d) Use of ALL operator in multiple-row subqueries: • ‘<ALL’ means less than maximum of returned values • ‘>ALL’ means more than the minimum of returned values 17
- 18. Multiple-Row Subqueries(cont’d) NULL values in multiple-row subqueries: • If one of the returned values by inner query is NULL value then the entire query returns no values •‘NOT IN’ operator is equivalent to ‘<>ALL’ •This issue can be resolved by using an additional WHERE clause in the inner query i.e WHERE mgr.manager_id IS NOT NULL 18
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