Lab # 6. Using Subqueries and Set Operators. Eng. Alaa O Shama

Transcription

1 The Islamic University of Gaza Faculty of Engineering Department of Computer Engineering ECOM 4113: Database Lab Lab # 6 Using Subqueries and Set Operators Eng. Alaa O Shama November, 2015

2 Objectives: Define subqueries and their types. Use a set operator to combine multiple queries into a single query. SUBQUERIES A subquery ia a SELECT statement that is embedded in the clause of another SELECT statement. You can build a powerful statements out of simple ones by using subqueries. You can write subqueries in the WHERE clause of another SQL statement to obtain values based on an unknown conditional value. Subquery Syntax: SELECT select_list FROM table WHERE expr operator (SELECT select_list FROM table);

3 The subquery (inner query ) executes before the main query (outer query). The result of the subquery is used by main query. Ex: Retrieve all employees who has a salary greater than the average salaries in department 80. To solve this query, we need to know the average salaries in department 80. First Query: Then, we can use this value in a new query to find the employees who has a salary greater than it. Second Query:

4 A better solution is to embed the first query in the second query in two parentheses. This solution is called SUBQUERY solution, the first query is called INNER QUERY and the second query is called OUTER QUERY. Types of SUBQUERIES Single Row SUBQUERIES: Queries that return only one row from the inner SELECT statement. Multiple Row SUBQUERIES: Queries that return more than one row from the inner SELECT statement. A single row SUBQUERY uses a single row operator (>, <>, etc...). However, multiple-row SUBQUERY uses multiple-row operator (IN, ALL, ANY), instead of a single-row operator.

5 The multiple-row comparison operators are: Operator Meaning IN (set) Equal to any member in the list. ANY (set) Must be preceded by =, <>, >, <, <=, >=. Compares a value to each value in a list or returned by a query. Evaluates to FALSE if the query returns no rows. ALL (set) Must be preceded by =, <>, >, <, <=, >=. Compares a value to every value in a list or returned by a query. Evaluates to TRUE if the query returns no rows. Ex: Retrieving all employees are not IT programmers and whose salary is less than that of any IT programmer. EX: Write a query to retrieve the employee number and last name of all employees who work in a department with any employee whose last name contains the letter u.

6 EX: Retrieve the last name, department number, and job ID of all employees whose department location ID is 1700.

7 Set Operators Set Operators are used to combine the results of two or more component queries into one result. Queries containing set operators are called compound queries. In oracle, there are 4 Set Operators: UNION: return all rows from multiple tables and eliminate any duplicate rows. UNION ALL: return all rows from multiple tables including all duplicate rows. INTERSECT: return all rows from multiple tables and eliminate any duplicate rows. MINUS: return all rows from multiple tables and eliminate any duplicate rows. Notes: Column count of all component queries must be the same. Data types of retrieved columns should match or at least should be implicitly convertible by database. Ex: write a query to retrieve department IDs for departments that do not contain the job ID ST_CLERK.

8 Ex: Retrieve name and id for all countries that have no departments located in them. Exercises: (must be in your report) 1. Write a query to retrieve all employees whose salary is more than the salary of any employee from department Write a query to retrieve departments names and cities for all departments that has more than 5 employees. 3. Write a query to retrieve name and id for all countries that have no departments located in them. 4. Write a query that prompts the user for an employee's last name. The query then displays the last name and hire date of any employee in the same department as the employee whose name they supply (excluding that employee). GOOD LUCK