7. Using Sub Queries
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This document discusses using subqueries to solve queries. It defines a subquery as a query within another query that can appear in the WHERE, HAVING, or FROM clauses. The document outlines the objectives of understanding subqueries and lists the types as single row or multiple row. It provides examples of each type, with single row subqueries returning a single value and using single comparison operators, while multiple row subqueries return multiple values and use operators like ANY, IN or ALL.
7. Using Sub Queries
- 2. USING SUB-QUERIES, TO SOLVE QUERIES.
- 3. OBJECTIVES At the end of this module, students should be able to: 1. Define sub-queries. 2. Know type of problems that sub-queries can solve. 3. List the types of sub-queries 4. Write single row and multiple row sub- queries.
- 4. SUB-QUERIES A sub-query –also called a nested SELECT statement- is defined as a query, within another query. It may appear in the WHERE clause, in the HAVING clause, or in the FROM clause. When placing a restriction on a query block, we always specify the conditions for the restriction in the WHERE clause.
- 5. EXAMPLE:
- 6. Under certain conditions, you may need to write a query to determine the WHERE condition.
- 7. EXERCISE: Using a single SELECT statement, query the employees table for those employees who earn a salary below Nancy’s salary. However, if you want to query for the details of all the staff who earn a salary less than that of Nancy you would use a sub-query for Nancy’s salary, and the result in the WHERE condition of our SELECT statement.
- 8. We have two types of sub-queries. The single row sub-queries and the multiple row sub- queries.
- 9. SINGLE ROW SUB_QUERIES The single row sub-queries, would return only one value, from the inner SELECT statement. In a single row sub-query, the outer query, contains a single row comparison operator, like an equal to, equal to or greater than, or less than.
- 10. EXAMPLE:
- 11. This is a single row sub-query because the inner SELECT statement will return only one value and the outer SELECT statement uses a single comparison operator to pick up that value.
- 12. MULTIPLE ROW SUB_QUERIES A multiple row sub-query on the contrary would return more than one row from the inner SELECT statement. The outer SELECT will therefore need a multiple comparison operator like ANY, IN or ALL to pick the multiple values returned by the inner query.
- 13. EXAMPLE:
- 14. The Oracle server first takes the inner query and searches for the DISTINCT values in the salary of employees, with STORE CLERK as the job_id. It then places this numbers in the bracket next to the ANY keyword. And that is what gives us the result displayed, on the previous screen.