解析Oracle练习题：解析实例及答案（oracle练习题及答案）
Solutions to Oracle Exercises：Parsing Examples & Answers
Oracle providess a wealth of exercises for those who want to improve their skill in the relational database management system. In particular, parsing exercises can help you develop a solid understanding of how to structure complex queries. To help build expertise in this field, as well as provide potential answers for those stuck at a particular problem, this article will look at some of the most common Oracle exercise questions and provide detailed explanations.
Let’s start with a basic example. Suppose we wish to retrieve the last worker who had an anniversary with the company within the past year in order to give them a special award. In this scenario, we might use the following query to get the required data:
SELECT LAST_NAME, HIRED_DATE
FROM EMPLOYEES
WHERE (SYSDATE – HIRED_DATE)/ 365 BETWEEN 1 and 2
We can break this down into the following steps:
1. Use SELECT to retrieve data from the EMPLOYEES table.
2. Retrieve the LAST_NAME and HIRED_DATE columns.
3. Use a WHERE clause to filter records based on the calculated duration between the SYSDATE and the HIRED_DATE value.
4. By checking if the calculated duration is between 1 and 2 (in years), we can effectively retrieve only those employees who had an anniversary in the past year.
We can test our query quickly, as long as we are sure about the data in the table. This way, we can check if our query works – and if we’re getting the desired results.
This is just one example of a parsing exercise. Let’s look at another. Suppose we want to display the first and last names of managers who have a salary greater than the average salary of their department. We can use the following query:
SELECT FIRST_NAME, LAST_NAME
FROM EMPLOYEES
WHERE SALARY > (SELECT AVG(SALARY)
FROM EMPLOYEES
WHERE DEPT_ID = EMPLOYEES.DEPT_ID)
The steps to understand this query are similar to the earlier one:
1. Use SELECT to retrieve data from the EMPLOYEES table
2. Retrieve the FIRST_NAME and LAST_NAME columns.
3. Use a WHERE clause to filter records based on salary, compared to the average salary of the same department.
4. To calculate average salary for a particular department, we’ll use a sub-query that evaluates the average salary for all employees belonging to a particular department – with the department ID being taken from the employee’s record.
Similar to the previous exercise, we can quickly test if the query is giving us the desired results, barring discrepancies in our data.
These are just a few examples of parsing exercises. Solutions to advanced exercises can be quite complex, but a thorough understanding of the concept and good practice should bring you closer to solving the problem. Good luck with your Oracle studies!
