A good example of this is an id column which is the table's primary key. In contrast, if the defining column is unique across the whole table, you can use an uncorrelated delete. If your driving column is unique for each group, but may have duplicates elsewhere in the table, you'll need a correlated delete. insert date, id, etc.) that is not one of the copied values.Īssuming you have this, there are two ways you can build your delete. To do this you'll need another column in the table (e.g. For example, you might want to preserve the oldest row. To do this, you must first decide which rows you want to keep. It's better to build a single statement which removes all the unwanted copies in one go. But this is unworkable if there are a large number of duplicates. If there is only a handful, you could do this by hand. For simplicity, I'm going to assume that either the rows are exact copies or you don't care which you remove. Now you've identified the copies, you often want to delete the extra rows. How to Delete the Duplicate Rows Delete key1 by Büşra ÖZCOŞKUN CC BY-SA 4.0 With a list of the duplicates in hand, it's time for the next step! Select f.*, count(*) over (partition by title, uk_release_date) ct Or you could use the with clause: with film_counts as ( So if you have more than one email address in your user accounts table, how do you know who is who? USER_ID EMAIL_ADDRESS FIRST_NAME LAST_NAME It is common for websites to use your email address as your username. Or maybe there are only one or two column values that are copies. But all the "real world" information is repeated: FILM_ID TITLE UK_RELEASE_DATE LENGTH_IN_MINUTES RATING This means the rows are no longer exact copies. For example, it's common for tables to have an ID column. For example, the two rows below hold identical information: TITLE UK_RELEASE_DATE LENGTH_IN_MINUTES BBFC_RATINGīut often you need to find copies on a subset of the columns. SELECT t.customer_id, MIN(t.transaction_date) AS first_transaction_date, MAX(t.transaction_date) AS last_transaction_date, MAX(t.transaction_amount) – MIN(t.transaction_amount) AS diff_amount, MAX(t.transaction_date) – MIN(t.transaction_date) AS diff_date FROM transaction t GROUP BY t.So you're looking for two (or more) rows that are copies of each other. SELECT p.product_id, p.product_name FROM products p LEFT OUTER JOIN transaction t ON p.product_id = t.product_id WHERE t.transaction_id IS NULL 4. SELECT p.product_id, p.product_name, total_amount FROM products p JOIN ( SELECT product_id, SUM(transaction_amount) AS total_amount FROM transaction GROUP BY product_id ORDER BY total_amount DESC ) t ON p.product_id = t.product_id WHERE ROWNUM = 1 3. SELECT * FROM ( SELECT transaction_id, customer_id, transaction_date, product_id, transaction_amount, ROW_NUMBER() OVER (ORDER BY transaction_date DESC) AS rn FROM transaction WHERE customer_id = 1 –(let 1) and transaction_date >= TRUNC(SYSDATE, ‘MM’) – 30 ) WHERE rn <= 10 order by transaction_id,customer_id 2. Or, you can also jump to our below two sections on SQL query interview questions for freshers and experienced professionals.ġ. Write a query to find the 3rd highest salary from a table without top or limit keyword.Write an SQL query to fetch only odd rows from the table.Write an SQL query to fetch duplicate records from EmployeeDetails.Write an SQL query to fetch all the Employees who are also managers.Fetch employee names and salaries even if the salary value is not present for the employee.Write an SQL query to fetch a project-wise count of employees.Fetch all employees from EmployeeDetails who have a salary record in EmployeeSalary.SQL query to fetch all the Employees from EmployeeDetails who joined in the Year 2020.SQL query to fetch all the employees who are not working on any project.SQL Query to fetch records that are present in one table but not in another table.You can try solving these questions and click on the links to go to their respective answers. If you want to skip the basic questions and start with some tricky SQL queries then you can directly move to our SQL queries interview questions for the experienced section.Ĭonsider the below two tables for reference while trying to solve the SQL queries for practice.įor your convenience, I have compiled the top 10 questions for you. In fact, I have been asked most of these questions during interviews in the different phases of my career. Whether you are a DBA, developer, tester, or data analyst, these SQL query interview questions and answers are going to help you. Hello friends! in this post, we will see some of the most common SQL queries asked in interviews.
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