Oracle Querying Duplicate Data
When we need to query duplicate data from the database, we can use various methods in Oracle to achieve it. In Oracle, duplicate data often refers to the same value appearing in the same column, and these values appear more than once.
The following introduces several ways to query Oracle duplicate data.
Method 1: Use GROUP BY and HAVING
The function of GROUP BY and HAVING is to group and filter data. We can use these two statements together to query duplicate data.
For example, we have a table named employees, which contains two fields: employee name (ename) and salary (sal). If we want to query the names and salaries of employees with duplicate salaries, we can use the following statement:
SELECT ename, sal, COUNT() FROM employees GROUP BY ename, sal HAVING COUNT() > ; 1;
After running the above query statement, we can get the names and salaries of all employees with duplicate salaries.
Method 2: Use window function
Window function is a method that can calculate aggregation or analysis functions in query results. We can use window functions to query duplicate data.
For example, we have a table named orders, which contains two fields: order number (order_id) and sales volume (sales). If we want to query the order number and sales volume with repeated sales, we can use the following statement:
SELECT order_id, sales FROM ( SELECT order_id, sales, COUNT(*) OVER(PARTITION BY sales) AS cnt FROM orders ) WHERE cnt > 1;
After running the above query statement, we can get the order number and sales volume of all repeated sales.
Method 3: Use self-join
Self-join is a method of querying data by connecting different instances of the same table. We can use self-join to query duplicate data.
For example, we have a table named customers, which contains two fields: customer name (name) and mobile phone number (phone). If we want to query the names and mobile phone numbers of customers with duplicate mobile phone numbers, we can use the following statement:
SELECT c1.name, c1.phone FROM customers c1 JOIN customers c2 ON c1.phone = c2.phone AND c1 .name <> c2.name;
After running the above query statement, we can get all the customer names and mobile phone numbers with duplicate mobile phone numbers.
Summary
In Oracle, we can use various statements such as GROUP BY and HAVING, window functions and self-joins to query repeated data. When using it, you need to choose the appropriate method according to the actual situation to achieve a more efficient query effect. At the same time, you also need to pay attention to the optimization of query statements to reduce query time and consumed resources as much as possible.
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